Sub queries in check constraint

Question

I have table designed in SQL-Server 2008 R2.

I have a column in that table which needs to be checked against another table when data is inserting.

ALTER TABLE Table1
        WITH CHECK ADD CONSTRAINT CK_Code
        CHECK (MyField in (Select Field From Table2))

This cause an error

Sub-queries are not allowed in this context. Only scalar expressions are allowed.

I have looked at this question about Check Constraint - Subqueries are not allowed in this context.

Is there any way of achieving this without using a trigger?

Answer 1

Note, what you really want is a foreign key constraint. That said, to get a "query" into a check you can write a function that contains the query and outputs a scalar value, then use that function in the check constraint.

CREATE FUNCTION myFunction (
    @field DATATYPE(?)
)
RETURNS VARCHAR(5)
AS
BEGIN
    IF EXISTS (SELECT* FROM Table2 WHERE MYFIELD = @field)
        return 'True'
    return 'False'
END

Something like that. Not tested.

Then you can add it to your check like so

ALTER TABLE Table1
    WITH CHECK ADD CONSTRAINT CK_Code
    CHECK (myFunction(MYFIELD) = 'True')

Answer 2

You can't have sub queries inside check constraints. What you can do is use a UDF that returns a scalar value inside the check constraint.

Step 1: Create the table

USE CTBX
GO

CREATE TABLE RawMaterialByGender 
(
RMGID int primary key identity(1,1),
RMID smallint foreign key references RawMaterialMaster(RMID),
LeveLMasterID smallint foreign key references LevelMaster(LevelTextID),
IsDeleted bit
)

Step 2 : Create the UDF that returns a scalar

Create FUNCTION [dbo].[IsValidLevelMasterGender](@LevelMasterID smallint)

    RETURNS bit
    AS
    BEGIN
     DECLARE @count smallint;
     DECLARE @return bit;

     SELECT @count = count(LevelTextID)      
     FROM [LevelMaster]
     WHERE LevelCategoryID = 3 AND IsActive = 1 AND LevelTextID=@LevelMasterID

     IF(@count = 0)
     SET @return = 'false';
     ELSE
     SET @return = 'true';

     RETURN @return;

    END;
    GO

Step 3 : Alter table to add the CHECK constraint

ALTER TABLE RawMaterialByGender 
ADD CONSTRAINT check_LevelMasterID CHECK (dbo.IsValidLevelMasterGender(LeveLMasterID) = 'true')

Answer 3

ALTER TABLE Table1
ADD CONSTRAINT FK_Table1_Code FOREIGN KEY (MyField)
REFERENCES Table2 (Field) ;

Ref: http://msdn.microsoft.com/en-us/library/ms190273.aspx
Note: I haven't checked the above for syntax.

Answer 4

First of all in your example you clearly need FK Constraint.

Another posibility is to use view with WITH CHECK OPTION and give access to user through it:

CREATE TABLE Table1(i INT PRIMARY KEY, CK_Code CHAR(1));
CREATE TABLE Table2(Field CHAR(1));
INSERT INTO Table2(Field) VALUES ('A'),('B'), ('C');
GO

CREATE VIEW v_Table1
AS
SELECT *
FROM Table1
WHERE CK_code IN (SELECT Field FROM Table2)  -- here goes your subquery check
WITH CHECK OPTION;

When you try to insert data that violates your "constraint" like:

INSERT INTO v_Table1(i, CK_Code)
VALUES(10, 'D');

You will get:

The attempted insert or update failed because the target view either specifies WITH CHECK OPTION or spans a view that specifies WITH CHECK OPTION and one or more rows resulting from the operation did not qualify under the CHECK OPTION constraint.

The statement has been terminated.

LiveDemo