链式变量赋值在 SQL 中是如何工作的?-数据库问题

How does chaining variable assignments work in SQL?(链式变量赋值在 SQL 中是如何工作的?)

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问题描述

我正在分析存储过程中的一些旧 SQL 代码.

I'm analyzing some old SQL code in a stored procedure.

Declare @Var1 money, 
    @Var2 money, 
    @Var3 money,

等等...

Select @Var1 = OldValue, 
       @Var2 = @Var1,

等等...

所以我想知道当它们都在同一个 select 语句中时这些赋值是如何工作的.我假设在调用 select 后 Var2 = OldValue，但我想确定.

So I'm wondering how these assignments work when they are both in the same select statement. I'm assuming Var2 = OldValue after the select is called, but I want to be sure.

围绕这种情况的规则是什么?赋值是否按照声明的顺序执行?如果是这样，在以下情况下会为 Var2 分配什么值:

What are the rules surrounding this situation? Are the assignments executed in the order that they are declared? If so, what value would be assigned to Var2 in the following case:

Select @Var2 = @Var1,
       @Var1 = OldValue,

谢谢！

推荐答案

DECLARE @Var1 MONEY = 100, @Var2 MONEY = 50 

SELECT @Var1 = @Var2, 
       @Var2 = @Var1 

SELECT  @Var1, @Var2

退货

--------------------- ---------------------
50.00                 50.00

所以在那种情况下，它们是按从左到右的顺序执行的但是这是不能依赖的！

So at in that case they were executed in Left to Right order but this cannot be relied upon!

如果有多个赋值单个 SELECT 语句中的子句，SQL Server 不保证评估顺序表达式.注意效果是仅在有引用时可见在作业中.

If there are multiple assignment clauses in a single SELECT statement, SQL Server does not guarantee the order of evaluation of the expressions. Note that effects are only visible if there are references among the assignments.

来源 http://msdn.microsoft.com/en-us/library/ms187953.aspx

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