Suppose you have the following test.products table: CREATE TABLE test.products( product_id INT IDENTITY PRIMARY KEY , product_name VARCHAR ( 255 ) NOT NULL , unit_price DEC ( 10 , 2 ) NOT … Example 2 – Add a Column-Level Constraint. This article explains the SQL NOT NULL, Unique and SQL Primary Key constraints in SQL Server with examples. The following SQL adds a constraint named "PK_Person" that is a PRIMARY KEY constraint on multiple columns (ID and LastName): The ADD CONSTRAINT command is used to create a constraint after a table is already created. ADD CONSTRAINT. Index independent of a constraint Perhaps your scripting rollout and rollback DDL SQL changes and you want to check for instance if a default constraint exists before attemping to drop it and its parent column. I was going through some sample Scripts provided by Microsoft SQL Server team on their site, and was checking the JSON Sample Queries procedures views and indexes.sql script file. To add a CHECK constraint to an existing table, you use the ALTER TABLE ADD CONSTRAINT statement. The NOT EXISTS in SQL Server will check the Subquery for rows existence, and if there are no rows then it will return TRUE, otherwise FALSE. Now let’s add a CHECK constraint to the Price column. You can specify a unique clustered index if a clustered index on the table does not already exist. As a possible answer to my own question, this could be achieved by adding a Check constraint with a function such as the following: CREATE FUNCTION dbo.CheckSequenceKey (@SequenceKey nvarchar(10)) RETURNS bit AS BEGIN DECLARE @retval bit IF EXISTS (SELECT 1 FROM dbo.Seqs S WHERE S.SequenceKey = @SequenceKey) SET @retval = 1 ELSE SET @retval = 0 RETURN @retval … Now that the constraint has been added, here’s what happens if we try to insert invalid data: To drop the constraint you will have to add thee code to ALTER THE TABLE to drop it, but this should work Code Snippet IF EXISTS ( SELECT * FROM sys.objects WHERE object_id = OBJECT_ID ( N '[dbo]. Or we can simply say, SQL Server Not Exists operator will return the results exactly opposite to the result returned by the Subquery. For example, you can use UNIQUE constraints to make sure that no duplicate values are entered in specific columns that do not participate in a primary key. The addition of the new column with the constraint is detected and included in the deployment as part of the table creation, however there is no if exists statement to check for its existence of the constraint and drop it if it exists. For more information, see Unique Constraints and Check Constraints. Constraints in SQL Server are predefined rules and restrictions that are enforced in a single column or multiple columns, regarding the values allowed in the columns, to maintain the integrity, accuracy, and reliability of that column’s data. To avoid this situation, usually, developers add T-SQL If Exists statement and drop the object if it is already available in the database. Let’s say we wish to deploy a stored procedure ‘stpGetAllMembers’ in the SQLShack test database. UNIQUE constraints. We are adding a new default constraint to a table. ALTER TABLE Event ADD CONSTRAINT chkPrice CHECK (Price > 0); This constraint will ensure that the price is always greater than zero. 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