WITH CHECK ADD CONSTRAINT followed by CHECK CONSTRAINT vs. ADD CONSTRAINT

Question

I\'m looking at the AdventureWorks sample database for SQL Server 2008, and I see in their creation scripts that they tend to use the following:

ALTER TABLE          


        

Answer 1

To demonstrate how this works--

CREATE TABLE T1 (ID INT NOT NULL, SomeVal CHAR(1));
ALTER TABLE T1 ADD CONSTRAINT [PK_ID] PRIMARY KEY CLUSTERED (ID);

CREATE TABLE T2 (FKID INT, SomeOtherVal CHAR(2));

INSERT T1 (ID, SomeVal) SELECT 1, 'A';
INSERT T1 (ID, SomeVal) SELECT 2, 'B';

INSERT T2 (FKID, SomeOtherVal) SELECT 1, 'A1';
INSERT T2 (FKID, SomeOtherVal) SELECT 1, 'A2';
INSERT T2 (FKID, SomeOtherVal) SELECT 2, 'B1';
INSERT T2 (FKID, SomeOtherVal) SELECT 2, 'B2';
INSERT T2 (FKID, SomeOtherVal) SELECT 3, 'C1';  --orphan
INSERT T2 (FKID, SomeOtherVal) SELECT 3, 'C2';  --orphan

--Add the FK CONSTRAINT will fail because of existing orphaned records
ALTER TABLE T2 ADD CONSTRAINT FK_T2_T1 FOREIGN KEY (FKID) REFERENCES T1 (ID);   --fails

--Same as ADD above, but explicitly states the intent to CHECK the FK values before creating the CONSTRAINT
ALTER TABLE T2 WITH CHECK ADD CONSTRAINT FK_T2_T1 FOREIGN KEY (FKID) REFERENCES T1 (ID);    --fails

--Add the CONSTRAINT without checking existing values
ALTER TABLE T2 WITH NOCHECK ADD CONSTRAINT FK_T2_T1 FOREIGN KEY (FKID) REFERENCES T1 (ID);  --succeeds
ALTER TABLE T2 CHECK CONSTRAINT FK_T2_T1;   --succeeds since the CONSTRAINT is attributed as NOCHECK

--Attempt to enable CONSTRAINT fails due to orphans
ALTER TABLE T2 WITH CHECK CHECK CONSTRAINT FK_T2_T1;    --fails

--Remove orphans
DELETE FROM T2 WHERE FKID NOT IN (SELECT ID FROM T1);

--Enabling the CONSTRAINT succeeds
ALTER TABLE T2 WITH CHECK CHECK CONSTRAINT FK_T2_T1;    --succeeds; orphans removed

--Clean up
DROP TABLE T2;
DROP TABLE T1;