SELECT while 循环中的语句不工作

Question

I have created the following Select statement which is working fine:

SELECT @Block = [Blok], @Year = [Aar] 
FROM [PT99991_Result].[dbo].[Testheader] 
WHERE N = @TestHeaderID

The problem is that this Select statement is used in a While loop where the database can change to another one during the loop. I have tried to modify the statement to the following but it's not working. I have also tried to use EXEC which takes care of my problem but then I'm facing a problem with the local variables @Block and @Year instead.

SET @DataBase = 'PT99991_RESULT' --This is only for test!

SELECT @Block = [Blok], @Year = [Aar] 
FROM '[' + @DataBase + '].[dbo].[Testheader]' 
WHERE N = @TestHeaderID

I'm not sure what I'm doing wrong?

Answer 1

First, generate a T-SQL template like this:

DECLARE @DynamicTSQLStatement NVARCHAR(MAX) = N'SELECT @Block = [Blok], @Year = [Aar] 
                                                FROM [@DataBase].[dbo].[Testheader] 
                                                WHERE N = @TestHeaderID';

Then, let's say that the variale @Datbase holds your current database name. If it is not extracted from sys.database you can perform additional validation to ensure nobody is doing something wrong.

IF NOT EXISTS(SELEFT 1 FROM sys.database WHERE [name] = @Datbase
BEGIN
    ....
END;

After the validation, you just replace the database name in the template:

SET @DynamicTSQLStatement = REPLACE(@DynamicTSQLStatement, '@Database', @Database);

Then, execute the code passing the parameters:

EXEC sp_executesql @DynamicTSQLStatement
                  ,N'@TestHeaderID INT'
                  ,N'@Block INT OUTPUT'
                  ,N'@Year INT OUTPUT'
                  ,@TestHeaderID
                  ,@Block OUTPUT
                  ,@Year OUTPUT;

of course on every loop iteration, reset the template.

Answer 2

Instead of while loop, You can go for undocumented stored procedure: ms_foreachdb and execute against the databases and finally apply filter for the specific database.

Caveat : Don't use this in production code, because, it uses undocumented stored procedure.

CREATE TABLE #test(dbname sysname, blok int, aar int)
DECLARE @db_list NVARCHAR(max) = 'DB1,DB2'

EXECUTE master.sys.sp_MSforeachdb 'USE [?]; declare @blok int, @aar int; INSERT INTO #test SELECT db_name(), blok, aar from [dbo].[Testheader] WHERE N = TestHeaderId;'

SELECT * FROM #test where dbname in 
(
SELECT value FROM string_split(@db_list,',')
)