多选与单选语句 [英] Multiple Select vs Single Select Statement

问题描述

我只是想知道执行多次选择与使用一次选择之间的区别是什么?我看到有人这样做了，我觉得有点像下面这样；

I was just wondering whats the difference between doing multiple selects vs. using a single select is there any performance gain? I saw someone did it, and I feel its like kinda out of hand such as below;

select @a = cBranchName from mstores where nBranchCode = 1
select @b = cBranchAddress from mstores where nBranchCode = 1
select @c = dCreateDate from mstores where nBranchCode = 1
select @d = AssignedCompanyId from mstores where nBranchCode = 1
select @e = StoreFormat from mstores where nBranchCode = 1

vs.

select @a = cBranchName 
  ,@b = cBranchAddress
  ,@c = dCreateDate
  ,@d = AssignedCompanyId
  ,@e = StoreFormat from mstores where nBranchCode = 1

我还假定5个select会进行5次访问数据库，如何证明单个select语句比5个select语句要快，

I also presume that the 5 select's make 5 trips to the database, how can I prove that single select statement is faster than the 5 select statements,

即使只是看一下，我已经认为单个选择语句比多个选择要快得多.

Even though just by looking at it, I already thought that the single select statement is significantly faster than the multiple selects.

推荐答案

主要区别:

1. 五次往返，而不是一次.光/电的速度可以照顾到这一点
2. 要检查，解析和执行5条语句(不是一个).您认为会更快些吗?
3. 一致性-如果两次选择之间发生更新会怎样?
4. 如果显示执行计划，您可以很清楚地看到正在进行的工作

我很茫然，无法理解为什么有人会认为5个查询比一个更好.

I'm quite at a loss to understand why anyone would ever have cause to think the 5 queries would be better than one.

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