如何使用外键引用创建检查约束

Question

I am creating a database with these two tables in H2:

CREATE TABLE INSTANCE
( 
     INSTANCE_ID BIGINT AUTO_INCREMENT PRIMARY KEY, 
     DATE DATE, 
     TIME TIME, 
     LOCATION VARCHAR(255), 
     PRICE INT, 
     CAPACITY INT, 
     EVENT_ID INT
);

CREATE TABLE RESERVATION
( 
     RESERVATION_ID BIGINT AUTO_INCREMENT PRIMARY KEY, 
     RESERVATION_ORDER INT NOT NULL, 
     INSTANCE_ID INT NOT NULL, 
     USER_ID INT NOT NULL
);

So for example there is an instance with capacity of 20.

Reservation order increments with each reservation to this instance.

So for example like this:

INSERT INTO RESERVATION VALUES (1,1,1,1);
INSERT INTO RESERVATION VALUES (2,2,1,3);
INSERT INTO RESERVATION VALUES (3,3,1,4);
INSERT INTO RESERVATION VALUES (4,4,1,5);
INSERT INTO RESERVATION VALUES (5,1,3,2);
INSERT INTO RESERVATION VALUES (6,2,3,5);
INSERT INTO RESERVATION VALUES (7,1,6,6);

My problem is, that I need to create check constraint that

INSTANCE.CAPACITY >= RESERVATION_ORDER 

so I cannot add another reservation, when there is already 20 of them made to that instance, but I have no idea how to do that. I used SQL only a few times in my life and I have a hard time referencing other tables with a foreign key.

I tried adding something like this when creating the table with no success

CHECK (RESERVATION_ORDER =< (SELECT CAPACITY FROM INSTANCE WHERE (INSTANCE_ID = INSTANCE_ID)))

Answer 1

In SQL, a check constraint can only refer to columns in the same row or user-defined functions.

You may be able to do what you want by defining a user-defined functions. Or you can define a trigger to enforce the constraint.

You cannot, however, do so with a simple check constraint.

The most common way to tackle this problem is the following:

1. Add a column to reservations which is the current number of active instances.
2. Add insert/update/delete triggers to instances and increment/decrement the aforementioned column appropriately.
3. Add a check constraint in reservervations that compares the calculated value to the computed count.

This check constraint will fail.

I prefer this approach over using UDFs in a check constraint because the count is easy to see and verify.

Answer 2

You can define UDF and call that UDF on RESERVATION table. Something like below :

CREATE FUNCTION dbo.CheckInstanceCapacity (@event_id int, @capacity int)
RETURNS int
AS 
BEGIN
  DECLARE @value int
    SELECT @value = CASE WHEN CAPACITY >= @capacity THEN 0 ELSE 1 END
    FROM INSTANCE
    WHERE EVENT_ID = @event_id 
  RETURN @value
END;

ALTER TABLE RESERVATION ADD CONSTRAINT InsCapacity 
CHECK (dbo.CheckInstanceCapacity(EVENT_ID, RESERVATION_ORDER) = 0); 

Answer 3

You need to add table names or aliases to distinguish INSTANCE_ID column from table RESERVATION from the same column from the table INSTANCE in your check constraint. You also need a foreign key constraint.

ALTER TABLE RESERVATION ADD
    FOREIGN KEY (INSTANCE_ID) REFERENCES INSTANCE(INSTANCE_ID);

ALTER TABLE RESERVATION ADD
    CHECK(RESERVATION_ORDER <=
        (SELECT CAPACITY FROM INSTANCE I
            WHERE (RESERVATION.INSTANCE_ID = I.INSTANCE_ID)));

I think you should also check that RESERVATION_ORDER is greater than or equal to 1 by replacing RESERVATION_ORDER <= (SELECT … with RESERVATION_ORDER BETWEEN 1 AND (SELECT … .

Most likely you also need a UNIQUE constraint to prevent duplicate rows in RESERVATION with the same INSTANCE_ID and RESERVATION_ORDER values.

ALTER TABLE RESERVATION ADD UNIQUE(INSTANCE_ID, RESERVATION_ORDER);

Please note that FOREIGN KEY and UNIQUE constraints are always ensured, but CHECK constraints with references to other tables in H2 are validated only when you insert or modify the rows with the constraint. CHECK constraints in H2 aren't re-validated when tables referenced from its subqueries are modified. So you should either not decrease CAPACITY in existing rows in INSTANCE table, or perform some additional validation when you do that, for example, in a trigger.