Question

Trying to have a single SQL script that works on a number of environments, but the USE statement causes an error for the other environment, i.e. in the example below, when running against SQL02, the USE [application], even when inside a BEGIN ... END?

-- connected to SQL02
DECLARE @STAGE INTEGER = 3
IF @@SERVERNAME = 'SQL02' 
    SET @STAGE = 1
IF @@SERVERNAME = 'SQL03' 
    SET @STAGE = 2

IF @STAGE = 1 
    BEGIN
        IF EXISTS ( SELECT  *
                    FROM    [application_tst].[sys].[database_principals]
                    WHERE   name = N'qauser' ) 
            BEGIN
                USE [application_tst]
                --rest of code
            END
    END

IF @STAGE = 2 
    BEGIN
        IF EXISTS ( SELECT  *
                    FROM    [application].[sys].[database_principals]
                    WHERE   name = N'qauser' ) 
            BEGIN
                USE [application]
                --rest of code
            END
    END

Answer 1

You will need to use explicit three part names everywhere and not use USE. USE is an execution time command and your batch will fail to compile as expected because you expect the name resolution to occur as if the USE was executed during compilation.

-- connected to SQL02
DECLARE @STAGE INTEGER = 3
IF @@SERVERNAME = 'SQL02' 
    SET @STAGE = 1
IF @@SERVERNAME = 'SQL03' 
    SET @STAGE = 2

IF @STAGE = 1 
    BEGIN
        IF EXISTS ( SELECT  *
                    FROM    [application_tst].[sys].[database_principals]
                    WHERE   name = N'qauser' ) 
            BEGIN
                select from [application_tst].dbo.table
                insert into [application_tst].dbo.table
                --rest of code, using explicit 3 part names
            END
    END

IF @STAGE = 2 
    BEGIN
        IF EXISTS ( SELECT  *
                    FROM    [application].[sys].[database_principals]
                    WHERE   name = N'qauser' ) 
            BEGIN
                select from [application].dbo.table
                insert into [application].dbo.table
                --rest of code, using explicit 3 part names
            END
    END

An alternative is to use dynamic sql, which will delay compilation until is invoked.