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我们来看看这两个陈述：
IF (CONDITION 1) OR (CONDITION 2)
...
IF (CONDITION 3) AND (CONDITION 4)
...
如果条件1为TRUE,是否会检查条件2？
如果条件3为假,将检查条件4吗？
WHERE上的条件怎么样？SQL Server引擎是否优化了WHERE子句中的所有条件？程序员是否应该按正确的顺序放置条件以确保SQL Server优化器以正确的方式解析它？
添加：
感谢Jack的链接,来自t-sql代码的惊喜：
IF 1/0 = 1 OR 1 = 1
SELECT 'True' AS result
ELSE
SELECT 'False' AS result
IF 1/0 = 1 AND 1 = 0
SELECT 'True' AS result
ELSE
SELECT 'False' AS result
在这种情况下,没有提出除零除外.
结论：
如果C/C++#/ VB有短路,为什么SQL Server不能拥有它？
To truly answer this let’s take a look at how both work with
conditions. C++/C#/VB all have short circuiting defined in the
language specifications to speed up code execution. Why bother
evaluating N OR conditions when the first one is already true or M
AND conditions when the first one is already false.
We as developers have to be aware that SQL Server works differently.
It is a cost based system. To get the optimal execution plan for our
query the query processor has to evaluate every where condition and
assign it a cost. These costs are then evaluated as a whole to form a
threshold that must be lower than the defined threshold SQL Server has
for a good plan. If the cost is lower than the defined threshold the
plan is used,if not the whole process is repeated again with a
different mix of condition costs. Cost here is either a scan or a seek
or a merge join or a hash join etc… Because of this the
short-circuiting as is available in C++/C#/VB simply isn’t possible.
You might think that forcing use of index on a column counts as short
circuiting but it doesn’t. It only forces the use of that index and
with that shortens the list of possible execution plans. The system is
still cost based.
As a developer you must be aware that SQL Server does not do
short-circuiting like it is done in other programming languages and
there’s nothing you can do to force it to.
解决方法
在SQL Server中无法保证在WHERE子句中处理语句的顺序或顺序.允许语句短路的单个表达式是CASE-WHEN.以下是我在Stackoverflow上发布的答案：
How SQL Server short-circuits WHERE condition evaluation
It does when it feels like it,but not in the way you immediately think of.
As a developer you must be aware that SQL Server does not do short-circuiting like it is done in other programming languages and there’s nothing you can do to force it to.
有关详细信息,请查看上述博客条目中的第一个链接,该链接指向另一个博客：
Does SQL Server Short-Circuit?
The final verdict? Well,I don’t really have one yet,but it is probably safe to say that the only time you can ensure a specific short-circuit is when you express multiple WHEN conditions in a CASE expression. With standard boolean expressions,the optimizer will move things around as it sees fit based on the tables,indexes and data you are querying.
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