Oracle Learning - 4

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.