Question

I tried to add a Check Constraint and I have only failed so far. What would be the way to get around this:

Msg 1046, Level 15, State 1, Line 6

Subqueries are not allowed in this context. Only scalar expressions
are allowed.

This is the code:

ALTER TABLE dbo.PropertySeasonDiscount ADD CONSTRAINT
[CC_PropertySeasonDiscount_MadeFrom_MadeTo]
CHECK (
    (SELECT COUNT(PropertySeasonDiscountId) FROM dbo.PropertySeasonDiscounts apsdeb 
        WHERE 
            (apsdeb.PropertySeasonId = PropertySeasonId) AND
            (
                (apsdeb.ValidForReservationsMadeTo >= ValidForReservationsMadeFrom AND ValidForReservationsMadeFrom >= apsdeb.ValidForReservationsMadeFrom) OR
                (apsdeb.ValidForReservationsMadeFrom <= ValidForReservationsMadeTo AND ValidForReservationsMadeTo <= apsdeb.ValidForReservationsMadeTo)
            )
    ) = 0
);

Answer 1

SQL Server does not currently support subqueries for CHECK CONSTRAINTs.

As you have discovered, there can be trouble with CHECK constraints involving UDFs when attempting to circumvent the subquery limitation.

The alternative constraint implementation strategies are triggered procedural and embedded procedural. The former is preferred because, in common with declarative constraints, they cannot be circumvented.

Implementing a triggered procedural strategy that is well optimized and handles concurrency issues is non-trivial but still doable. I highly recommend the book Applied Mathematics for Database Professionals By Lex de Haan, Toon Koppelaars, chapter 11 (the code examples are Oracle but can be easily ported to SQL Server).