Mysql : How to check uniqueness of pair

Question

   CREATE TABLE nodes (
        id INTEGER PRIMARY KEY,
        name VARCHAR(10) NOT NULL,
        feat1 CHAR(1), -- e.g., age
        feat2 CHAR(1)  -- e.g., school attended or company
   );

   CREATE TABLE edges (
        a INTEGER NOT NULL REFERENCES nodes(id) ON UPDATE CASCADE ON DELETE CASCADE,
        b INTEGER NOT NULL REFERENCES nodes(id) ON UPDATE CASCADE ON DELETE CASCADE,
        PRIMARY KEY (a, b)
   );

   CREATE INDEX a_idx ON edges (a);
   CREATE INDEX b_idx ON edges (b);

If we want to represent an undirected graph, we need to add a CHECK constraint on the uniqueness of the pair.

Since the SQL standard does not allow a subquery in the CHECK constraint,How can i check uniqueness of the pair?

Answer 1

You could rig up a trigger that fails upon seeing either (A,B) or (B,A) :

Here is the Trigger:

DELIMITER $$
CREATE TRIGGER edges_bi BEFORE INSERT
ON edges FOR EACH ROW
BEGIN
    DECLARE found_count,dummy,diff,SomethingsWrong INT DEFAULT 0;
    DECLARE errmsg VARCHAR(128);
    SET diff = new.a - new.b;
    IF diff = 0 THEN
        SET errmsg = CONCAT('[',new.a,',',new.b,'] is Vertex, Not Edge');
        SET SomethingsWrong = 1;
    END IF;
    SELECT COUNT(1) INTO found_count FROM edges
    WHERE (a=NEW.a AND b=NEW.b) OR (a=NEW.b AND b=NEW.a);
    IF found_count = 1 THEN
        SET errmsg = CONCAT('[',new.a,',',new.b,'] Already Exists');
        SET SomethingsWrong = 1;
    END IF;
    IF SomethingsWrong = 1 THEN
        SELECT errmsg INTO dummy FROM edges WHERE 1=1;
    END IF;
END; $$
DELIMITER ;

Here is a sample table:

DROP DATABASE if exists saurabh;
CREATE DATABASE saurabh;
USE saurabh
CREATE TABLE edges
(
   a INTEGER NOT NULL,
   b INTEGER NOT NULL,
   PRIMARY KEY (a,b),
   UNIQUE KEY (b,a)
);

Notice that I have a PRIMARY KEY and a UNIQUE KEY with the columns of the PRIMARY KEY reversed

Let's create the table:

mysql> DROP DATABASE if exists saurabh;
Query OK, 1 row affected (0.01 sec)

mysql> CREATE DATABASE saurabh;
Query OK, 1 row affected (0.00 sec)

mysql> USE saurabh
Database changed
mysql> CREATE TABLE edges
    -> (
    ->    a INTEGER NOT NULL,
    ->    b INTEGER NOT NULL,
    ->    PRIMARY KEY (a,b),
    ->    UNIQUE KEY (b,a)
    -> );
Query OK, 0 rows affected (0.12 sec)

mysql>

Let's create the Trigger:

mysql> DELIMITER $$
mysql> CREATE TRIGGER edges_bi BEFORE INSERT
    -> ON edges FOR EACH ROW
    -> BEGIN
    ->     DECLARE found_count,dummy,diff,SomethingsWrong INT DEFAULT 0;
    ->     DECLARE errmsg VARCHAR(128);
    ->     SET diff = new.a - new.b;
    ->     IF diff = 0 THEN
    ->         SET errmsg = CONCAT('[',new.a,',',new.b,'] is Vertex, Not Edge');
    ->         SET SomethingsWrong = 1;
    ->     END IF;
    ->     SELECT COUNT(1) INTO found_count FROM edges
    ->     WHERE (a=NEW.a AND b=NEW.b) OR (a=NEW.b AND b=NEW.a);
    ->     IF found_count = 1 THEN
    ->         SET errmsg = CONCAT('[',new.a,',',new.b,'] Already Exists');
    ->         SET SomethingsWrong = 1;
    ->     END IF;
    ->     IF SomethingsWrong = 1 THEN
    ->         SELECT errmsg INTO dummy FROM edges WHERE 1=1;
    ->     END IF;
    -> END; $$
Query OK, 0 rows affected (0.11 sec)

mysql> DELIMITER ;

Here is some sample data:

INSERT INTO edges (a,b) VALUES (5,3);
INSERT INTO edges (a,b) VALUES (3,3);
INSERT INTO edges (a,b) VALUES (3,5);
INSERT INTO edges (a,b) VALUES (5,5);
SELECT * FROM edges;

Let's try loading these into the edges table:

mysql> INSERT INTO edges (a,b) VALUES (5,3);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO edges (a,b) VALUES (3,3);
ERROR 1366 (HY000): Incorrect integer value: '[3,3] is Vertex, Not Edge' for column 'dummy' at row 1
mysql> INSERT INTO edges (a,b) VALUES (3,5);
ERROR 1366 (HY000): Incorrect integer value: '[3,5] Already Exists' for column 'dummy' at row 1
mysql> INSERT INTO edges (a,b) VALUES (5,5);
ERROR 1366 (HY000): Incorrect integer value: '[5,5] is Vertex, Not Edge' for column 'dummy' at row 1
mysql> SELECT * FROM edges;
+---+---+
| a | b |
+---+---+
| 5 | 3 |
+---+---+
1 row in set (0.00 sec)

Note that blocking A=B conditions prevents any self-loops

CAVEAT

This trigger does not work if

• you start with an empty table
• enter (3,3) as the first row

because the BEFORE INSERT trigger does not fire on an empty table.

Once you enter a valid row with A <> B then all checks are performed properly.

Give it a Try !!!

Answer 2

MySQL does not support CHECK constraints.

You can create a BEFORE INSERT and BEFORE UPDATE triggers to check this situation, and throw an error if needed.

Example:

CREATE TABLE edges(
  a INT(11) NOT NULL,
  b INT(11) NOT NULL
);

DELIMITER $$

CREATE TRIGGER trigger1
BEFORE INSERT
ON edges
FOR EACH ROW
BEGIN
  SET @cnt = NULL;

  SELECT COUNT(*) INTO @cnt FROM edges
    WHERE a = new.a AND b = new.b OR a = new.b AND b = new.a;

  IF @cnt > 0 THEN
    SIGNAL SQLSTATE '02000' SET MESSAGE_TEXT = 'Error: uniqueness of pair';
  END IF;
END
$$

DELIMITER ;

Also, create similar BEFORE UPDATE trigger to avoid NEW wrong values on updating, or just use a stored procedure because the code is the same.

Answer 3

CHECK is not supported on CREATE TABLE by MySQL, as the documentation dictates

The CHECK clause is parsed but ignored by all storage engines

In fact there is an open bug report on that issue since 2004 (!).

The approach I would take is to create a stored procedure trigger on insert and on update that deliberate fails if the pair exists.