ALTER Table – [ WITH { CHECK | NOCHECK } ] { CHECK | NOCHECK } CONSTRAINT – A Deep Dive

In this post we will understand what is [with Check/No] , where they will be used and what is check/nocheck constraint and how we can utilise them. Yesterday one of my fellow Senior architect asked this question so I thought of sharing the details with you guys. In this post we shall not discussing the types of constraints and how we can use each types of constraint with real life scenarios.

So basically we need use alter table command to modifies a table definition
1. by adding constraints or dropping constraints.
2. by disabling or enabling constraints.

Syntax of Alter Table with CONSTRAINT

 ```-- ALTER TABLE [ database_name . [ schema_name ] . | schema_name . ] table_name .... | [ WITH { CHECK | NOCHECK } ] { CHECK | NOCHECK } CONSTRAINT { ALL | constraint_name [ ,...n ] } -- ```

Let’s first understand what is CHECK OR NOCHECK CONSTRAINT or how we can enable or disable the constraint.

CHECK CONSTRAINT – is used to enable the constraint. Below is the syntax to enable a single constraint.

 ```-- ALTER TABLE testConstraints CHECK CONSTRAINT chk_Vals GO -- ```

NOCHECK CONSTRAINT – is used to disable the constraint. Below is the syntax to disable a single constraint.

 ```-- ALTER TABLE testConstraints NOCHECK CONSTRAINT chk_Vals GO -- ```

Now Let’s understand the concepts via examples

Let’s first create a sample table and insert some data into it.

 ```-- /* Create a sample table*/ CREATE TABLE testConstraints ( Id INT PRIMARY KEY ,Vals INT ) GO /*Insert some data in the table*/ INSERT INTO testConstraints VALUES (1,1), (2,2), (3,2) GO -- ```

Now let’s create a unique constraint on column Vals with name ak_Vals

/* Alter Table command to create a new constraint */

 ```-- ALTER TABLE testConstraints ADD CONSTRAINT chk_Vals CHECK (Vals > 5); GO -- ```

OUTPUT

 ```-- Msg 547, Level 16, State 0, Line 19 The ALTER TABLE statement conflicted with the CHECK constraint "chk_Vals". The conflict occurred in database "master", table "dbo.testConstraints", column 'Vals'. -- ```

So the output in layman terms is that we cannnot create the check constraint because we have conflicting data in our vals column. This also means that when we create a constraint SQL will verify the existing data with the constraint we are creating and it will only create the constraint if it succeeds.

Now let’s delete all the existing data from our table testConstraints and insert some fresh data which is in accordance with our check constraint

 ```-- TRUNCATE TABLE testConstraints GO INSERT INTO testConstraints VALUES (1,10), (2,21), (3,25) GO -- ```

Now let’s again create the new CHECK constaint using below command

 ```-- ALTER TABLE testConstraints ADD CONSTRAINT chk_Vals CHECK (Vals > 5); GO -- ```

This time we are able to sucessfully create the constraint because the existing data passed the constraint check.

Now let’s try to insert a value in vals column which is less than 5. So in this case I am inserting 4, Let’s see what happens.</span

 ```-- INSERT INTO testConstraints VALUES (4,4) GO --OUTPUT Msg 547, Level 16, State 0, Line 50 The INSERT statement conflicted with the CHECK constraint "chk_Vals". The conflict occurred in database "master", table "dbo.testConstraints", column 'Vals'. The statement has been terminated. -- ```

This means that the constraint worked and SQL does not allowed us to insert the data less than 5.

So for that lets now disable the index. The system for the same is –

Disable single constraint

 ```-- INSERT INTO testConstraints VALUES (4,4) GO ALTER TABLE testConstraints NOCHECK CONSTRAINT chk_Vals GO -- ```

Now let’s again try to insert the data using below command.

 ```-- INSERT INTO testConstraints VALUES (4,4) GO -- (1 row affected) -- ```

This time we are able to insert the data. Now let’s again enable the constraint.

Enable single constraint

 ```-- ALTER TABLE testConstraints CHECK CONSTRAINT chk_Vals GO --OUTPUT Commands completed successfully. -- ```

The constraint is enabled sucessfully. Also note that we have data in the table that conflicts with the CHECK constraint “chk_Vals” still SQL SERVER here does not gave any issues. This is because SQL SERVER has not compared the existing data according to the constraints we have created. So this is the default behaviour when you enable the constraint.

Now lets again try to insert conflicting data.

 ```-- INSERT INTO testConstraints VALUES (5,2) GO -OUTPUT Msg 547, Level 16, State 0, Line 89 The INSERT statement conflicted with the CHECK constraint "chk_Vals". The conflict occurred in database "master", table "dbo.testConstraints", column 'Vals'. The statement has been terminated. -- ```

Here clearly SQL SERVER checked the newly inserted data with the constraint and since the data fails SQL will not insert this row.

Lets checks the data in the table.

 ```-- SELECT * FROM testConstraints GO Id Vals ----------- ----------- 1 10 2 21 3 25 4 4 (4 rows affected) -- ```

With above output it is clear that (5,2) row has not been inserted.

So what we need to do if we need to check the existing data while enabling the constraint. For that we need to use the following command.

 ```-- ALTER TABLE testConstraints WITH CHECK CHECK CONSTRAINT chk_Vals GO --OUTPUT Msg 547, Level 16, State 0, Line 116 The ALTER TABLE statement conflicted with the CHECK constraint "chk_Vals". The conflict occurred in database "master", table "dbo.testConstraints", column 'Vals'. -- ```

So when you execute the command it will say that table has some data that conflicts with the constraint.

Another thing is that if you do not want SQL SERVER to verify the existing data with the constraint then you can use below.

 ```-- ALTER TABLE testConstraints WITH NOCHECK CHECK CONSTRAINT chk_Vals GO -- ```

OR

 ```-- ALTER TABLE testConstraints CHECK CONSTRAINT chk_Vals GO -- ```

Both the above commands will execute sucessfully. So it is clear that when you enable a constraint and if you provide WITH NOCHECK or if you dont provide this option SQL Server will not verify the existing data with the constraint

Notes –

Now there are two options available for CHECK CONSTRAINT.

1. CHECK CONSTRAINT – WITH CHECK
2. CHECK CONSTRAINT – WITH NOCHECK

CHECK CONSTRAINT – WITH CHECK

1. It will enable the constraint.
2. It can only be used with FOREIGN KEY and CHECK constraints.
3. Future inserts or updates will be validated against the constraint conditions
4. Existing data will also be validated against the constraint conditions.

Syntax-

 ```-- ALTER TABLE testConstraints WITH CHECK CHECK CONSTRAINT chk_Vals GO -- ```

CHECK CONSTRAINT – WITH NOCHECK

1. It will enable the constraint.
2. It can only be used with FOREIGN KEY and CHECK constraints.
3. Future inserts or updates will be validated against the constraint conditions
4. Existing data will NOT be validated against the constraint conditions.

Syntax-

 ```-- ALTER TABLE testConstraints WITH NOCHECK CHECK CONSTRAINT chk_Vals GO -- ```

CHECK CONSTRAINT (DEFAULT)

1. It is same as CHECK CONSTRAINT – WITH NOCHECK.
2. It will enable the constraint.
3. It can only be used with FOREIGN KEY and CHECK constraints.
4. Future inserts or updates will be validated against the constraint conditions
5. Existing data will NOT be validated against the constraint conditions.

Syntax-

 ```-- ALTER TABLE testConstraints CHECK CONSTRAINT chk_Vals GO -- ```

NOCHECK CONSTRAINT

1. It will disable the constraint.
2. It can only be used with FOREIGN KEY and CHECK constraints.
3. Here future inserts or updates to the column are not validated against the constraint conditions.
4. Note – Constraint types – DEFAULT, PRIMARY KEY, and UNIQUE cannot be disabled.

Syntax-

 ```-- ALTER TABLE testConstraints NOCHECK CONSTRAINT chk_Vals GO -- ```

Reference

Pawan Khowal

Pawan is a SQL Server Developer. If you need any help in writing code/puzzle or training please email at – pawankkmr”AT”gmail.com. Meanwhile please go throgh the top pages from his blog.

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