delete from msgmessag where exists (select 1 from msgmessag

来源：学生作业帮编辑：拍题作业网作业帮分类：综合作业时间：2024/05/01 16:05:38

delete from msgmessag where exists (select 1 from msgmessag where contentid='9')
在oracle中执行这条语句后的结果不是应该：将msgmessag中contentid='9'的记录删除掉么?怎么会把msgmessag 这个表中所有的记录都给删掉呢?

是会删除所有记录的,因为子查询这条记录是存在的.
你的查询可解释为删除如果子查询中（msgmessag表）存在等于9的记录,就删除主表msgmessag.
应该与删除表建立关联：
delete from msgmessag where exists (select 1 from msgmessag A where contentid='9' AND A.CONTENTID=MSGMESSAG.CONTENTID)
该查询可解释为：删除如果msgmessag（主查询）CONTENTID字段与子查询相同就删除该记录.（子查询中的CONTENTID字段等于9并且与主查询相同）.
效果等同于delete from msgmessag where contentid='9

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