Aggregates on Distinct Values
The five aggregate functions can all be used in two ways:
To perform calculations on all rows, specify the ALL argument or specify no argument at all (because ALL is the default behavior).
To only include unique values, specify the DISTINCT argument.
 | ALL Is Default The ALL argument need not be specified because it is the default behavior. If DISTINCT is not specified, ALL is assumed. |
The following example uses the AVG() function to return the average product price offered by a specific vendor. It is the same SELECT statement used above, but here the DISTINCT argument is used so that the average only takes into account ...
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