所以要想达到释放磁盘空间的目的,delete 以后执行 optimize table 操作。 示例:查看表占用硬盘空间大小的 SQL 语句如下:(用 M 做展示单位,数据库名:csjdemo,表名:demo2) select concat(round(sum(DATA_LENGTH/1024/1024),2),'M') as table_size from information_schema.tables where table_schema='csjdemo'...
删除主表中的数据时,触发器将自动删除从表中与之关联的数据。 DELETEFROMtable1WHEREid=1; 1. 上述代码删除了主表table1中id为1的数据。由于我们在主表上创建了触发器,所以触发器将自动删除从表table2中与被删除主表数据关联的数据。 5. 序列图 下面是整个流程的序列图,用于展示各个对象之间的交互: MysqlDev...
DELETEFROMtable1FROMtable1,table2/* join of 2 tables */ 如果两个表引用具有相同的别名,则两者引用同一个表: 代码语言:javascript 代码运行次数:0 运行 AI代码解释 DELETEFROMtable1ASxFROMtable1ASx,table2/* join of 2 tables */ 如果两个表引用都有别名,并且别名不同,则 IRIS将执行表的两个实例的联...
分析select*fromt_table_1wheretask_idin(selectidfromt_table_2whereuid = #{uid}) 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...
Partitioned TablesDELETEsupports explicit partition selection using the PARTITIONoption, which takes a listofthe comma-separated namesofoneormore partitionsorsubpartitions (orboth)fromwhichtoselectrowstobe dropped. Partitionsnotincludedinthe list are ignored. Given a ...
DELETEFROMemployees;Code language:SQL (Structured Query Language)(sql) 3) Deleting related rows from multiple tables It becomes more complicated when you want to delete a row in a table that is associated with other rows in another table. ...
第2,创建完成后,我们查看下数据表占用的空间,如图所示: 其中,查询前需要对表进行分析,使用命令为:ANALYZE TABLE test ESTIMATE STATISTICS;查询语句为:SELECT blocks, empty_blocks, num_rows FROM user_tables WHERE table_name = 'TEST'; 注意上面三个字段的结果:BLOCKS=0; EMPTY_BLOCKS=13312; NUM_ROWS=0,...
The first DELETE statement shows the ISO-compatible subquery solution, and the second DELETE statement shows the Transact-SQL FROM extension to join the two tables.SQL Copy -- SQL-2003 Standard subquery DELETE FROM Sales.SalesPersonQuotaHistory WHERE BusinessEntityID IN (SELECT BusinessEntityID ...
BEGINLOOPDELETEFROMyour_table_nameWHERErownum<=50000; EXITWHENSQL%ROWCOUNT=0;COMMIT;ENDLOOP;END; 释放表空间 存放大数据量的表,其表空间占用也比较大,删除数据后并不会自动释放这些记录占用的表空间,所以,即便表里面数据量很少,查询效率依旧很慢,所以,需要释放表空间。
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。 EXPLAIN SELECT * from t_table_1 where task_id in (select id from t_table_2 where uid = 1) ...