MySQL if statement subquery value

问题

I want to do something like this:

SELECT IF((SELECT something FROM table) AS tmp) > 0, tmp, (SELECT bla FROM oter_table))

Unfortunately the 'as tmp' and returing the tmp is not working. How can I get this to work? without doing repeating the query:

SELECT IF((SELECT something FROM table) > 0, (SELECT something FROM table), (SELECT bla FROM oter_table))

回答1:

You can use User-Defined Variables:

SELECT IF(
          @val:=(SELECT something FROM table1) > 0, 
          @val, 
          (SELECT bla FROM table2)
       )

SQL Fiddle

回答2:

SELECT IF( (SELECT something AS tmp FROM first_table) > 0, tmp, (SELECT bla FROM other_table) )

EDIT This is not going to work. You need to quantify the selection somewhere with a "WHERE" clause, like so:

SELECT IF( (SELECT something FROM first_table) > 0, (SELECT something FROM first_table), (SELECT something_else FROM second_table) ) FROM first_table WHERE something = 45 

来源：https://stackoverflow.com/questions/17723808/mysql-if-statement-subquery-value