Grouped Set Functions - the GROUP BY Clause

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Normally, set functions return a single value, calculated from the set of all values in the column or expression.

If the SELECT statement includes a GROUP BY clause, set functions will be applied to groups of values. Columns used for GROUP BY do not have to be included in the SELECT list.

Find the number of rows in each category within the FORMATS table:

 SELECT category_id, COUNT(display_order)
    FROM formats
    GROUP BY category_id;

Returns:

CATEGORY_ID
1	4
2	4
3	2

Restrictions when Using GROUP BY

Using a GROUP BY clause places some restrictions on the SELECT statement. Only constants, columns used in the GROUP BY clause, and columns used in set functions may be included in the SELECT list.

Column References

In a statement with column references in the SELECT list, all columns not used in set functions must be used as grouping columns.

Null Values

For grouping purposes, NULL values are regarded as equivalent. Thus for the example table:

SAMPLE
1.0
2.0
2.0
2.0
3.0
3.0
4.0
5.0
-
-

The following statement:

 SELECT sample, COUNT(*) as number
    ...
    GROUP BY sample;

Returns:

SAMPLE	NUMBER
1.0	1
2.0	3
3.0	2
4.0	1
5.0	1
-	2

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