Write a SQL query to delete all duplicate email entries in a table namedPerson, keeping only unique emails based on itssmallestId. +---+---+ | Id | Email | +---+---+ | 1 | john@example.com | | 2 | bob@example.com | | 3 | john@example.com | +---+---+ Id is the ...
Now we have realized that custid 1, 4 & 5 are duplicate. The self-join statement accompanied by delete statement will give us the desired output of keeping the last duplicate record by eliminating all the previous duplicate records. We will use theCommon Table Expression (CTE)and put the Sel...
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 InnerjoinTableonTable.country=#TMP.country andTable.nam...
You can then remove the offending rows using uncorrelated deletes (if the driving column is unique across the whole table) or correlated deletes (if it's only unique within each duplicate group). Whichever approach you take, it's a good idea to create an unvalidated unique constraint first....
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. ...
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的一个bu...
开始我们拿sql到数据库查询平台查库执行计划,无奈这个平台有bug,delete语句无法查看,所以我们改成select,“应该”是一样。这个“应该”加了双引号,导致我们走了一点弯路。 EXPLAINSELECT*fromt_table_1wheretask_idin(selectidfromt_table_2whereuid=1)
Most of the Database Developers have such a requirement to delete duplicate records from the Database. Like SQL Server, ROW_NUMBER() PARTITION BY is also available in PostgreSQL. I have prepared this script, using simple inner query with the use of ROW_NUMBER() PARTITION BY clause. Create...
on duplicate key update column = value [, column = value] ...代码语言:javascript 代码运行次数:0 运行 AI代码解释 mysql> insert students values (31, 444, '狄仁杰', 56789) -> on duplicate key update id = 30, sn = 555, name = '狄仁杰', qq = 56789; Query OK, 2 rows affected (0.0...
2.有慢sql,这点比较奇怪,通过explain select语句是走索引的,但数据库慢日志记录到,全表扫描,不会错。 那是select和delete的执行计划不同吗?正常来说应该是一样的,delete无非就是先查,加锁,再删。 拿到本地环境执行再次查看执行计划,发现确实不同,select的是一样的,但delete的变成全表扫描了。