|Built-in Functions / Standard Built-in Functions|
AVG is an aggregate function that evaluates the average of an expression over a set of rows. AVG is allowed only on expressions that evaluate to numeric data types.
AVG ( [ DISTINCT | ALL ] Expression )
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).
SELECT AVG (DISTINCT flying_time), SUM (DISTINCT miles) FROM Flights
The expression can contain multiple column references or expressions, but it cannot contain another aggregate or subquery. It must evaluate to an SQL-92 numeric data type. You can therefore call methods that evaluate to SQL-92 data types. If an expression evaluates to NULL, the aggregate skips that value.
SELECT AVG(c1) FROM (VALUES (1), (1), (1), (1), (2)) AS myTable (c1)
SELECT AVG(CAST (c1 AS DOUBLE PRECISION)) FROM (VALUES (1), (1), (1), (1), (2)) AS myTable (c1)