Or we can simply say, SQL Server Not Exists operator will return the results exactly opposite to the result returned by the Subquery. The ADD CONSTRAINT command is used to create a constraint after a table is already created. To avoid this situation, usually, developers add T-SQL If Exists statement and drop the object if it is already available in the database. 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. 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. The following SQL adds a constraint named "PK_Person" that is a PRIMARY KEY constraint on multiple columns (ID and LastName): We are adding a new default constraint to a table. Constraints are rules that the SQL Server Database Engine enforces for you. Index independent of a constraint 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. You can specify a unique clustered index if a clustered index on the table does not already exist. ADD CONSTRAINT. To add a CHECK constraint to an existing table, you use the ALTER TABLE ADD CONSTRAINT statement. When you create a UNIQUE constraint, a unique nonclustered index is created to enforce a UNIQUE constraint by default. 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]. Now let’s add a CHECK constraint to the Price column. Let’s say we wish to deploy a stored procedure ‘stpGetAllMembers’ in the SQLShack test database. Example 2 – Add a Column-Level Constraint. ALTER TABLE Event ADD CONSTRAINT chkPrice CHECK (Price > 0); This constraint will ensure that the price is always greater than zero. Now that the constraint has been added, here’s what happens if we try to insert invalid data: 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. 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 … 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 … 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. For more information, see Unique Constraints and Check Constraints. UNIQUE constraints. [CONSTRAINT_NAME]' ) AND type in ( N 'U' )) Most schema checks can be done using a collection of information schema views which SQL Server has built in. 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