问题描述
下图是 Microsoft SQL Server 2008 R2 系统视图的一部分.从图中我们可以看出,sys.partitions
和sys.allocation_units
的关系取决于sys.allocation_units.type
的值.所以为了把它们结合在一起,我会写一些类似的东西:
The following image is a part of Microsoft SQL Server 2008 R2 System Views. From the image we can see that the relationship between sys.partitions
and sys.allocation_units
depends on the value of sys.allocation_units.type
. So to join them together I would write something similar to this:
SELECT *
FROM sys.indexes i
JOIN sys.partitions p
ON i.index_id = p.index_id
JOIN sys.allocation_units a
ON CASE
WHEN a.type IN (1, 3)
THEN a.container_id = p.hobt_id
WHEN a.type IN (2)
THEN a.container_id = p.partition_id
END
但是上面的代码给出了一个语法错误.我猜这是因为 CASE
语句.谁能帮忙解释一下?
But the upper code gives a syntax error. I guess that's because of the CASE
statement.
Can anyone help to explain a little?
添加错误信息:
消息 102,级别 15,状态 1,第 6 行 '=' 附近的语法不正确.
Msg 102, Level 15, State 1, Line 6 Incorrect syntax near '='.
推荐答案
CASE
表达式从子句的 THEN
部分返回一个值.你可以这样使用它:
A CASE
expression returns a value from the THEN
portion of the clause. You could use it thusly:
SELECT *
FROM sys.indexes i
JOIN sys.partitions p
ON i.index_id = p.index_id
JOIN sys.allocation_units a
ON CASE
WHEN a.type IN (1, 3) AND a.container_id = p.hobt_id THEN 1
WHEN a.type IN (2) AND a.container_id = p.partition_id THEN 1
ELSE 0
END = 1
请注意,您需要对返回值进行处理,例如将其与 1 进行比较.您的语句试图返回赋值或相等性测试的值,这在 CASE
/THEN
子句的上下文中都没有意义.(如果 BOOLEAN
是一种数据类型,那么相等性测试就有意义.)
Note that you need to do something with the returned value, e.g. compare it to 1. Your statement attempted to return the value of an assignment or test for equality, neither of which make sense in the context of a CASE
/THEN
clause. (If BOOLEAN
was a datatype then the test for equality would make sense.)
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