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SQL语句理解 sElECt * From B whErE (sElECt Count...

子查询中select count(1) as num from A where A.ID = B.ID就是统计B表和A表用ID连接的行数子查询的行数=0,就可以实现从B表中排除A表数据

select * from a,b where a.id=b.id ---等值连接select * from a inner join b on a.id = b.id -----内连接内连接与等值连接效果是相同的,执行效率也是一样的.只不过内连接是由sql 1999规则定的书写方式,其实这两个是一样的.

select count(*) from aa where (select * from aa where a=1)错了 select count(*) from aa where a=1 select count(*) from aa where a in (select a from aa where a=1)

select * from (select *from b) as xxx whereselect * from (select *from b) xxx where确实是一样的,但是第三句相当于select * from (select *from b) where这个有错误.

explain extended select count(*) from tablets_history;解释成select count(0) as `count(*)` from `paiwei`.`tablets_history`;explain extended select count(1) from tablets_history;解释成select count(1) as `count(1)` from `paiwei`.`tablets_history`;这两

当我们只关心数据表有多少记录行而不需要知道具体的字段值时,类似“select 1 from tblname”是一个很不错的sql语句写法,它通常用于子查询.这样可以减少系统开销,提高运行效率,因为这样子写的sql语句,数据库引擎就不会去检索数据

1)去掉r表中的所有列的相同的值的记录2)查询r中只要不为空的行数3)我想你应该是问count(1)什么的把,如果你的数据表没有主键,那么count(1)比count(*)快 如果有主键的话,那主键(联合主键)作为count的条件也比count(*)要快 如果你的表只有一个字段的话那count(*)就是最快的啦

你好!select count(*) from A where A.date > (select max(systemdate) from B)如有疑问,请追问.

SELECT count (*) as a SUM ( CASE WHEN str=xx THEN 1 ELSE 0 END ) as bFROM tab1

你执行这个语句取得结果赋值给一个数据容器.比如datatable,然后去这个datatable的0行的scount列的值就行了. 具体是什么类型的,看你需要了,你也可以转成string类型,也可以是integer类型.

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