AVG is allowed only on expressions that evaluate to numeric data types.
See
The DISTINCT qualifier eliminates duplicates. The ALL qualifier retains duplicates. ALL is the default value if neither ALL nor DISTINCT is specified. For example, if a column contains the values 1.0, 1.0, 1.0, 1.0, and 2.0, AVG(col) returns a smaller value than AVG(DISTINCT col).
Only one DISTINCT aggregate expression per
The expression can contain multiple column references or expressions, but it cannot contain another aggregate or subquery. It must evaluate to an SQL numeric data type. You can therefore call methods that evaluate to SQL data types. If an expression evaluates to NULL, the aggregate skips that value.
The resulting data type is the same as
the expression on which it operates (it will never overflow). The following
query, for example, returns the INTEGER 1, which might not be what
you would expect:
CAST
the expression to another data type if you want more precision: