数据库优化点滴

原来四舍五入也有个国际惯例，叫奇进偶舍，意思是当舍入位前面一位是奇数时，就进，为偶数时，就舍，这也是体现公平性的原理。

可是国际惯例往往在国内很多企业行不通，为了应付他们的要求，采用Math.Round(price,2,MidpointRounding.AwayFromZero)就可以了。

SELECT Sum([TradeNum]), Sum([Payment]), [TradeTime],

(select sum([Payment]) from [Order] as o where [TradeStatus] =2 and o.ID=[Order].ID) as NoPayment

FROM [Order]

GROUP BY date(tradetime)

优化如下：

SELECT Sum([TradeNum]), Sum([Payment]), [TradeTime],O.NoPayment

FROM [Order] O,

(select ID,sum([Payment])NoPayment  from [Order] as o where [TradeStatus] =2 GROUP BY ID) T WHERE O.ID=T.ID

GROUP BY DATEPART(DD,tradetime) ,O.NoPayment

总结：在select 中如果需要到表中查询的话， 如果想办法把它拆分到from中来。

SQL中分布式查询用这个

BEGIN DISTRIBUTED TRANSACTION