如果选择true则返回mysql中的select

Question

为什么不使用此查询（未定义@param为true）：

    SELECT 
    c.ID ,
    IF((SELECT b.f1 into @param from db.tbl2 as b Where b.f2 = c.f2 AND b.f3 = c.f3),@param,c.off)  as 'off' 
    FROM db.tbl1 as c 

Answer 1

请改用左连接。像这样修改您的查询：

SELECT c.ID ,COALESCE(b.f1,c.off) as 'off' 
FROM db.tbl1 as c 
LEFT JOIN tbl2 as b ON b.f2 = c.f2 AND b.f3 = c.f3

不要让它变得复杂。