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')