Question

I have three tables, table A has a unique primary key that is auto-incremented, and the other two (table B and C) have primary keys that have a foreign key constraint that ties them to the first tables primary key.

I want to make a constraint that maintains that for all rows in the second and third tables they cannot contain any duplicate, and for all records in table A there is a matching record in B or C.

So basically a record of type A can be a type B or C and must be one of B or C.

Is there away to make this constraint without triggers in MySQL? or are triggers necessary?

Thanks for any help.

Answer 1

You can use a “type” table:

CREATE TABLE Type
  ( type_code CHAR(1) NOT NULL
  , PRIMARY KEY (type_code)
  ) ;

with exactly 2 rows (as many as the different subtype tables you need:

INSERT INTO Type (type_code)
VALUES ('B'), ('C') ;

The supertype table (that includes a column that references “Type”):

CREATE TABLE A
  ( a_id INT NOT NULL AUTO_INCREMENT
  , type_code CHAR(1) NOT NULL
  , PRIMARY KEY (a_id)
  , UNIQUE KEY (type_code, a_id)
  , FOREIGN KEY (type_code)
      REFERENCES Type (type_code)
  ) ;

The subtype tables (that are now referencing the combination of A’s Primary key and type_code:

CREATE TABLE B
  ( a_id INT NOT NULL
  , type_code CHAR(1) NOT NULL DEFAULT 'B'
  , PRIMARY KEY (type_code, a_id)
  , FOREIGN KEY (type_code, a_id)
      REFERENCES A (type_code, a_id)
  , CHECK (type_code = 'B')
  ) ;

CREATE TABLE C
  ( a_id INT NOT NULL
  , type_code CHAR(1) NOT NULL DEFAULT 'C'
  , PRIMARY KEY (type_code, a_id)
  , FOREIGN KEY (type_code, a_id)
      REFERENCES A (type_code, a_id)
  , CHECK (type_code = 'C')
  ) ;

The above would work fine, if only MySQL had implemeneted CHECK constraints. But it hasn’t. So, to be absolutely sure that all your specifications are enforced, and not 'B' type data is inserted in C table, you’ll have to add 2 more “type” tables (and remove the useless in MySQL CHECK constraints):

CREATE TABLE TypeB
  ( type_code CHAR(1) NOT NULL
  , PRIMARY KEY (type_code)
  ) ;

CREATE TABLE TypeC
  ( type_code CHAR(1) NOT NULL
  , PRIMARY KEY (type_code)
  ) ;

with exactly 1 rows each:

INSERT INTO TypeB (type_code)
VALUES ('B') ;

INSERT INTO TypeC (type_code)
VALUES ('C') ;

and the additional FKs:

ALTER TABLE B
  ADD FOREIGN KEY (type_code)
    REFERENCES TypeB (type_code) ;

ALTER TABLE C
  ADD FOREIGN KEY (type_code)
    REFERENCES TypeC (type_code) ;

With these constraints, every row of table A, will be either of type B or C and it will be in the respective table (B or C) and never in both.

If you also want to ensure that they will be in exactly one table (and never in neither B nor C), that should be taken care when inserting in A (all insertions should be done with a transaction that enforces that requirement).