Finding Cross Dependency Tables in SQL - sql

Question

I need to List all the tables which depends on each other
Example
TableA has foreign Key relation to TableB
TableB has foreign Key relation to TableA
So far I got the script to find the foreign key relations for the tables but how can I filter them down to get just the cross table reference one.
my script so are is:
SELECT CAST(p.name AS VARCHAR(255)) AS [Primary Table] ,
CAST(c.name AS VARCHAR(255)) AS [Foreign Table]
FROM sysobjects f
INNER JOIN sysobjects c ON f.parent_obj = c.id
INNER JOIN sysreferences r ON f.id = r.constid
INNER JOIN sysobjects p ON r.rkeyid = p.id
INNER JOIN syscolumns rc ON r.rkeyid = rc.id
AND r.rkey1 = rc.colid
INNER JOIN syscolumns fc ON r.fkeyid = fc.id
AND r.fkey1 = fc.colid
LEFT JOIN syscolumns rc2 ON r.rkeyid = rc2.id
AND r.rkey2 = rc.colid
LEFT JOIN syscolumns fc2 ON r.fkeyid = fc2.id
AND r.fkey2 = fc.colid
WHERE f.type = 'F'
ORDER BY CAST(p.name AS VARCHAR(255))

Answer 1

This simple select statement returns the circular direct foreign key references in a database:
IF OBJECT_ID('dbo.d') IS NOT NULL DROP TABLE dbo.d;
IF OBJECT_ID('dbo.c') IS NOT NULL DROP TABLE dbo.c;
IF OBJECT_ID('dbo.b') IS NOT NULL
BEGIN
ALTER TABLE dbo.a DROP CONSTRAINT [dbo.a(bid)->dbo.b(bid)];
DROP TABLE dbo.b;
END
IF OBJECT_ID('dbo.a') IS NOT NULL DROP TABLE dbo.a;
CREATE TABLE dbo.a(aid INT PRIMARY KEY CLUSTERED, bid INT);
CREATE TABLE dbo.b(aid INT CONSTRAINT [dbo.b(aid)->dbo.a(aid)] REFERENCES dbo.a(aid), bid INT PRIMARY KEY CLUSTERED);
ALTER TABLE dbo.a ADD CONSTRAINT [dbo.a(bid)->dbo.b(bid)] FOREIGN KEY(bid) REFERENCES dbo.b(bid);
CREATE TABLE dbo.c(cid INT PRIMARY KEY CLUSTERED);
CREATE TABLE dbo.d(did INT PRIMARY KEY CLUSTERED, cid INT CONSTRAINT [dbo.d(cid)->dbo.c(cid)] REFERENCES dbo.c(cid));
SELECT *
FROM sys.foreign_keys fk1
JOIN sys.foreign_keys fk2
ON fk1.parent_object_id = fk2.referenced_object_id
AND fk2.parent_object_id = fk1.referenced_object_id;
From here you can join to the DMVs sys.tables and sys.columns to get the additional information like table and column names.
Two things to be aware of:
You should stop using the compatibility views. They are there to support old scripts that where written for SQL 2000 and should not be used in new development.
You can have up to 16 columns in a foreign key. Your script supports only two. However, you are not even returning the column names so you should not join to sys.columns at all, if you don't need the column names.
If you just need the names of the tables you can get away without an additional join by using this select statement instead:
SELECT
QUOTENAME(OBJECT_SCHEMA_NAME(fk1.parent_object_id))+'.'+QUOTENAME(OBJECT_NAME(fk1.parent_object_id))+
' <-> '+
QUOTENAME(OBJECT_SCHEMA_NAME(fk1.referenced_object_id))+'.'+QUOTENAME(OBJECT_NAME(fk1.referenced_object_id))
FROM sys.foreign_keys fk1
JOIN sys.foreign_keys fk2
ON fk1.parent_object_id = fk2.referenced_object_id
AND fk2.parent_object_id = fk1.referenced_object_id
AND fk1.parent_object_id < fk1.referenced_object_id;
I also added an additional condition to the WHERE clause of the query to include each pair only once.