Question

Query Output:
> 

SELECT
  @foo := 1,
  @foo,
  (SELECT @foo),
  (SELECT foo FROM (SELECT @foo AS foo) subselect)

+ -------------- + --------- + ------------------ + ----------------------------------------------------- +
| @foo := 1      | @foo      | (SELECT @foo)      | (SELECT foo FROM (SELECT @foo AS foo) subselect)      |
+ -------------- + --------- + ------------------ + ----------------------------------------------------- +
| 1              | 1         | 1                  | 0                                                     |
+ -------------- + --------- + ------------------ + ----------------------------------------------------- +
1 rows

Well… I just want to know why fourth column value is 0 instead 1.

Answer 1

Because this FROM (SELECT @foo AS foo) is evaluated before this @foo := 1. Basically anything in FROM will be evaluated before your SELECT.

It actually should be null but Im guessing you’ve assigned your session variable a value of zero somewhere else.