MySQL if语句子查询值

我想这样做：

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

不幸的是，“作为tmp”和重新执行tmp不起作用。我怎样才能让它工作？在不重复查询的情况下：

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

编辑这不起作用。您需要使用“WHERE”子句对选择进行量化，如下所示：

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 

您可以使用：

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您是否阅读了

JOIN

语句？实际情况有点复杂，因此我想知道如何修复上述情况（没有JOIN）。也许不使用RDBMS？

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 

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