SQL 根据sql中的case语句创建select语句
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create select statement based on case statement in sql
提问by Ben
Is there a clean way in sql to create a select statement based on a case statement?
sql中是否有一种干净的方法可以根据case语句创建select语句？
I'm looking for the basic logic to be something like
我正在寻找类似的基本逻辑
CASE @tableToUse
WHEN 'Table1' SELECT * FROM Table1
WHEN 'Table2' SELECT * FROM table2
DEFAULT 'Table3' SELECT * FROM table3
END CASE
回答by TLiebe
In T-Sql (MS SQL Server) you can dynamically compose your SQL statement and then use the sp_executesqlmethod to execute it. Based on your case statement you can build a SELECT statement using a variable and then use sp_executesql to run it. The one thing to be aware of is that you need to make sure you clean any input you get from users if you are using text directly from users to prevent SQL injection attacks.
在 T-Sql (MS SQL Server) 中，您可以动态地编写 SQL 语句，然后使用sp_executesql方法执行它。根据您的 case 语句，您可以使用变量构建 SELECT 语句，然后使用 sp_executesql 运行它。需要注意的一件事是，如果您直接使用来自用户的文本来防止 SQL 注入攻击，则需要确保清除从用户那里获得的任何输入。
For example,
例如，
DECLARE @sql NVARCHAR(200)
SELECT @sql = CASE @tableToUse
WHEN 'Table1' THEN 'SELECT * FROM Table1'
WHEN 'Table2' THEN 'SELECT * FROM Table2'
ELSE 'Table3' THEN 'SELECT * FROM Table2'
END
EXECUTE sp_executesql @sql
You should be able to do something similar in other versions of SQL.
您应该能够在其他版本的 SQL 中执行类似的操作。
回答by JohnFx
Sorry, Case statements in tSQL (which I assume you are asking about) are for evaluating expressions only and can't be used for control flow.
抱歉，tSQL 中的 Case 语句（我假设您正在询问）仅用于评估表达式，不能用于控制流。
You are pretty much stuck with IF..THEN
你几乎被 IF..THEN 困住了
IF (@Something='Table1')
SELECT Field1 from table1
ELSE
IF (@Something='Table2')
SELECT Field1 from table2
ELSE
SELECT Field1 From table3
回答by Astro
Well, this is old, but in case it is helpful:
嗯，这是旧的，但如果它有帮助：
CASE WHEN <Variabe> THEN (SELECT <Column> FROM <Table>) END
回答by Baaju
You can do this ! only that you cannot do a "select *" as that might return more than one value that shoot an error even if that has to be a table with a single column
你可以这样做 ！只是您不能执行“选择 *”，因为它可能会返回多个会引发错误的值，即使它必须是具有单列的表
SELECT columnName = CASE WHEN 'foo' THEN select <Column_Name> from tableName where columnName = 'this'
WHEN 'bar' THEN select <Column_Name> from tableName where columnName = 'that'
END
FROM tableName
回答by HLGEM
Why not just join to the testTable on username?
为什么不直接加入用户名的 testTable？
