是否可以在 sql From 子句中使用 Case 语句
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Is it possible to use a Case statement in a sql From clause
提问by Zambian
Is it possible to use a Case statement in a sql From clause using SQL 2005? For example, I'm trying something like:
是否可以在使用 SQL 2005 的 sql From 子句中使用 Case 语句？例如，我正在尝试类似的事情：
SELECT Md5 FROM
CASE
WHEN @ClientType = 'Employee' THEN @Source = 'HR'
WHEN @ClientType = 'Member' THEN @Source = 'Other'
END CASE
WHERE Current = 2;
采纳答案by jhale
Assuming SQL Server:
假设 SQL Server：
You would need to use dynamic SQL. Build the string and then call sp_executesql with the string.
您将需要使用动态 SQL。构建字符串，然后使用该字符串调用 sp_executesql。
Edit: Better yet, just use if statements to execute the appropriate statement and assign the value to a variable. You should avoid dynamic SQL if possible.
编辑：更好的是，只需使用 if 语句来执行适当的语句并将值分配给变量。如果可能，您应该避免使用动态 SQL。
回答by yukondude
I don't believe that's possible. For one thing, query optimizers assume a specific list of table-like things in the FROM clause.
我不相信这是可能的。一方面，查询优化器假定 FROM 子句中有一个特定的类似表的列表。
The most simple workaround that I can think of would be a UNION between the two tables:
我能想到的最简单的解决方法是在两个表之间建立 UNION：
SELECT md5
FROM hr
WHERE @clienttype = 'Employee'
AND current = 2
UNION
SELECT md5
FROM other
WHERE @clienttype = 'Member'
AND current = 2;
Only one half of the UNION could be True, given the @clienttype predicate.
给定@clienttype 谓词，只有一半的 UNION 可以为 True。
回答by Kevin Conner
No, you can't pick a table to query using a CASE statement. CASE statements only go within expressions, such as for a column's value or as part of your WHERE expression.
不，您不能使用 CASE 语句选择要查询的表。CASE 语句仅用于表达式中，例如用于列的值或作为 WHERE 表达式的一部分。
This should do it, if you are looking for just one value:
如果您只寻找一个值，则应该这样做：
IF @ClientType = 'Employee' BEGIN
SET @Source = (SELECT Md5 FROM HR WHERE Current = 2)
END
ELSE IF @ClientType = 'Member' BEGIN
SET @Source = (SELECT Md5 FROM Other WHERE Current = 2)
END
回答by marc_s
Since you don't specify what SQL backend you're going against, this will be hard to properly answer....
由于您没有指定要反对的 SQL 后端，因此很难正确回答....
As far as I can tell, you will not be able to do this neither against MS SQL Server, nor against Interbase/Firebird. I can't speak for other backend servers, though...
据我所知，无论是针对 MS SQL Server 还是针对 Interbase/Firebird，您都无法做到这一点。不过，我不能代表其他后端服务器...
Marc
马克
回答by Tor Haugen
I think it's pretty safe to say the answer is no way. And that's regardless of SQL dialect.
我认为可以肯定地说答案是不可能的。这与 SQL 方言无关。
