Now the situation is that the duplicate row is in the local temporary table. All we need to now is to delete records from main table customers1 as per matching custid of the local temporary table. Delete from customers1 where custid in (select Custid from #Temp_customers1) Will the above...
How to Delete Duplicate Records in SQL Server Usually on daily basis we usually need to perform removing duplicate records from a table. This post can help you to understand “How to Delete Duplicate Records in SQL Server”. Here consider an example for removing duplicate records. IF EXISTS(SE...
Write a SQL query to delete all duplicate email entries in a table named Person, keeping only unique emails based on its smallest Id. +---+---+ | Id | Email | +---+---+ | 1 | john@example.com | | 2 | bob@example.com | | 3 | john@example.com | +---+---+ Id is ...
关闭后上面的sql就变成先扫描外部的t_table_1,然后再逐行去匹配子查询了,假设t_table_1的数据量非常大,那全表扫描时间就会很长,我们可以通过optimizer_trace证明一下。 optimizer_trace是mysql一个跟踪功能,可以跟踪优化器做的各种决策,包括sql改写,成本计算,索引选择详细过程,并将跟踪结果记录到INFORMATION_SCHEMA.OP...
SELECT class_num, class_name FROM class WHERE class_num IN (SELECT class_num FROM roster); semijoin有以下几种策略,以下是官方的解释:Duplicate Weedout: Run the semijoin as if it was a join and remove duplicate records using a temporary table. FirstMatch: When scanning the inner tables for...
"How do I find duplicate rows using SQL?" This is often closely followed with: "How do I delete all but one of the copies?" In this post we'll look at how you can use SQL to: Find duplicate rows Delete duplicate rows Stop people storing new duplicates!
不记得当时是不是也是用这种方法了.把SQL Query放在这边方便大家,如果你需要. Select0asid,country,name,tel,address Into#TMP FromTable Wherecountry='china' Groupbycountry,name,tel,address Havingcount(*)>1 Update#TMPset#TMP.id=Table.id From#TMP ...
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: ...
name | varchar(20) | NO | | NULL | | | chinese | float | YES | | 0 | | | math | float | YES | | 0 | | | english | float | YES | | 0 | | +---+---+---+---+---+---+ 5 rows in set (0.00 sec) 插入测试数据: 代码语言:javascript 代码运行次数:0 运行 AI代码...
...其中before触发器类似于SQL Server中的instead of触发器,作用在检查约束之前。而after触发器和SQL Server中一样,在检查约束之后才生效。...注意,delete触发器只在表中记录被删除的时候才会被激活。例如delete语句、replace语句。...在insert into... on duplicate key update语句中,插入没有重复值冲突的记录时...