Using the GROUP BY clause

The GROUP BY clause enables you to establish data groups based on columns. The grouping criterion is defined by the GROUP BY clause, which is followed by the WHERE clause in the SQL execution path. Following this execution path, the result set rows are grouped based on like values of grouping columns and the WHERE clause restricts the entries in each group.

Note

All columns that are used besides the aggregate functions must be included in the GROUP BY clause. The GROUP BY clause does not support the use of column aliases; you must use the actual column names. The GROUP BY columns may or may not appear in the SELECT list. The GROUP BY clause can only be used with aggregate functions such as  SUM , AVG , COUNT , MAX , and MIN .

The following statement illustrates the syntax of the GROUP BY clause:

SELECT expression1, expression2, ... expression_n,
aggregate_function (expression)
FROM tables
WHERE conditions
GROUP BY expression1, expression2, ... expression_n;

The expression1, expression2, ... expression_n commands are expressions that are not encapsulated within an aggregate function and must be included in the GROUP BY clause.

Let's take a look at these commands:

• aggregate_function: This performs many functions, such as SUM (http://www.techonthenet.com/oracle/functions/sum.php), COUNT (http://www.techonthenet.com/oracle/functions/count.php), MIN (http://www.techonthenet.com/oracle/functions/min.php), MAX (http://www.techonthenet.com/oracle/functions/max.php), or AVG (http://www.techonthenet.com/oracle/functions/avg.php).
• tables: This is where you can retrieve records from. There must be at least one table listed in the FROM clause.
• conditions: This is a condition that must be met for the records to be selected.

The GROUP BY clause must appear right after the FROM or WHERE clause. Followed by the GROUP BY clause is one column or a list of comma-separated columns. You can also put an expression in the GROUP BY clause.

As mentioned in the previous paragraph, the GROUP BY clause divides rows returned from the SELECT statement into groups. For each group, you can apply an aggregate function, for example, to calculate the sum of items or count the number of items in the groups.

Let's look at a GROUP BY query example that uses the SUM function (http://www.techonthenet.com/oracle/functions/sum.php). This example uses the SUM function to return the name of the product and the total sales (for the product).

SELECT product, SUM(sale) AS "Total sales"
FROM order_details
GROUP BY product;

In the select statement, we have sales where we applied the SUM function and the other field product is not part of SUM, we must use in the GROUP BY clause.