用戶變量

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 

嗯...我只是想知道爲什麼第四列的值是0，而不是1

Answer 1

因爲這是FROM (SELECT @foo AS foo)這@foo := 1之前評估。基本上，FROM中的任何內容都將在SELECT之前進行評估。

它實際上應該是空的，但我猜你已經將會話變量賦值爲零。