Referential integrity checking in table definition

Question

I have a table of STUDENTS which contains student information. Each student might or might not be a CANDIDATE in a college voting event, depending on the value of CANDIDATE BIT.

I'd like to have a table VOTE which keeps the votes of students, but I'd like to check if the student which is being inserted as VOTES_FOR is actually a CANDIDATE or not. In plain english: Check if the person being voted for is actually a candidate or not.

Any idea on how I can check for this constraint? I want to do this task using simple regular DDL commands not stored procedure and PL/SQL or T-SQL

CREATE TABLE STUDENTS(
    ID INT NOT NULL UNIQUE AUTO_INCREMENT = 1,
    FIRST_NAME NVARCHAR(20) NOT NULL ,
    LAST_NAME NVARCHAR(20) NOT NULL,
    CANDIDATE BIT DEFAULT '0',
    PRIMARY KEY(ID)
);    
CREATE TABLE VOTE(
    STUDENT_ID references STUDENTS(ID),
    VOTES_FOR references STUDENTS(ID),
    PRIMARY KEY STUDENT_ID
);

Answer 1

Declare a UNIQUE constraint on the pair of columns "ID" and "CANDIDATE", and reference that pair from the table "VOTES". Use a CHECK constraint in "VOTES" to guarantee that votes are recorded only for a candidate.

CREATE TABLE STUDENTS(
    ID INTEGER PRIMARY KEY,
    FIRST_NAME VARCHAR2(20) NOT NULL,
    LAST_NAME VARCHAR2(20) NOT NULL,
    CANDIDATE CHAR(1) DEFAULT 'f' NOT NULL 
      CHECK (CANDIDATE IN ('t', 'f')),
    UNIQUE (ID, CANDIDATE)
);

CREATE TABLE VOTES(
    STUDENT_ID INTEGER PRIMARY KEY
      REFERENCES STUDENTS (ID),
    VOTES_FOR INTEGER NOT NULL,
    CANDIDATE CHAR(1) DEFAULT 't' 
      CHECK (CANDIDATE = 't'),
    FOREIGN KEY (VOTES_FOR, CANDIDATE)
      REFERENCES STUDENTS (ID, CANDIDATE)
);

"IS_CANDIDATE" is probably a better name than "CANDIDATE"; "CANDIDATE_ID" is probably a better name than "VOTES_FOR".

Answer 2

Right on for seeking a DDL solution for this type of problem.

I'd push towards incorporating a Candidate table. My reasoning is this will allow you to declare unique traits such as start and end candidacy times that would enable a student to try to become a candidate the following year or later.

Also this gives you an ID for that candidate to refer to.

CREATE TABLE student {
    id INTEGER PRIMARY KEY,
    fname VARCHAR(20) NOT NULL,
    lname VARCHAR(20) NOT NULL
};

CREATE TABLE candidate {
    id INTEGER PRIMARY KEY,
    student_id INTEGER(11) NOT NULL,
    start_year INTEGER(4) NOT NULL,
    start_month INTEGER(2) NOT NULL,
    start_day INTEGER(2) NOT NULL,
    end_year INTEGER(4) NULL,
    end_month INTEGER(2) NULL,
    end_day INTEGER(2) NULL
}

CREATE TABLE vote {
    student_id INTEGER(11) NOT NULL,
    candidate_id INTEGER(11) NOT NULL
};

Answer 3

If you're using Oracle and the database you want to look into a Ref Constraint here's an exmample Ref Constraint Example

Although the situation may be a little more complex than Oracle constraints allow for. This article written by Tony Andrews provides an ingenious workaround for achieving your desired results.

Answer 4

Have you tried using the SQL CHECK constraint?

http://www.w3schools.com/sql/sql_check.asp

You're going to need a composite foreign key to provide the check you want.