Must declare even if I have already declared - what am i doing wrong?

[Kiru_2024]

Below is the script and its throwing "Must declare the scalar variable "@sql"." error.

EXEC sp_MSforeachdb '

IF OBJECT_ID(''tempdb..##FKConstraints'') IS NOT NULL

    DROP TABLE ##FKConstraints;

CREATE TABLE ##FKConstraints

(

    ServerName VARCHAR(40),

    DatabaseName VARCHAR(20),

    SQLText VARCHAR(MAX)

);

DECLARE @FinalSQL NVARCHAR(MAX) = N''

IF DB_ID() > 4

BEGIN

    DECLARE @sql VARCHAR(MAX) = ''

    SELECT @sql

        = @sql + CHAR(13) + CHAR(10) + CHAR(13) + CHAR(10) + ''ALTER TABLE ['' + s.name + '']'' + ''.'' + ''['' + o.name + '']''

          + ''WITH CHECK CHECK CONSTRAINT'' + i.name + CHAR(13) + CHAR(10)

    FROM sys.foreign_keys i

        INNER JOIN sys.objects o

            ON i.parent_object_id = o.object_id

        INNER JOIN sys.schemas s

            ON o.schema_id = s.schema_id

    WHERE i.is_not_trusted = 1

          AND is_not_for_replication = 0;

    IF @@ROWCOUNT > 0

        INSERT INTO ##FKConstraints

        (

            ServerName,

            DatabaseName,

            SQLText

        )

        VALUES

        (@@SERVERNAME, DB_NAME(), @sql);

END;

--SELECT * FROM ##FKConstraints

SELECT @FinalSQL = N''USE'' + N'' ''+ DatabaseName + N''   '' + CHAR(13) + CHAR(10) + SQLText + CHAR(13) + CHAR(10)

FROM ##FKConstraints;

EXEC sp_executesql @FinalSQL;'

[#2]

I'm not able to debug the entirety of your script, but it would appear you have an issue with nested strings and escaping the ' character.

[#3]

Your script

EXEC sp_MSforeachdb '
IF OBJECT_ID(''tempdb..##FKConstraints'') IS NOT NULL
    DROP TABLE ##FKConstraints;
CREATE TABLE ##FKConstraints
(
    ServerName VARCHAR(40),
    DatabaseName VARCHAR(20),
    SQLText VARCHAR(MAX)
);
DECLARE @FinalSQL NVARCHAR(MAX) = N''
IF DB_ID() > 4
BEGIN
    DECLARE @sql VARCHAR(MAX) = ''
    SELECT @sql
        = @sql + CHAR(13) + CHAR(10) + CHAR(13) + CHAR(10) + ''ALTER TABLE ['' + s.name + '']'' + ''.'' + ''['' + o.name + '']''
          + ''WITH CHECK CHECK CONSTRAINT'' + i.name + CHAR(13) + CHAR(10)
    FROM sys.foreign_keys i
        INNER JOIN sys.objects o
            ON i.parent_object_id = o.object_id
        INNER JOIN sys.schemas s
            ON o.schema_id = s.schema_id
    WHERE i.is_not_trusted = 1
          AND is_not_for_replication = 0;
    IF @@ROWCOUNT > 0
        INSERT INTO ##FKConstraints
        (
            ServerName,
            DatabaseName,
            SQLText
        )
        VALUES
        (@@SERVERNAME, DB_NAME(), @sql);
END;
--SELECT * FROM ##FKConstraints
SELECT @FinalSQL = N''USE'' + N'' ''+ DatabaseName + N''   '' + CHAR(13) + CHAR(10) + SQLText + CHAR(13) + CHAR(10)
FROM ##FKConstraints;
EXEC sp_executesql @FinalSQL;'

What's executed by sp_MSForEachDb

IF OBJECT_ID('tempdb..##FKConstraints') IS NOT NULL
    DROP TABLE ##FKConstraints;
CREATE TABLE ##FKConstraints
(
    ServerName VARCHAR(40),
    DatabaseName VARCHAR(20),
    SQLText VARCHAR(MAX)
);
DECLARE @FinalSQL NVARCHAR(MAX) = N'
IF DB_ID() > 4
BEGIN
    DECLARE @sql VARCHAR(MAX) = '
    SELECT @sql
        = @sql + CHAR(13) + CHAR(10) + CHAR(13) + CHAR(10) + 'ALTER TABLE [' + s.name + ']' + '.' + '[' + o.name + ']'
          + 'WITH CHECK CHECK CONSTRAINT' + i.name + CHAR(13) + CHAR(10)
    FROM sys.foreign_keys i
        INNER JOIN sys.objects o
            ON i.parent_object_id = o.object_id
        INNER JOIN sys.schemas s
            ON o.schema_id = s.schema_id
    WHERE i.is_not_trusted = 1
          AND is_not_for_replication = 0;
    IF @@ROWCOUNT > 0
        INSERT INTO ##FKConstraints
        (
            ServerName,
            DatabaseName,
            SQLText
        )
        VALUES
        (@@SERVERNAME, DB_NAME(), @sql);
END;
--SELECT * FROM ##FKConstraints
SELECT @FinalSQL = N'USE' + N' '+ DatabaseName + N'   ' + CHAR(13) + CHAR(10) + SQLText + CHAR(13) + CHAR(10)
FROM ##FKConstraints;

Pay particular attention to the area around "Declare @sql" on line 12 and how that breaks when '' is converted back to '.