Generally, when loading a large data set, we disable constraints and drop indexes for speeding up the loading process. Constraints help us to enforce integrity rules making sure that no inconsistent data is exist though it slows down update operations on the table. There are different types of constraints for addressing different types of integrity such as;
- Entity integrity - referring row level integrity
- Domain integrity - referring column level integrity
- Referential integrity - referring integrity with relationships
In addition to that, constraints can be used for addressing user-defined integrity too.
SQL Server supports following constraints types;
- PRIMARY KEY constraint
- UNIQUE KEY constraint
- FOREIGN KEY constraint
- CHECK constraint
- NOT NULL constraint
- DEFAULT constraint
However, note that we cannot disable all constraints. Only CHECK and FOREIGN KEY constraints can be disabled. For disabling other constraints, either constraints have to be modified or dropped. For example, if you need to disable either PRIMARY KEY or UNIQUE KEY constraint, index attached for them should be dropped.
Here is the code for disabling CHECK constraint.
-- Create a table with a check constraint USE tempdb; GO CREATE TABLE dbo.Customer ( CustomerId int not null , CustomerName varchar(10) not null , CreditAmount decimal(18,2) not null , CONSTRAINT Customer_SalaryCheck CHECK (CreditAmount < 100000) ); GO -- Inserting valid data INSERT INTO dbo.Customer VALUES (1, 'Joe Brown', 65000); INSERT INTO dbo.Customer VALUES (2, 'Mary Smith', 75000); -- Inserting invalid data -- This will not be possible as it violates the constraint INSERT INTO dbo.Customer VALUES (3, 'Joe Brown', 110000); -- Disabling the constraint and inserting the same again -- Now it allows to insert a row with an invalid value ALTER TABLE dbo.Customer NOCHECK CONSTRAINT Customer_SalaryCheck; GO INSERT INTO dbo.Customer VALUES (3, 'Joe Brown', 110000); GO -- Enabling the constraint. -- Note that updated records are not validated ALTER TABLE dbo.Customer CHECK CONSTRAINT Customer_SalaryCheck; GO -- Now table contains invalid data.
Note that it does not validate records inserted when enabling it again. If you want to check all records for integrity when enabling, use following code.
-- If you want to make sure no invalid data exist -- and validate all records agains the constraint -- disable it again and enable it with CHECK option ALTER TABLE dbo.Customer NOCHECK CONSTRAINT Customer_SalaryCheck; GO ALTER TABLE dbo.Customer WITH CHECK CHECK CONSTRAINT Customer_SalaryCheck; GO -- This will throw and error because current rowset has an invalid value. -- This can be corrected by either updating it or deleting it. DELETE FROM dbo.Customer WHERE CustomerId = 3; -- Now constraint can be enabled with CHECK option ALTER TABLE dbo.Customer WITH CHECK CHECK CONSTRAINT Customer_SalaryCheck; GO -- And if you run sys.check_constraint again, you will see 0 for is_not_trusted column. DROP TABLE dbo.Customer;