COUNT DISTINCT and COUNT UNIQUE Functions

The COUNT DISTINCT function returns the number of unique values in the column or expression, as the following example shows. If the COUNT DISTINCT function encounters NULL values, it ignores them:

SELECT COUNT (DISTINCT item_num) FROM items

NULLs are ignored unless every value in the specified column is NULL. If every column value is NULL, the COUNT DISTINCT function returns zero (0).

The UNIQUE keyword has the same meaning as the DISTINCT keyword in COUNT functions. The UNIQUE keyword instructs the database server to return the number of unique non-NULL values in the column or expression. The following example calls the COUNT UNIQUE function, but it is equivalent to the preceding example that calls the COUNT DISTINCT function:

SELECT COUNT (UNIQUE item_num) FROM items