Question

如果从以下查询中删除exists语句，我的输出将如何更改。

UPDATE AR_CRDT C
set (OPERATOR_ID,UPDT_TS) = ( SELECT AD.OPERATOR_ID,sysdate
                FROM
                (
                    Select A.*
                    FROM AR1_CUSTOMER_CREDIT A      
                 )AD 
                 WHERE AD.CREDIT_ID = C.CRDT_ID

                 )
where exists    ( SELECT 1
                FROM
                (
                    Select A.*
                    FROM AR1_CUSTOMER_CREDIT A          
                 )AD 
                  WHERE AD.CREDIT_ID = C.CRDT_ID
                 );

Answer 1

如果您删除了exists，那么OPERATOR_ID和UPDT_TS会针对条件不匹配的行设置为NULL。

为什么要用额外的子查询来写这个？

UPDATE AR_CRDT C
    SET (OPERATOR_ID, UPDT_TS) = (SELECT AD.OPERATOR_ID, sysdate
                                  FROM AR1_CUSTOMER_CREDIT A      
                                  WHERE A.CREDIT_ID = C.CRDT_ID
                                 )
    WHERE EXISTS (SELECT 1
                  FROM AR1_CUSTOMER_CREDIT A          
                  WHERE A.CREDIT_ID = C.CRDT_ID
                 );

Answer 2

EXISTS当前导致AR_CRDT的更新仅限于AR_CRDT记录与AR1_CUSTOMER_CREDIT记录相关联的记录。

删除该子句意味着将更新AR_CRDT的所有记录。一些将具有子查询的值，一些将为NULL，其中AR_CRDT和AR1_CUSTOMER_CREDIT之间没有链接。

存在vs没有存在于oracle中

2 个答案: