Question

I have table Animal.

I want to return everything from this table + one column which denotes if record is referenced anywhere else as a foreign key.

I.E.:

Animal_Id    Name  
    1        Cat
    2        Dog
    3        Parrot

I want to return this:

AnimalId    Name     Referenced 
    1        Cat         true
    2        Dog         false
    3        Parrot      true

by ‘referenced’ I mean if a specific Animal_Id was referenced in any other table in the database as a foreign key. I think I first can query information_schema and find out what tables contain this foreign key and then use loop and dynamic sql to execute
select count(*) from eachTable where AnimalID = 1

Does anyone have a snippet on how to do that?

Answer 1

This should do it:

SELECT OO.Animal_ID, OO.Name, CASE WHEN XX.REFERENCED IS NULL THEN 'false' ELSE 'true' END Referenced
FROM   Animal OO
       OUTER APPLY (SELECT SUM(1) REFERENCED
                    FROM   (SELECT FkAnimal_ID FROM AnimalRef1 RR WHERE RR.FkAnimal_ID = OO.Animal_ID UNION ALL
                            SELECT FkAnimal_ID FROM AnimalRef2 RR WHERE RR.FkAnimal_ID = OO.Animal_ID UNION ALL
                            SELECT FkAnimal_ID FROM AnimalRef3 RR WHERE RR.FkAnimal_ID = OO.Animal_ID) II) XX

If you don’t know all the FK tables, you can use system meta-data tables to generate that collection of UNION ALL queries into a table, which you could then copy & paste into your query:

WITH AKT AS ( SELECT f.name AS ForeignKey
                    ,OBJECT_NAME(f.parent_object_id) AS TableName
                    ,COL_NAME(fc.parent_object_id, fc.parent_column_id) AS ColumnName
                    ,OBJECT_NAME (f.referenced_object_id) AS ReferenceTableName
                    ,COL_NAME(fc.referenced_object_id, fc.referenced_column_id) AS ReferenceColumnName
              FROM   sys.foreign_keys AS f
                     INNER JOIN sys.foreign_key_columns AS fc ON f.OBJECT_ID = fc.constraint_object_id
              WHERE  f.referenced_object_id = object_id('Animal'))
SELECT 'SELECT ' + ColumnName + ' FROM ' + TableName + ' WHERE  RR.' + ColumnName + ' = OO.' + ReferenceColumnName + ' UNION ALL'
FROM   AKT

And for the whole thing in a single query using a recursive CTE:

DECLARE @QUERY NVARCHAR(MAX)

WITH AKT AS ( SELECT ROW_NUMBER() OVER (ORDER BY f.name) RN, f.name AS ForeignKey
                    ,OBJECT_NAME(f.parent_object_id) AS TableName
                    ,COL_NAME(fc.parent_object_id, fc.parent_column_id) AS ColumnName
                    ,SCHEMA_NAME(oo.schema_id) SchemaName
                    ,OBJECT_NAME (f.referenced_object_id) AS ReferenceTableName
                    ,COL_NAME(fc.referenced_object_id, fc.referenced_column_id) AS ReferenceColumnName
              FROM   sys.foreign_keys AS f
                     INNER JOIN sys.foreign_key_columns AS fc ON f.OBJECT_ID = fc.constraint_object_id
                     INNER JOIN sys.objects oo ON oo.object_id = fc.referenced_object_id
              WHERE  f.referenced_object_id = object_id('Animal'))

    ,bs AS (SELECT AKT.RN
                  ,'SELECT ' + ColumnName + ' FROM ' + SchemaName + '.' + TableName + ' WHERE ' + ColumnName + ' = OO.' + ReferenceColumnName  SubQuery
            FROM   AKT)
    ,re AS (SELECT bs.RN, CAST(RTRIM(bs.SubQuery) AS VARCHAR(MAX)) Joined
            FROM   bs
            WHERE  bs.RN = 1
            UNION  ALL
            SELECT bs2.RN, CAST(re.Joined + ' UNION ALL ' + ISNULL(RTRIM(bs2.SubQuery), '') AS VARCHAR(MAX)) Joined
            FROM   re, bs bs2 
            WHERE  re.RN = bs2.RN - 1 )
    ,fi AS (SELECT ROW_NUMBER() OVER (ORDER BY RN DESC) RNK, Joined
            FROM   re)
SELECT @QUERY  = 'SELECT OO.Animal_ID, OO.Name, CASE WHEN XX.REFERENCED IS NULL THEN ''No'' ELSE ''Yes'' END Referenced
FROM   Animal OO
       OUTER APPLY (SELECT SUM(1) REFERENCED
                    FROM   (' + Joined + ') II) XX'
FROM   fi
WHERE  RNK = 1

EXEC (@QUERY)