How to create an alias of database in SQL Server

Question

We have a very old software has been created around 10 years ago and we don\'t have source code.

The software uses two databases, DB01 and DB02

Answer 1

Here's a stored proc to do it. Simply add it to your database and call it with the target database. It will create synonyms for all tables in the target database, and create the schemas if they don't exist. I've left a commented out section in case someone knows of a way to get the create schemas working without a cursor.

CREATE PROCEDURE CreateSynonymsForTargetDatabase (
    @databaseName sysname
)
AS BEGIN
DECLARE @TSQL nvarchar(max) = N''
DECLARE @rn char(2),
    @SchemaName sysname;

    SET @rn = char(13) + char(10)   

    CREATE TABLE #DBSynonym(        
        [Schema] sysname NOT NULL,
        [Table] sysname NOT NULL
    )

    SET @TSQL = N'
        INSERT INTO #DBSynonym ([Schema], [Table])
        SELECT Schemas.name, Tables.name
        FROM [' + @databaseName + '].sys.tables 
        INNER JOIN [' + @databaseName + '].sys.schemas on tables.schema_id = schemas.schema_id      
    '

    EXEC (@TSQL)
    SET @TSQL = N''

    DECLARE MissingSchemasCursor CURSOR
    READ_ONLY
    FOR 
        SELECT newSchemas.[Schema]
        FROM #DBSynonym newSchemas
        LEFT JOIN sys.schemas on newSchemas.[Schema] = schemas.name
        WHERE schemas.schema_id is null
        GROUP BY newSchemas.[Schema]

    OPEN MissingSchemasCursor
    FETCH NEXT FROM MissingSchemasCursor INTO @SchemaName
    WHILE (@@fetch_status <> -1)
    BEGIN
        IF (@@fetch_status <> -2)
        BEGIN
            SET @TSQL = N'CREATE SCHEMA ' + QUOTENAME(@SchemaName) + N';'

            EXEC sp_executesql @TSQL
        END
        FETCH NEXT FROM MissingSchemasCursor INTO @SchemaName
    END
    CLOSE MissingSchemasCursor
    DEALLOCATE MissingSchemasCursor

    /*
    SELECT @TSQL = @TSQL +
        N'
        GO
        CREATE SCHEMA ' + QUOTENAME([Schema]) + N';'
    FROM #DBSynonym newSchemas
    LEFT JOIN sys.schemas on newSchemas.[Schema] = schemas.name
    WHERE schemas.schema_id is null
    GROUP BY newSchemas.[Schema]

    PRINT 'CREATE SCHEMAS : ' + ISNULL(@TSQL,'')
    EXEC sp_executesql @TSQL
    */
    SET @TSQL = N''

    SELECT @TSQL = @TSQL +
        N'
        CREATE SYNONYM ' + QUOTENAME([Schema]) + N'.' + QUOTENAME([Table]) + N'
        FOR ' + QUOTENAME(@databaseName) + N'.' + QUOTENAME([Schema]) + N'.' + QUOTENAME([Table]) + N';'
    FROM #DBSynonym


    EXEC sp_executesql @TSQL
    SET @TSQL = N''

END
GO

Use it as follows :

EXEC CreateSynonymsForTargetDatabase 'targetDbName'