mysql 实用sql 汇总

update select 从查询结果中更新数据

示例场景

两个表 ordrs, sum_orders， sum_orders 中的count是 orders中相同project_id的count之和；cost同理

一条语句直接更新sum_orders中的count & cost

#orders

 

#sum_orders

 

示例代码

UPDATE sum_orders a
INNER JOIN ( SELECT project_id, sum( count ) AS sum_count, sum( cost ) AS sum_cost FROM orders GROUP BY project_id ) b ON a.project_id = b.project_id 
SET a.count = b.sum_count,
a.cost = b.sum_cost;

再加条件

UPDATE T1, T2, [INNER JOIN | LEFT JOIN] T1 ON T1.C1 = T2. C1 

SET T1.C2 = T2.C2, T2.C3 = expr 

WHERE condition

 

delete join 写法

delete d
from member_log d
left join member m ON d.uid=m.uid
where date(d.date)='2014' and m.uid is null ;

 

粗略估量查询行数--快速

select TABLE_ROWS from information_schema.`TABLES` WHERE TABLE_NAME = 'xxxx'

数据来描述表的信息，包括表的名字、表的大小等信息，这些信息被称为元数据，所以应该先想到的是mysql是如何存储表的元数据的