mysql - IFNULL in mysql speed issue

Question

select IFNULL(col1, (select col2 from table2 where ...)) from table1 

Will it run select for table2 if table1 would have not null value? This is a speed issue question. I have no appropriate database to check.

Answer 1

只有当 col1 为 null 但每次都为 null 时，才会执行子查询。因此，如果 col1 中有许多空值，这将变得非常慢。

以下可能更适合您：

select coalesce(col1,col2) from table1,table2 
                     where (relationship between both tables)