MySQL的if语句子查询值

Question

我想要做这样的事情：

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

不幸的是，“作为TMP”和returing tmp的不工作。我怎样才能使这个工作？没有做重复查询：

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)) 

编辑这是行不通的。你需要一个 “WHERE” 条款某处量化的选择，像这样：

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