SQL查询中条件表达式的顺序? [英] The order of condition expressions in SQL queries?

问题描述

我知道至少在C和C#中条件的顺序很重要.例如，以下两个条件是不同的:
1.(str1.Length> 0 && str1 [0] ==``A'')
这很好，是正确的.
2.(str1 [0] ==''A''&& str1.Length> 0)
这太荒谬了，因为执行命令的顺序(在这种情况下)是从左到右，如果str1.Length = 0，则将引发异常，因此它将无法正常工作.这只是一个简单的例子.但是我想知道的是，在WHERE，HAVING，ON或其他可能的条件之后，SQL查询在条件下的工作方式是否相同?
非常感谢！

I know the order of conditions is important in at least C and C#. For example, the two following conditions are different:
1. (str1.Length > 0&&str1[0] == ''A'')
This works well and is the correct one.
2. (str1[0] == ''A''&&str1.Length>0)
This is too ridiculous because the order of executing commands (in this case) is from left to right, if str1.Length = 0, an exception will be thrown, so it can''t work. This is only a simple example. But what I want to know is Do SQL queries work the same way with conditions after WHERE, HAVING, ON or anywhere else possible?
Thank you so much!

推荐答案

您是否想要坏消息?抱歉，SQL的行为与C，C ++和C#不同.在这种情况下，AND子句始终从左至右执行，任何错误结果都会中止该操作(称为评估短路"). SQL不会那样做.它的作用更为复杂.此处有描述: SQL Server如何短路WHERE条件评估 [

Do you want the bad news? Sorry, but SQL does not behave the same as C, C++ and C#. In those, AND clauses are always executed left - to right, and any false result will abort the operation (called "evaluation short-circuiting"). SQL does not do that. What it does do is more complex. There is a description here: How SQL Server short circuits WHERE condition evaluation[^]

Gets my five for a very good question, though!

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