Joe Celko's SQL for Smarties, 3rd Edition

15.5. EXISTS() and Referential Constraints

Standard SQL was designed so that the declarative referential constraints could be expressed as EXISTS() predicates in a CHECK() clause. For example:

CREATE TABLE Addresses
(addressee_name CHAR(25) NOT NULL PRIMARY KEY,
 street_loc CHAR(25) NOT NULL,
 city_name CHAR(20) NOT NULL,
 state_code CHAR(2) NOT NULL
            REFERENCES ZipCodeData(state_code),
...);

could be written as:

CREATE TABLE Addresses
(addressee_name CHAR(25) NOT NULL PRIMARY KEY,
 street_loc CHAR(25) NOT NULL,
 city_name CHAR(20) NOT NULL,
 state_code CHAR(2) NOT NULL,
 CONSTRAINT valid_state_code
  CHECK (EXISTS(SELECT *
                  FROM ZipCodeData AS Z1
                 WHERE Z1.state_code = Addresses.state_code)),
...);

There is no advantage to this expression for the DBA, ...