mysql查询中的括号给出不同的结果

问题描述

我有一个查询，当我将其放在括号中时会给我不同的结果，但是当我在不带括号的情况下运行它会给我不同的结果。我想在两者之间应用Union，所以我必须使用括号，因为没有Union无效。 查询如下：

SELECT distinct
   recurring_billing.id,recurring_billing.kid_id,recurring_billing.class_id,recurring_billing.app_id,recurring_billing.Region_ID,CC.month_name,CC.billing_year,CASE
      WHEN
         CC.month_name = recurring_billing.billing_month 
         AND CC.billing_year = recurring_billing.billing_year 
      THEN
         recurring_billing.billing_status 
      ELSE
         'Pending' 
   END
   AS billing_status,tblkids.kid_name,tblkids.kid_Lastname,tblkids.kid_EMail,tbl_app.app_CCExp AS cc_exp,tbl_app.app_CCName AS cc_name,tbl_app.app_CCNumber AS cc_number,tbl_app.app_CCType AS cc_type,tblclasses.cla_EndDate,tblclassdays.classday_day,CC.remainingclasses,tbl_app.cost_per_class,CC.remainingclasses * tbl_app.cost_per_class AS cost_amount,CASE
      WHEN
         OP.override_amt IS NOT NULL 
      THEN
         OP.override_amt 
      ELSE
         CC.remainingclasses * tbl_app.cost_per_class 
   END
   AS pmt_amount,CASE
      WHEN
         OP.process_payment = False 
      THEN
         OP.process_payment 
      ELSE
         True 
   END
   AS process_payment 
FROM
   recurring_billing 
   LEFT JOIN
      tblkidsxclass 
      ON recurring_billing.kid_id = tblkidsxclass.kxc_kidid 
      AND recurring_billing.app_id = tblkidsxclass.kxc_appid 
   LEFT JOIN
      tbl_app 
      ON recurring_billing.app_ID = tbl_app.app_ID 
   LEFT JOIN
      tblkids 
      ON recurring_billing.kid_id = tblkids.kid_ID 
   LEFT JOIN
      tblclasses 
      ON recurring_billing.class_id = tblclasses.cla_ID 
   LEFT JOIN
      tblclassdays 
      ON tblclasses.cla_ID = tblclassdays.classday_classID 
   INNER JOIN
      (
         SELECT
            MONTHNAME(classday_day) AS month_name,YEAR(classday_day) AS billing_year,cla_ID,COUNT(classday_classid) AS remainingclasses,c.cost_per_class AS cost_per_class,COUNT(classday_classid) * cost_per_class AS TotalClassCost 
         FROM
            tblclassdays 
            JOIN
               (
                  SELECT
                     tblclasses.cla_ID,tblclasses.cost_per_class,tblclasses.cla_nextclass AS next1,tblclasses_1.cla_nextclass AS next2 
                  FROM
                     tblclasses 
                     LEFT JOIN
                        tblclasses AS tblclasses_1 
                        ON tblclasses.cla_nextclass = tblclasses_1.cla_ID 
                  WHERE
                     tblclasses.cla_ID IN 
                     (
                        SELECT disTINCT
                           recurring_billing.class_id 
                        FROM
                           recurring_billing 
                           LEFT JOIN
                              tblkidsxclass 
                              ON recurring_billing.kid_id = tblkidsxclass.kxc_kidid 
                              AND recurring_billing.app_id = tblkidsxclass.kxc_appid                                        -- LEFT JOIN tblkids ON recurring_billing.kid_id = tblkids.kid_ID
                           LEFT JOIN
                              tblclasses 
                              ON recurring_billing.class_id = tblclasses.cla_ID                                         -- LEFT JOIN tblclassdays ON tblclasses.cla_ID = tblclassdays.classday_classID
                        WHERE
                           ISNULL(tblkidsxclass.kxc_dropoutdate) 
                           AND tblkidsxclass.pmt_option = 'Recurring' 
                           AND tblclasses.cla_active = TRUE                                     -- AND tblclasses.cla_EndDate >= Now()
                        GROUP BY
                           recurring_billing.id
                     )
               )
               c 
               ON tblclassdays.classday_classid IN 
               (
                  c.cla_ID,c.next1,c.next2
               )
         WHERE
            tblclassdays.classday_noclass = FALSE 
            AND MONTH(classday_day) = 11 
            AND YEAR(classday_day) = 2020 
            AND 
            (
               CONCAT(CAST(classday_day AS DATE),' ',CAST(classday_endtime AS TIME)) > CAST(CURDATE() AS DATETIME)
            )
         GROUP BY
            cla_ID
      )
      CC 
      ON CC.cla_ID = tblclassdays.classday_classID 
   LEFT JOIN
      override_payments OP 
      ON recurring_billing.app_id = OP.app_id 
      AND recurring_billing.kid_id = OP.kid_id 
      AND CC.month_name = OP.billing_month 
      AND CC.billing_year = OP.billing_year 
WHERE
   ISNULL(tblkidsxclass.kxc_dropoutdate) 
   AND tblkidsxclass.pmt_option = 'Recurring' 
   AND tblclasses.cla_active = TRUE 
   AND recurring_billing.kid_id NOT IN 
   (
      SELECT
         kid_id 
      from
         recurring_billing 
      where
         billing_month = 'November' 
         and billing_year = '2020'
   )
   -- AND tblclasses.cla_EndDate >= Now()
GROUP BY
   recurring_billing.id 
ORDER BY
   recurring_billing.id ASC

如果用括号括起来，则会出现相同的查询。

解决方法

UNION语句不需要括号。以下2个并集语句均起作用并产生相同的结果。

SELECT 1 UNION SELECT 2;
(SELECT 1) UNION (SELECT 2);