以Duplicate Weedout为例,mysql会先将roster的记录以class_num为主键添加到一张临时表,达到去重的目的。接着扫描临时表,每行去匹配外层表,满足条件则放到结果集,最终返回。 具体使用哪种策略是优化器根据具体情况分析得出的,可以从explain的extra字段看到。 那么为什么delete没有使用semijoin优化呢? 这其实是mysql的一...
假设我们有一个名为users的表,其中有一个名为email的列,我们想要根据email列来判断重复数据,并删除重复的记录。 DELETEFROMusersWHEREemailIN(SELECTemailFROM(SELECTemail,COUNT(*)FROMusersGROUPBYemailHAVINGCOUNT(*)>1)ASduplicate_emails); 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 以上的SQL语句中,我们首先...
comparison of truncate vs delete in mysql/sqlserver [duplicate] DELETE DELETE is a DML Command. DELETE statement is executed using a row lock, each row in the table is locked for deletion. We can specify filters in where clause It deletes specified data if where condition exists. Delete acti...
I am trying to find if data (let's say rows) duplicate in Table 1 first and remove them. Then I want to see if Table 2 duplicates any of Table 1's rows and remove them from Table 2. Then I will have both tables as clean as I can make things. I can then export the data for...
回到这条简单sql,包含子查询,按照我们的理解,mysql应该是先执行子查询:select id from t_table_2 where uid = #{uid},然后再执行外部查询:select * from t_table_1 where task_id in,但这不一定,例如我关了这个参数: setoptimizer_switch='semijoin=off'; ...
deletep1fromPerson p1, Person p2wherep1.id>p2.idandp1.Email=p2.Email; 此答案通过测试,但是效率较低. 答案二 后续思考中发现,可以使用临时表解决SELECT与DELETE同时存在的问题,答案如下所示: DELETEFROMPersonWHEREIdIN(SELECTIdFROM(SELECTp1.IdasIdFROMPerson p1, Person p2WHEREp1.Email=p2.EmailANDp...
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INSERT操作在插入或更新记录时,检查到 duplicate key或者有一个被标记删除的duplicate key(本文的案例),对于普通的INSERT/UPDATE,会加LOCK_S属性锁next-key lock。而对于类似REPLACE INTO或者INSERT … ON DUPLICATE这样的SQL加的是X锁。而针对不同的索引类型也有所不同: ...
因为a字段是一个唯一索引,所以insert语句会在插入前进行一次duplicate key的检查,需要申请S锁防止其他事务对a字段进行重复插入。而插入意向锁与T1已经insert语句必须等待前面 sess2中delete 获取a=5的行锁并且释放锁。于是,sess2(delete) 等待sess1(delete) ,sess1(insert)等待sess2(delete),循环等待,造成死锁。
The rows returned are the duplicates and what I see returned are from table1. So I actually want to delete them from table2 if I understand this correctly, thus I should be returning table2 inmy SELECT, right? Then I could just delete those. Or am I over simplifying my position?