如果连接返回false，则立即使用其他连接。它可能吗？

Question

我的代码工作并不像我想：

SELECT 
      r.id, 
      IF(exp.id != 0, 1, 
      IF(exp2.id != 0, 1, 
       IF(exp3.id != 0, 1, 0) 
      ) 
     ) AS white_list 

FROM rates AS r 

LEFT JOIN 
exclusion_policies AS exp ON (
    1 = 1 
    AND exp.client_id = r.client_id 
    AND exp.mcc   = r.mcc 
    AND exp.mnc   = r.mnc 
    AND exp.active  = 1 
) 
LEFT JOIN 
exclusion_policies AS exp2 ON (
    1 = 1 
    AND exp2.client_id = r.client_id 
    AND exp2.mcc   = r.mcc 
    AND exp2.active  = 1 
) 
LEFT JOIN 
exclusion_policies AS exp3 ON (
    1 = 1 
    AND exp3.client_id = r.client_id 
    AND exp3.active  = 1 
) 

我想，如果先加入exp为true，则停止verfy EXP2和EXP3： somethig像：

CASE WHEN 
    LEFT JOIN 
    exclusion_policies AS exp ON (
    1 = 1 
    AND exp.client_id = r.client_id 
    AND exp.mcc   = r.mcc 
    AND exp.mnc   = r.mnc 
    AND exp.active  = 1 
) 
THEN true // exp is not null, ONLY 
ELSE 

    CASE WHEN 
    LEFT JOIN 
     exclusion_policies AS exp2 ON (
     1 = 1 
     AND exp2.client_id = r.client_id 
     AND exp2.mcc   = r.mcc 
     AND exp2.active  = 1 
    ) 
    THEN true // exp2 is not null, ONLY 
    ELSE 

     CASE WHEN 
     LEFT JOIN 
     exclusion_policies AS exp3 ON (
     1 = 1 
     AND exp3.client_id = r.client_id 
     AND exp3.active  = 1 
    ) 
     THEN true // exp3 is not null, ONLY 
     ELSE false // exp, exp2, exp3 are null 
     END 

    END 
END 

我需要得到真正的（1），IF

IF(exp.id != 0, 1, 
    IF(exp2.id != 0, 1, 
    IF(exp3.id != 0, 1, 0) 
) 
) AS white_list 

顺便说一下，也许REVERS更加优化LEFT JOIN EXP3，2，1只的第三个，任何方式

Answer 1

您可以通过在ON子句中放入正确的逻辑来创建条件连接。如果ON条件为真，就会进行加入。像下面的代码。我还没有测试过，但你可以尝试一下，然后再次发布。

SELECT 
    r.id, 
    IF(exp.id != 0, 1, 
    IF(exp2.id != 0, 1, 
     IF(exp3.id != 0, 1, 0) 
    ) 
    ) AS white_list 

FROM rates AS r 
LEFT JOIN exclusion_policies AS exp ON 
     exp.client_id = r.client_id AND 
     exp.mcc   = r.mcc AND 
     exp.mnc   = r.mnc AND 
     exp.active  = 1 
LEFT JOIN exclusion_policies AS exp2 ON 
     NOT (exp2.client_id = r.client_id AND 
      exp2.mcc   = r.mcc AND 
      exp2.mnc   = r.mnc AND 
      exp2.active  = 1) AND 
     exp2.client_id = r.client_id AND 
     exp2.mcc   = r.mcc AND 
     exp2.active  = 1 
LEFT JOIN exclusion_policies AS exp3 ON 
     NOT (exp3.client_id = r.client_id AND 
      exp3.mcc   = r.mcc AND 
      exp3.mnc   = r.mnc AND 
      exp3.active  = 1) AND 
     NOT (exp3.client_id = r.client_id AND 
      exp3.mcc   = r.mcc AND 
      exp3.active  = 1) AND 
     exp3.client_id = r.client_id AND 
     exp3.active  = 1 

Answer 2

我sloved它，

感谢某人

IF(exp3.id != 0, 1,IF(exp2.id != 0, 1, IF(exp.id != 0, 1, 0))) 
 
FROM rates AS r 
 
    LEFT JOIN 
 
    exclusion_policies AS exp ON (
 
     exp.client_id  = r.client_id AND 
 
     exp.mcc    = r.mcc AND 
 
     exp.mnc    = r.mnc AND 
 
     exp.active   = 1 
 
    ) 
 
    LEFT JOIN 
 
    exclusion_policies AS exp2 ON (
 
     exp2.client_id  = r.client_id AND 
 
     exp2.mcc   = r.mcc AND 
 
     exp2.mnc   IS NULL AND 
 
     exp2.active  = 1 
 
    ) 
 
    LEFT JOIN 
 
    exclusion_policies AS exp3 ON (
 
     exp3.client_id  = r.client_id AND 
 
     exp3.mcc   IS NULL AND 
 
     exp3.mnc   IS NULL AND 
 
     exp3.active  = 1 
 
    )

Answer 3

你想获得一个1如有下列情况为真：

• 存在exclusion_policies一个记录具有相同client_id，mcc，mnc和active = 1，或
• 存在exclusion_policies一个记录具有相同client_id，mcc和active = 1，或
• 存在exclusion_policies一个记录具有相同的client_id和active = 1。

但你实际上是说你想1当存在exclusion_policies记录具有相同client_id和active = 1。似乎没有必要检查更具体的案例，因为它们是否属实并不影响基于上次检查的结果。

换句话说：这表达

IF(exp.id != 0, 1, 
    IF(exp2.id != 0, 1, 
     IF(exp3.id != 0, 1, 0) 
    ) 
) 

将只有当返回0，当且仅当存在exclusion_policies没有记录与同client_id和active = 1

所以你还不如执行该多更简单的SQL语句：

SELECT r.id, 
      COALESCE(MAX(exp.active), 0) AS white_list 
FROM  rates AS r 
LEFT JOIN exclusion_policies AS exp 
     ON exp.client_id = r.client_id 
      exp.active = 1 
GROUP BY r.id