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...
mysqlsqlduplicatessubquerysql-delete 444 我有一个包含以下字段的表格: id (Unique) url (Unique) title company site_id 现在,我需要删除具有相同标题、公司和站点ID的行。一种方法是使用以下SQL语句以及一个脚本(PHP)来完成: SELECT title, site_id, location, id, count( * ) FROM jobs GROUP BY ...
With Duplicates as (select distinct a.custid as Customer_ID from customers2 a join customers2 b on a.custid <> b.custid and a.CustName = b.CustName and a.CustCity = b.CustCity and a.Passport_Number = b.Passport_Number ) Delete from Customers2 where custid in (select Customer_ID fr...
This returns you a single row for each combination. This includes the rows without duplicates. To return just the copied values you need to filter the results. This is those where the count is greater than one. You can do this with a having clause, like so: Copy code snippet Copied to ...
LooseScan: Scan a subquery table using an index that enables a single value to be chosen from each subquery's value group. Materialize the subquery into an indexed temporary table that is used to perform a join, where the index is used to remove duplicates. The index might also be used ...
delete对SQL的影响 未删除前的SQL执行情况 #插入100W数据 mysql> call insert_user_data(1000000); Query OK, 0 rows affected (35.99 sec) #添加相关索引 mysql> alter table user add index idx_name(name), add index idx_phone(phone); Query OK, 0 rows affected (6.00 sec) Records: 0 Duplicates:...
Scan a subquery tableusingan index that enables asinglevaluetobe chosenfromeachsubquery's value group.Materialize the subqueryintoan indexed temporary table thatisusedtoperform ajoin,wherethe indexisusedtoremove duplicates. The index might also be used laterforlookupswhenjoining the temporary tablewith...
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)
How to Delete Duplicate Records in Sql Server, removing duplicates using cte, delete duplicate rows from a table in sql SERVER using a CTE.
Query OK, 5 rows affected (0.03 sec) Records: 5 Duplicates: 0 Warnings: 0 SELECT * FROM t1; +---+---+ | id | c1 | +---+---+ | 1 | 1 | | 2 | 2 | | 3 | 3 | | 4 | 4 | | 5 | 5 | +---+---+ 5 rows in set (0.00 sec) DELETE FROM t1 WHERE id =...