MySQL的if語句子查詢值

Question

我想要做這樣的事情：

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

不幸的是，「作爲TMP」和returing tmp的不工作。我怎樣才能使這個工作？沒有做重複查詢：

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

Answer 1

您可以使用User-Defined Variables：

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

SQL Fiddle

Answer 2

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

編輯這是行不通的。你需要一個 「WHERE」 條款某處量化的選擇，像這樣：

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