BusinessObjects Topics

Tuesday, November 11, 2008

Oracle Learning - 13

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Oracle/PLSQL: Set Transaction

There are three transaction control functions. These are:
1. SET TRANSACTION READ ONLY;
2. SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
3. SET TRANSACTION USE ROLLBACK SEGMENT name;

Oracle/PLSQL: Lock Table

The syntax for a Lock table is:
LOCK TABLE tables IN lock_mode MODE [NOWAIT];
Tables is a comma-delimited list of tables.
Lock_mode is one of:
ROW SHARE
ROW EXCLUSIVE
SHARE UPDATE
SHARE
SHARE ROW EXCLUSIVE
EXCLUSIVE.
NoWait specifies that the database should not wait for a lock to be released.
Oracle/PLSQL Topics: Cursors

A cursor is a mechanism by which you can assign a name to a "select statement" and manipulate the information within that SQL statement.
A cursor is a SELECT statement that is defined within the declaration section of your PLSQL code. We'll take a look at three different syntaxes for cursors.
Cursor without parameters (simplest)
The basic syntax for a cursor without parameters is:
CURSOR cursor_name
IS
SELECT_statement;

For example, you could define a cursor called c1 as below.
CURSOR c1
IS
SELECT course_number
from courses_tbl
where course_name = name_in;
The result set of this cursor is all course_numbers whose course_name matches the variable called name_in.

Below is a function that uses this cursor.
CREATE OR REPLACE Function FindCourse
( name_in IN varchar2 )
RETURN number
IS
cnumber number;
CURSOR c1
IS
SELECT course_number
from courses_tbl
where course_name = name_in;

BEGIN
open c1;
fetch c1 into cnumber;

if c1%notfound then
cnumber := 9999;
end if;

close c1;
RETURN cnumber;
END;

Cursor with parameters
The basic syntax for a cursor with parameters is:
CURSOR cursor_name (parameter_list)
IS
SELECT_statement;

For example, you could define a cursor called c2 as below.
CURSOR c2 (subject_id_in IN varchar2)
IS
SELECT course_number
from courses_tbl
where subject_id = subject_id_in);
The result set of this cursor is all course_numbers whose subject_id matches the subject_id passed to the cursor via the parameter.

Cursor with return clause
The basic syntax for a cursor with a return clause is:
CURSOR cursor_name
RETURN field%ROWTYPE
IS
SELECT_statement;
For example, you could define a cursor called c3 as below.
CURSOR c3
RETURN courses_tbl%ROWTYPE
IS
SELECT *
from courses_tbl
where subject = 'Mathematics;
The result set of this cursor is all columns from the course_tbl where the subject is Mathematics.
Oracle/PLSQL: OPEN Statement

Once you've declared your cursor, the next step is to open the cursor.
The basic syntax to OPEN the cursor is:
OPEN cursor_name;

For example, you could open a cursor called c1 with the following command:
OPEN c1;

Below is a function that demonstrates how to use the OPEN statement:
CREATE OR REPLACE Function FindCourse
( name_in IN varchar2 )
RETURN number
IS
cnumber number;
CURSOR c1
IS
SELECT course_number
from courses_tbl
where course_name = name_in;

BEGIN
open c1;
fetch c1 into cnumber;

if c1%notfound then
cnumber := 9999;
end if;

close c1;
RETURN cnumber;
END;

Oracle/PLSQL: FETCH Statement

The purpose of using a cursor, in most cases, is to retrieve the rows from your cursor so that some type of operation can be performed on the data. After declaring and opening your cursor, the next step is to FETCH the rows from your cursor.
The basic syntax for a FETCH statement is:
FETCH cursor_name INTO ;

For example, you could could have a cursor defined as:
CURSOR c1
IS
SELECT course_number
from courses_tbl
where course_name = name_in;
The command that would be used to fetch the data from this cursor is:
FETCH c1 into cnumber;
This would fetch the first course_number into the variable called cnumber;

Below is a function that demonstrates how to use the FETCH statement.
CREATE OR REPLACE Function FindCourse
( name_in IN varchar2 )
RETURN number
IS
cnumber number;
CURSOR c1
IS
SELECT course_number
from courses_tbl
where course_name = name_in;

BEGIN
open c1;
fetch c1 into cnumber;

if c1%notfound then
cnumber := 9999;
end if;

close c1;
RETURN cnumber;
END;
Oracle/PLSQL: CLOSE Statement

The final step of working with cursors is to close the cursor once you have finished using it.
The basic syntax to CLOSE the cursor is:
CLOSE cursor_name;

For example, you could close a cursor called c1 with the following command:
CLOSE c1;

Below is a function that demonstrates how to use the CLOSE statement:
CREATE OR REPLACE Function FindCourse
( name_in IN varchar2 )
RETURN number
IS
cnumber number;
CURSOR c1
IS
SELECT course_number
from courses_tbl
where course_name = name_in;

BEGIN
open c1;
fetch c1 into cnumber;

if c1%notfound then
cnumber := 9999;
end if;

close c1;
RETURN cnumber;
END;

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Oracle/PLSQL: Sequences (Autonumber)

In Oracle, you can create an autonumber field by using sequences. A sequence is an object in Oracle that is used to generate a number sequence. This can be useful when you need to create a unique number to act as a primary key.
The syntax for a sequence is:
CREATE SEQUENCE sequence_name
MINVALUE value
MAXVALUE value
START WITH value
INCREMENT BY value
CACHE value;
For example:
CREATE SEQUENCE supplier_seq
MINVALUE 1
MAXVALUE 999999999999999999999999999
START WITH 1
INCREMENT BY 1
CACHE 20;
This would create a sequence object called supplier_seq. The first sequence number that it would use is 1 and each subsequent number would increment by 1 (ie: 2,3,4,...}. It will cache up to 20 values for performance.
If you omit the MAXVALUE option, your sequence will automatically default to:
MAXVALUE 999999999999999999999999999
So you can simplify your CREATE SEQUENCE command as follows:
CREATE SEQUENCE supplier_seq
MINVALUE 1
START WITH 1
INCREMENT BY 1
CACHE 20;
Now that you've created a sequence object to simulate an autonumber field, we'll cover how to retrieve a value from this sequence object. To retrieve the next value in the sequence order, you need to use nextval.
For example:
supplier_seq.nextval
This would retrieve the next value from supplier_seq. The nextval statement needs to be used in an SQL statement. For example:
INSERT INTO suppliers
(supplier_id, supplier_name)
VALUES
(supplier_seq.nextval, 'Kraft Foods');
This insert statement would insert a new record into the suppliers table. The supplier_id field would be assigned the next number from the supplier_seq sequence. The supplier_name field would be set to Kraft Foods.

Frequently Asked Questions

One common question about sequences is:
Question: While creating a sequence, what does cache and nocache options mean? For example, you could create a sequence with a cache of 20 as follows:
CREATE SEQUENCE supplier_seq
MINVALUE 1
START WITH 1
INCREMENT BY 1
CACHE 20;

Or you could create the same sequence with the nocache option:
CREATE SEQUENCE supplier_seq
MINVALUE 1
START WITH 1
INCREMENT BY 1
NOCACHE;

Answer: With respect to a sequence, the cache option specifies how many sequence values will be stored in memory for faster access.
The downside of creating a sequence with a cache is that if a system failure occurs, all cached sequence values that have not be used, will be "lost". This results in a "gap" in the assigned sequence values. When the system comes back up, Oracle will cache new numbers from where it left off in the sequence, ignoring the so called "lost" sequence values.
Note: To recover the lost sequence values, you can always execute an ALTER SEQUENCE command to reset the counter to the correct value.
Nocache means that none of the sequence values are stored in memory. This option may sacrifice some performance, however, you should not encounter a gap in the assigned sequence values.

Question: How do we set the LASTVALUE value in an Oracle Sequence?
Answer: You can change the LASTVALUE for an Oracle sequence, by executing an ALTER SEQUENCE command.
For example, if the last value used by the Oracle sequence was 100 and you would like to reset the sequence to serve 225 as the next value. You would execute the following commands.
alter sequence seq_name
increment by 124;
select seq_name.nextval from dual;
alter sequence seq_name
increment by 1;
Now, the next value to be served by the sequence will be 225.

Oracle/PLSQL: Commit

The syntax for the COMMIT statement is:
COMMIT [WORK] [COMMENT text];
The Commit statement commits all changes for the current session. Once a commit is issued, other users will be able to see your changes.
Oracle/PLSQL: Commit

The syntax for the COMMIT statement is:
COMMIT [WORK] [COMMENT text];
The Commit statement commits all changes for the current session. Once a commit is issued, other users will be able to see your changes.

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Oracle/PLSQL: FOR Loop

The syntax for the FOR Loop is:
FOR loop_counter IN [REVERSE] lowest_number..highest_number
LOOP
{.statements.}
END LOOP;
You would use a FOR Loop when you want to execute the loop body a fixed number of times.

Let's take a look at an example.
FOR Lcntr IN 1..20
LOOP
LCalc := Lcntr * 31;
END LOOP;
This example will loop 20 times. The counter will start at 1 and end at 20.

The FOR Loop can also loop in reverse. For example:
FOR Lcntr IN REVERSE 1..15
LOOP
LCalc := Lcntr * 31;
END LOOP;
This example will loop 15 times. The counter will start at 15 and end at 1. (loops backwards)

Oracle/PLSQL: CURSOR FOR Loop

The syntax for the CURSOR FOR Loop is:
FOR record_index in cursor_name
LOOP
{.statements.}
END LOOP;
You would use a CURSOR FOR Loop when you want to fetch and process every record in a cursor. The CURSOR FOR Loop will terminate when all of the records in the cursor have been fetched.
Here is an example of a function that uses a CURSOR FOR Loop:
CREATE OR REPLACE Function TotalIncome
( name_in IN varchar2 )
RETURN varchar2
IS
total_val number(6);

cursor c1 is
select monthly_income
from employees
where name = name_in;

BEGIN
total_val := 0;
FOR employee_rec in c1
LOOP
total_val := total_val + employee_rec.monthly_income;
END LOOP;

RETURN total_val;
END;
In this example, we've created a cursor called c1. The CURSOR FOR Loop will terminate after all records have been fetched from the cursor c1.
Oracle/PLSQL: While Loop

The syntax for the WHILE Loop is:
WHILE condition
LOOP
{.statements.}
END LOOP;
You would use a WHILE Loop when you are not sure how many times you will execute the loop body. Since the WHILE condition is evaluated before entering the loop, it is possible that the loop body may not execute even once.
Let's take a look at an example:
WHILE monthly_value <= 4000
LOOP
monthly_value := daily_value * 31;
END LOOP;
In this example, the WHILE Loop would terminate once the monthly_value exceeded 4000.

Oracle/PLSQL: Repeat Until Loop

Oracle doesn't have a Repeat Until loop, but you can emulate one. The syntax for emulating a REPEAT UNTIL Loop is:
LOOP
{.statements.}
EXIT WHEN boolean_condition;
END LOOP;
You would use an emulated REPEAT UNTIL Loop when you do not know how many times you want the loop body to execute. The REPEAT UNTIL Loop would terminate when a certain condition was met.
Let's take a look at an example:
LOOP
monthly_value := daily_value * 31;
EXIT WHEN monthly_value > 4000;
END LOOP;
In this example, the LOOP would repeat until the monthly_value exceeded 4000.
Oracle/PLSQL: Exit Statement

The syntax for the EXIT statement is:
EXIT [WHEN boolean_condition];
The EXIT statement is most commonly used to terminate LOOP statements.
Let's take a look at an example:
LOOP
monthly_value := daily_value * 31;
EXIT WHEN monthly_value > 4000;
END LOOP;
In this example, the LOOP would terminate when the monthly_value exceeded 4000.

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Oracle/PLSQL: Case Statement

In Oracle 9i, you can use the case statement within an SQL statement. It has the functionality of an IF-THEN-ELSE statement.
The syntax for the case statement is:
CASE expression
WHEN condition_1 THEN result_1
WHEN condition_2 THEN result_2
...
WHEN condition_n THEN result_n
ELSE result END
expression is the value that you are comparing to the list of conditions. (ie: condition_1, condition_2, ... condition_n)
condition_1 to condition_n must all be the same datatype. Conditions are evaluated in the order listed. Once a condition is found to be true, the case statement will return the result and not evaluate the conditions any further.
result_1 to result_n must all be the same datatype. This is the value returned once a condition is found to be true.

Note:
If no condition is found to be true, then the case statement will return the value in the ELSE clause.
If the ELSE clause is omitted and no condition is found to be true, then the case statement will return NULL.
You can have up to 255 comparisons in a case statement. Each WHEN ... THEN clause is considered 2 comparisons.

For Example:
You could use the case statement in an SQL statement as follows:
select table_name,
CASE owner
WHEN 'SYS' THEN 'The owner is SYS'
WHEN 'SYSTEM' THEN 'The owner is SYSTEM'
ELSE 'The owner is another value' END
from all_tables;

The above case statement is equivalent to the following IF-THEN-ELSE statement:
IF owner = 'SYS' THEN
result := 'The owner is SYS';
ELSIF owner = 'SYSTEM' THEN
result := 'The owner is SYSTEM'';
ELSE
result := 'The owner is another value';
END IF;

The case statement will compare each owner value, one by one.

One thing to note is that the ELSE clause within the case statement is optional. You could have omitted it. Let's take a look at the SQL statement above with the ELSE clause omitted.
Your SQL statement would look as follows:
select table_name,
CASE owner
WHEN 'SYS' THEN 'The owner is SYS'
WHEN 'SYSTEM' THEN 'The owner is SYSTEM' END
from all_tables;
With the ELSE clause omitted, if no condition was found to be true, the case statement would return NULL.

Oracle/PLSQL: GOTO Statement

The GOTO statement causes the code to branch to the label after the GOTO statement.
For example:
GOTO label_name;

Then later in the code, you would place your label and code associated with that label.
Label_name: {statements

Oracle/PLSQL: Loop Statement

The syntax for the LOOP statement is:
LOOP
{.statements.}
END LOOP;
You would use a LOOP statement when you are not sure how many times you want the loop body to execute and you want the loop body to execute at least once.
The LOOP statement is terminated when it encounters either an EXIT statement or when it encounters an EXIT WHEN statement that evaluated to TRUE.
Let's take a look at an example:
LOOP
monthly_value := daily_value * 31;
EXIT WHEN monthly_value > 4000;
END LOOP;
In this example, the LOOP would terminate when the monthly_value exceeded 4000.

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Oracle/PLSQL: IS NULL

In other languages, a null value is found using the = null syntax. However in PLSQL to check if a value is null, you must use the "IS NULL" syntax.
To check for equality on a null value, you must use "IS NULL".
For example,
IF Lvalue IS NULL then
.
END IF;
If Lvalue contains a null value, the "IF" expression will evaluate to TRUE.

You can also use "IS NULL" in an SQL statement. For example:
select * from suppliers
where supplier_name IS NULL;
This will return all records from the suppliers table where the supplier_name contains a null value.

Oracle/PLSQL: IS NOT NULL

In other languages, a not null value is found using the != null syntax. However in PLSQL to check if a value is not null, you must use the "IS NOT NULL" syntax.
For example,
IF Lvalue IS NOT NULL then
.
END IF;
If Lvalue does not contain a null value, the "IF" expression will evaluate to TRUE.

You can also use "IS NOT NULL" in an SQL statement. For example:
select * from suppliers
where supplier_name IS NOT NULL;
This will return all records from the suppliers table where the supplier_name does not contain a null value.

Oracle/PLSQL: IF-THEN-ELSE Statement

There are three different syntaxes for these types of statements.
Syntax #1: IF-THEN
IF condition THEN
{...statements...}
END IF;

Syntax #2: IF-THEN-ELSE
IF condition THEN
{...statements...}
ELSE
{...statements...}
END IF;

Syntax #3: IF-THEN-ELSIF
IF condition THEN
{...statements...}
ELSIF condition THEN
{...statements...}
ELSE
{...statements...}
END IF;

Here is an example of a function that uses the IF-THEN-ELSE statement:
CREATE OR REPLACE Function IncomeLevel
( name_in IN varchar2 )
RETURN varchar2
IS
monthly_value number(6);
ILevel varchar2(20);
cursor c1 is
select monthly_income
from employees
where name = name_in;
BEGIN
open c1;
fetch c1 into monthly_value;
close c1;
IF monthly_value <= 4000 THEN ILevel := 'Low Income'; ELSIF monthly_value > 4000 and monthly_value <= 7000 THEN ILevel := 'Avg Income'; ELSIF monthly_value > 7000 and monthly_value <= 15000 THEN
ILevel := 'Moderate Income';
ELSE
ILevel := 'High Income';
END IF;
RETURN ILevel;
END;
In this example, we've created a function called IncomeLevel. It has one parameter called name_in and it returns a varchar2. The function will return the income level based on the employee's name.

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SQL: CREATE Table

The basic syntax for a CREATE TABLE is:
CREATE TABLE table_name
(column1 datatype null/not null,
column2 datatype null/not null,
...
);
Each column must have a datatype. The column should either be defined as "null" or "not null" and if this value is left blank, the database assumes "null" as the default.

For example:

CREATE TABLE supplier
( supplier_id numeric(10) not null,
supplier_name varchar2(50) not null,
contact_name varchar2(50)
)

SQL: CREATE Table from another table

You can also create a table from an existing table by copying the existing table's columns.
It is important to note that when creating a table in this way, the new table will be populated with the records from the existing table (based on the SELECT Statement).

Syntax #1 - Copying all columns from another table
The basic syntax is:
CREATE TABLE new_table
AS (SELECT * FROM old_table);

For example:
CREATE TABLE suppliers
AS (SELECT *
FROM companies
WHERE id > 1000);
This would create a new table called suppliers that included all columns from the companies table.
If there were records in the companies table, then the new suppliers table would also contain the records selected by the SELECT statement.

Syntax #2 - Copying selected columns from another table
The basic syntax is:
CREATE TABLE new_table
AS (SELECT column_1, column2, ... column_n FROM old_table);

For example:
CREATE TABLE suppliers
AS (SELECT id, address, city, state, zip
FROM companies
WHERE id > 1000);
This would create a new table called suppliers, but the new table would only include the specified columns from the companies table.
Again, if there were records in the companies table, then the new suppliers table would also contain the records selected by the SELECT statement.

Syntax #3 - Copying selected columns from multiple tables
The basic syntax is:
CREATE TABLE new_table
AS (SELECT column_1, column2, ... column_n
FROM old_table_1, old_table_2, ... old_table_n);

For example:
CREATE TABLE suppliers
AS (SELECT companies.id, companies.address, categories.cat_type
FROM companies, categories
WHERE companies.id = categories.id
AND companies.id > 1000);
This would create a new table called suppliers based on columns from both the companies and categories tables.

SQL: ALTER Table

The ALTER TABLE command allows you to add, modify, or drop a column from an existing table.

Adding column(s) to a table
Syntax #1
To add a column to an existing table, the ALTER TABLE syntax is:
ALTER TABLE table_name
ADD column_name column-definition;
For example:
ALTER TABLE supplier
ADD supplier_name varchar2(50);
This will add a column called supplier_name to the supplier table.

Syntax #2
To add multiple columns to an existing table, the ALTER TABLE syntax is:

ALTER TABLE table_name
ADD ( column_1 column-definition,
column_2 column-definition,
...
column_n column_definition );
For example:

ALTER TABLE supplier
ADD ( supplier_name varchar2(50),
city varchar2(45) );
This will add two columns (supplier_name and city) to the supplier table.

Modifying column(s) in a table
Syntax #1
To modify a column in an existing table, the ALTER TABLE syntax is:
ALTER TABLE table_name
MODIFY column_name column_type;
For example:
ALTER TABLE supplier
MODIFY supplier_name varchar2(100) not null;
This will modify the column called supplier_name to be a data type of varchar2(100) and force the column to not allow null values.

Syntax #2
To modify multiple columns in an existing table, the ALTER TABLE syntax is:

ALTER TABLE table_name
MODIFY ( column_1 column_type,
column_2 column_type,
...
column_n column_type );
For example:

ALTER TABLE supplier
MODIFY ( supplier_name varchar2(100) not null,
city varchar2(75) );
This will modify both the supplier_name and city columns.

Drop column(s) in a table
Syntax #1
To drop a column in an existing table, the ALTER TABLE syntax is:
ALTER TABLE table_name
DROP COLUMN column_name;
For example:
ALTER TABLE supplier
DROP COLUMN supplier_name;
This will drop the column called supplier_name from the table called supplier.

Rename column(s) in a table
(NEW in Oracle 9i Release 2)
Syntax #1
Starting in Oracle 9i Release 2, you can now rename a column.
To rename a column in an existing table, the ALTER TABLE syntax is:
ALTER TABLE table_name
RENAME COLUMN old_name to new_name;
For example:
ALTER TABLE supplier
RENAME COLUMN supplier_name to sname;
This will rename the column called supplier_name to sname.

SQL: DROP Table

The basic syntax for a DROP TABLE is:
DROP TABLE table_name;

For example:
DROP TABLE supplier;
This would drop table called supplier.

SQL: Global Temporary tables

Global temporary tables are distinct within SQL sessions.
The basic syntax is:
CREATE GLOBAL TEMPORARY TABLE table_name ( ...);

For example:

CREATE GLOBAL TEMPORARY TABLE supplier
( supplier_id numeric(10) not null,
supplier_name varchar2(50) not null,
contact_name varchar2(50)
)
This would create a global temporary table called supplier .

SQL: Local Temporary tables

Local temporary tables are distinct within modules and embedded SQL programs within SQL sessions.
The basic syntax is:
DECLARE LOCAL TEMPORARY TABLE table_name ( ...);

SQL: VIEWS

A view is, in essence, a virtual table. It does not physically exist. Rather, it is created by a query joining one or more tables.
The syntax for a VIEW is:
CREATE VIEW view_name AS
SELECT columns
FROM table
WHERE predicates;

For example:
CREATE VIEW sup_orders AS
SELECT supplier.supplier_id, orders.quantity, orders.price
FROM supplier, orders
WHERE supplier.supplier_id = orders.supplier_id
and supplier.supplier_name = 'IBM';
This would create a virtual table based on the result set of the select statement. You can now query the view as follows:
SELECT *
FROM sup_orders;

Frequently Asked Questions

Question: Can you update the data in a view?
Answer: A view is created by joining one or more tables. When you update record(s) in a view, it updates the records in the underlying tables that make up the view.
So, yes, you can update the data in a view providing you have the proper privileges to the underlying tables.

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SQL: INTERSECT Query

The INTERSECT query allows you to return the results of 2 or more "select" queries. However, it only returns the rows selected by all queries. If a record exists in one query and not in the other, it will be omitted from the INTERSECT results.
Each SQL statement within the INTERSECT query must have the same number of fields in the result sets with similar data types.
The syntax for an INTERSECT query is:
select field1, field2, . field_n
from tables
INTERSECT
select field1, field2, . field_n
from tables;

Example #1
The following is an example of an INTERSECT query:
select supplier_id
from suppliers
INTERSECT
select supplier_id
from orders;
In this example, if a supplier_id appeared in both the suppliers and orders table, it would appear in your result set.

Example #2 - With ORDER BY Clause
The following is an INTERSECT query that uses an ORDER BY clause:
select supplier_id, supplier_name
from suppliers
where supplier_id > 2000
INTERSECT
select company_id, company_name
from companies
where company_id > 1000
ORDER BY 2;
Since the column names are different between the two "select" statements, it is more advantageous to reference the columns in the ORDER BY clause by their position in the result set. In this example, we've sorted the results by supplier_name / company_name in ascending order, as denoted by the "ORDER BY 2".
The supplier_name / company_name fields are in position #2 in the result set.

SQL: MINUS Query

The MINUS query returns all rows in the first query that are not returned in the second query.
Each SQL statement within the MINUS query must have the same number of fields in the result sets with similar data types.
The syntax for an MINUS query is:
select field1, field2, . field_n
from tables
MINUS
select field1, field2, . field_n
from tables;

Example #1
The following is an example of an MINUS query:
select supplier_id
from suppliers
MINUS
select supplier_id
from orders;
In this example, the SQL would return all supplier_id values that are in the suppliers table and not in the orders table. What this means is that if a supplier_id value existed in the suppliers table and also existed in the orders table, the supplier_id value would not appear in this result set.

Example #2 - With ORDER BY Clause
The following is an MINUS query that uses an ORDER BY clause:
select supplier_id, supplier_name
from suppliers
where supplier_id > 2000
MINUS
select company_id, company_name
from companies
where company_id > 1000
ORDER BY 2;
Since the column names are different between the two "select" statements, it is more advantageous to reference the columns in the ORDER BY clause by their position in the result set. In this example, we've sorted the results by supplier_name / company_name in ascending order, as denoted by the "ORDER BY 2".
The supplier_name / company_name fields are in position #2 in the result set.
SQL: UPDATE Statement

The UPDATE statement allows you to update a single record or multiple records in a table.
The syntax the UPDATE statement is:
UPDATE table
SET column = expression
WHERE predicates;

Example #1 - Simple example
Let's take a look at a very simple example.
UPDATE supplier
SET name = 'HP'
WHERE name = 'IBM';
This statement would update all supplier names in the supplier table from IBM to HP.

Example #2 - More complex example
You can also perform more complicated updates.
You may wish to update records in one table based on values in another table. Since you can't list more than one table in the UPDATE statement, you can use the EXISTS clause.
For example:

UPDATE supplier
SET supplier_name = ( SELECT customer.name
FROM customers
WHERE customers.customer_id = supplier.supplier_id)
WHERE EXISTS
( SELECT customer.name
FROM customers
WHERE customers.customer_id = supplier.supplier_id);
Whenever a supplier_id matched a customer_id value, the supplier_name would be overwritten to the customer name from the customers table.
Learn more about the EXISTS condition.

SQL: INSERT Statement

The INSERT statement allows you to insert a single record or multiple records into a table.
The syntax for the INSERT statement is:
INSERT INTO table
(column-1, column-2, ... column-n)
VALUES
(value-1, value-2, ... value-n);

Example #1 - Simple example
Let's take a look at a very simple example.
INSERT INTO supplier
(supplier_id, supplier_name)
VALUES
(24553, 'IBM');
This would result in one record being inserted into the supplier table. This new record would have a supplier_id of 24553 and a supplier_name of IBM.

Example #2 - More complex example
You can also perform more complicated inserts using sub-selects.
For example:
INSERT INTO supplier
(supplier_id, supplier_name)
SELECT account_no, name
FROM customers
WHERE city = 'Newark';
By placing a "select" in the insert statement, you can perform multiples inserts quickly.
With this type of insert, you may wish to check for the number of rows being inserted. You can determine the number of rows that will be inserted by running the following SQL statement before performing the insert.
SELECT count(*)
FROM customers
WHERE city = 'Newark';

Frequently Asked Questions

Question: I am setting up a database with clients. I know that you use the "insert" statement to insert information in the database, but how do I make sure that I do not enter the same client information again?
Answer: You can make sure that you do not insert duplicate information by using the EXISTS condition.
For example, if you had a table named clients with a primary key of client_id, you could use the following statement:
INSERT INTO clients
(client_id, client_name, client_type)
SELECT supplier_id, supplier_name, 'advertising'
FROM suppliers
WHERE not exists (select * from clients
where clients.client_id = suppliers.supplier_id);
This statement inserts multiple records with a subselect.
If you wanted to insert a single record, you could use the following statement:
INSERT INTO clients
(client_id, client_name, client_type)
SELECT 10345, 'IBM', 'advertising'
FROM dual
WHERE not exists (select * from clients
where clients.client_id = 10345);
The use of the dual table allows you to enter your values in a select statement, even though the values are not currently stored in a table.
Learn more about the EXISTS condition.

SQL: DELETE Statement

The DELETE statement allows you to delete a single record or multiple records from a table.
The syntax for the DELETE statement is:
DELETE FROM table
WHERE predicates;

Example #1 - Simple example
Let's take a look at a simple example:
DELETE FROM supplier
WHERE supplier_name = 'IBM';
This would delete all records from the supplier table where the supplier_name is IBM.
You may wish to check for the number of rows that will be deleted. You can determine the number of rows that will be deleted by running the following SQL statement before performing the delete.
SELECT count(*)
FROM supplier
WHERE supplier_name = 'IBM';

Example #2 - More complex example
You can also perform more complicated deletes.
You may wish to delete records in one table based on values in another table. Since you can't list more than one table in the FROM clause when you are performing a delete, you can use the EXISTS clause.
For example:
DELETE FROM supplier
WHERE EXISTS
( select customer.name
from customer
where customer.customer_id = supplier.supplier_id
and customer.customer_name = 'IBM' );
This would delete all records in the supplier table where there is a record in the customer table whose name is IBM, and the customer_id is the same as the supplier_id.
Learn more about the EXISTS condition.
If you wish to determine the number of rows that will be deleted, you can run the following SQL statement before performing the delete.
SELECT count(*) FROM supplier
WHERE EXISTS
( select customer.name
from customer
where customer.customer_id = supplier.supplier_id
and customer.customer_name = 'IBM' );

Frequently Asked Questions

Question: How would I write an SQL statement to delete all records in TableA whose data in field1 & field2 DO NOT match the data in fieldx & fieldz of TableB?
Answer: You could try something like this:
DELETE FROM TableA
WHERE NOT EXISTS
( select *
from TableB
where TableA .field1 = TableB.fieldx
and TableA .field2 = TableB.fieldz );

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SQL: UNION Query

The UNION query allows you to combine the result sets of 2 or more "select" queries. It removes duplicate rows between the various "select" statements.
Each SQL statement within the UNION query must have the same number of fields in the result sets with similar data types.
The syntax for a UNION query is:
select field1, field2, . field_n
from tables
UNION
select field1, field2, . field_n
from tables;

Example #1
The following is an example of a UNION query:
select supplier_id
from suppliers
UNION
select supplier_id
from orders;
In this example, if a supplier_id appeared in both the suppliers and orders table, it would appear once in your result set. The UNION removes duplicates.

Example #2 - With ORDER BY Clause
The following is a UNION query that uses an ORDER BY clause:
select supplier_id, supplier_name
from suppliers
where supplier_id > 2000
UNION
select company_id, company_name
from companies
where company_id > 1000
ORDER BY 2;
Since the column names are different between the two "select" statements, it is more advantageous to reference the columns in the ORDER BY clause by their position in the result set. In this example, we've sorted the results by supplier_name / company_name in ascending order, as denoted by the "ORDER BY 2".
The supplier_name / company_name fields are in position #2 in the result set.

Frequently Asked Questions

Question: I need to compare two dates and return the count of a field based on the date values. For example, I have a date field in a table called last updated date. I have to check if trunc(last_updated_date >= trun(sysdate-13).
Answer: Since you are using the COUNT function which is an aggregate function, we'd recommend using a UNION query. For example, you could try the following:
SELECT a.code as Code, a.name as Name, count(b.Ncode)
FROM cdmaster a, nmmaster b
WHERE a.code = b.code
and a.status = 1
and b.status = 1
and b.Ncode <> 'a10'
and trunc(last_updated_date) <= trunc(sysdate-13)
group by a.code, a.name
UNION
SELECT a.code as Code, a.name as Name, count(b.Ncode)
FROM cdmaster a, nmmaster b
WHERE a.code = b.code
and a.status = 1
and b.status = 1
and b.Ncode <> 'a10'
and trunc(last_updated_date) > trunc(sysdate-13)
group by a.code, a.name;
The UNION query allows you to perform a COUNT based on one set of criteria.
trunc(last_updated_date) <= trunc(sysdate-13)
As well as perform a COUNT based on another set of criteria.
trunc(last_updated_date) > trunc(sysdate-13)

SQL: UNION ALL Query

The UNION ALL query allows you to combine the result sets of 2 or more "select" queries. It returns all rows (even if the row exists in more than one of the "select" statements).
Each SQL statement within the UNION ALL query must have the same number of fields in the result sets with similar data types.
The syntax for a UNION ALL query is:
select field1, field2, . field_n
from tables
UNION ALL
select field1, field2, . field_n
from tables;

Example #1
The following is an example of a UNION ALL query:
select supplier_id
from suppliers
UNION ALL
select supplier_id
from orders;
If a supplier_id appeared in both the suppliers and orders table, it would appear multiple times in your result set. The UNION ALL does not remove duplicates.

Example #2 - With ORDER BY Clause
The following is a UNION query that uses an ORDER BY clause:
select supplier_id, supplier_name
from suppliers
where supplier_id > 2000
UNION ALL
select company_id, company_name
from companies
where company_id > 1000
ORDER BY 2;
Since the column names are different between the two "select" statements, it is more advantageous to reference the columns in the ORDER BY clause by their position in the result set. In this example, we've sorted the results by supplier_name / company_name in ascending order, as denoted by the "ORDER BY 2".
The supplier_name / company_name fields are in position #2 in the result set.

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SQL: Joins

A join is used to combine rows from multiple tables. A join is performed whenever two or more tables is listed in the FROM clause of an SQL statement.
There are different kinds of joins. Let's take a look at a few examples.

Inner Join (simple join)
Chances are, you've already written an SQL statement that uses an inner join. It is is the most common type of join. Inner joins return all rows from multiple tables where the join condition is met.
For example,
SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date
FROM suppliers, orders
WHERE suppliers.supplier_id = orders.supplier_id;
This SQL statement would return all rows from the suppliers and orders tables where there is a matching supplier_id value in both the suppliers and orders tables.

Let's look at some data to explain how inner joins work:
We have a table called suppliers with two fields (supplier_id and supplier_ name).
It contains the following data:

supplier_id supplier_name
10000 IBM
10001 Hewlett Packard
10002 Microsoft
10003 Nvidia

We have another table called orders with three fields (order_id, supplier_id, and order_date).
It contains the following data:

order_id supplier_id order_date
500125 10000 2003/05/12
500126 10001 2003/05/13

If we ran the SQL statement below:
SELECT suppliers.supplier_id, suppliers.supplier_name, orders.order_date
FROM suppliers, orders
WHERE suppliers.supplier_id = orders.supplier_id;

Our result set would look like this:

supplier_id name order_date
10000 IBM 2003/05/12
10001 Hewlett Packard 2003/05/13
The rows for Microsoft and Nvidia from the supplier table would be omitted, since the supplier_id's 10002 and 10003 do not exist in both tables.

Outer Join
Another type of join is called an outer join. This type of join returns all rows from one table and only those rows from a secondary table where the joined fields are equal (join condition is met).
For example,
select suppliers.supplier_id, suppliers.supplier_name, orders.order_date
from suppliers, orders
where suppliers.supplier_id = orders.supplier_id(+);
This SQL statement would return all rows from the suppliers table and only those rows from the orders table where the joined fields are equal.
The (+) after the orders.supplier_id field indicates that, if a supplier_id value in the suppliers table does not exist in the orders table, all fields in the orders table will display as in the result set.
The above SQL statement could also be written as follows:
select suppliers.supplier_id, suppliers.supplier_name, orders.order_date
from suppliers, orders
where orders.supplier_id(+) = suppliers.supplier_id

Let's look at some data to explain how outer joins work:
We have a table called suppliers with two fields (supplier_id and name).
It contains the following data:

supplier_id supplier_name
10000 IBM
10001 Hewlett Packard
10002 Microsoft
10003 Nvidia

We have a second table called orders with three fields (order_id, supplier_id, and order_date).
It contains the following data:

order_id supplier_id order_date
500125 10000 2003/05/12
500126 10001 2003/05/13

If we ran the SQL statement below:
select suppliers.supplier_id, suppliers.supplier_name, orders.order_date
from suppliers, orders
where suppliers.supplier_id = orders.supplier_id(+);

Our result set would look like this:

supplier_id supplier_name order_date
10000 IBM 2003/05/12
10001 Hewlett Packard 2003/05/13
10002 Microsoft
10003 Nvidia
The rows for Microsoft and Nvidia would be included because an outer join was used. However, you will notice that the order_date field for those records contains a value.

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SQL: GROUP BY Clause

The GROUP BY clause can be used in a SELECT statement to collect data across multiple records and group the results by one or more columns.
The syntax for the GROUP BY clause is:
SELECT column1, column2, ... column_n, aggregate_function (expression)
FROM tables
WHERE predicates
GROUP BY column1, column2, ... column_n;
aggregate_function can be a function such as SUM, COUNT, MIN, or MAX.

Example using the SUM function
For example, you could also use the SUM function to return the name of the department and the total sales (in the associated department).
SELECT department, SUM(sales) as "Total sales"
FROM order_details
GROUP BY department;
Because you have listed one column in your SELECT statement that is not encapsulated in the SUM function, you must use a GROUP BY clause. The department field must, therefore, be listed in the GROUP BY section.

Example using the COUNT function
For example, you could use the COUNT function to return the name of the department and the number of employees (in the associated department) that make over $25,000 / year.
SELECT department, COUNT(*) as "Number of employees"
FROM employees
WHERE salary > 25000
GROUP BY department;

Example using the MIN function
For example, you could also use the MIN function to return the name of each department and the minimum salary in the department.
SELECT department, MIN(salary) as "Lowest salary"
FROM employees
GROUP BY department;

Example using the MAX function
For example, you could also use the MAX function to return the name of each department and the maximum salary in the department.
SELECT department, MAX(salary) as "Highest salary"
FROM employees
GROUP BY department;
SQL: ORDER BY Clause

The ORDER BY clause allows you to sort the records in your result set. The ORDER BY clause can only be used in SELECT statements.
The syntax for the ORDER BY clause is:
SELECT columns
FROM tables
WHERE predicates
ORDER BY column ASC/DESC;
The ORDER BY clause sorts the result set based on the columns specified. If the ASC or DESC value is omitted, the system assumed ascending order.
ASC indicates ascending order. (default)
DESC indicates descending order.

Example #1
SELECT supplier_city
FROM supplier
WHERE supplier_name = 'IBM'
ORDER BY supplier_city;
This would return all records sorted by the supplier_city field in ascending order.

Example #2
SELECT supplier_city
FROM supplier
WHERE supplier_name = 'IBM'
ORDER BY supplier_city DESC;
This would return all records sorted by the supplier_city field in descending order.

Example #3
You can also sort by relative position in the result set, where the first field in the result set is 1. The next field is 2, and so on.
SELECT supplier_city
FROM supplier
WHERE supplier_name = 'IBM'
ORDER BY 1 DESC;
This would return all records sorted by the supplier_city field in descending order, since the supplier_city field is in position #1 in the result set.

Example #4
SELECT supplier_city, supplier_state
FROM supplier
WHERE supplier_name = 'IBM'
ORDER BY supplier_city DESC, supplier_state ASC;
This would return all records sorted by the supplier_city field in descending order, with a secondary sort by supplier_state in ascending order.

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SQL: BETWEEN Condition

The BETWEEN condition allows you to retrieve values within a range.
The syntax for the BETWEEN condition is:
SELECT columns
FROM tables
WHERE column1 between value1 and value2;
This SQL statement will return the records where column1 is within the range of value1 and value2 (inclusive). The BETWEEN function can be used in any valid SQL statement - select, insert, update, or delete.

Example #1 - Numbers
The following is an SQL statement that uses the BETWEEN function:
SELECT *
FROM suppliers
WHERE supplier_id between 5000 AND 5010;
This would return all rows where the supplier_id is between 5000 and 5010, inclusive. It is equivalent to the following SQL statement:
SELECT *
FROM suppliers
WHERE supplier_id >= 5000
AND supplier_id <= 5010;

Example #2 - Dates
You can also use the BETWEEN function with dates.
SELECT *
FROM orders
WHERE order_date between to_date ('2003/01/01', 'yyyy/mm/dd')
AND to_date ('2003/12/31', 'yyyy/mm/dd');
This SQL statement would return all orders where the order_date is between Jan 1, 2003 and Dec 31, 2003 (inclusive).
It would be equivalent to the following SQL statement:
SELECT *
FROM orders
WHERE order_date >= to_date('2003/01/01', 'yyyy/mm/dd')
AND order_date <= to_date('2003/12/31','yyyy/mm/dd');

Example #3 - NOT BETWEEN
The BETWEEN function can also be combined with the NOT operator.
For example,
SELECT *
FROM suppliers
WHERE supplier_id not between 5000 and 5500;
This would be equivalent to the following SQL:
SELECT *
FROM suppliers
WHERE supplier_id < 5000
OR supplier_id > 5500;
In this example, the result set would exclude all supplier_id values between the range of 5000 and 5500 (inclusive).
SQL: EXISTS Condition

The EXISTS condition is considered "to be met" if the subquery returns at least one row.
The syntax for the EXISTS condition is:
SELECT columns
FROM tables
WHERE EXISTS ( subquery );
The EXISTS condition can be used in any valid SQL statement - select, insert, update, or delete.

Example #1
Let's take a look at a simple example. The following is an SQL statement that uses the EXISTS condition:
SELECT *
FROM suppliers
WHERE EXISTS
(select *
from orders
where suppliers.supplier_id = orders.supplier_id);
This select statement will return all records from the suppliers table where there is at least one record in the orders table with the same supplier_id.

Example #2 - NOT EXISTS
The EXISTS condition can also be combined with the NOT operator.
For example,
SELECT *
FROM suppliers
WHERE not exists (select * from orders Where suppliers.supplier_id = orders.supplier_id);
This will return all records from the suppliers table where there are no records in the orders table for the given supplier_id.

Example #3 - DELETE Statement
The following is an example of a delete statement that utilizes the EXISTS condition:
DELETE FROM suppliers
WHERE EXISTS
(select *
from orders
where suppliers.supplier_id = orders.supplier_id);

Example #4 - UPDATE Statement
The following is an example of an update statement that utilizes the EXISTS condition:

UPDATE supplier
SET supplier_name = ( SELECT customer.name
FROM customers
WHERE customers.customer_id = supplier.supplier_id)
WHERE EXISTS
( SELECT customer.name
FROM customers
WHERE customers.customer_id = supplier.supplier_id);

Example #5 - INSERT Statement
The following is an example of an insert statement that utilizes the EXISTS condition:
INSERT INTO supplier
(supplier_id, supplier_name)
SELECT account_no, name
FROM suppliers
WHERE exists (select * from orders Where suppliers.supplier_id = orders.supplier_id);

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