Sql query result if no records found

what will be the output o开发者_Go百科f the below given query, if no matching records are found

select avg(salary) from employee where dept='sales'

Will it return 0 or null ?

It returns null: http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_avg

Have a look at this.

It returns NULL

Easy test in SQL Server:

DECLARE @T TABLE (id int)
INSERT INTO @T
SELECT NULL

SELECT AVG(id) FROM @T

Returns NULL

By default in MySQL and i'm pretty sure it is too in MSSQL, all aggregation functions return NULL if no data or if ANY OF THE DATA IS NULL, you cannot compute an aggregation on missing data.