MySQL Unique Constraint – How to Apply Unique Constraint Over Two Columns

You may want to try creating a trigger that will check for the presence of (col1,col2) existing as (col2,col1)

Here is an example:

use test
drop table if exists ali;
create table ali
(
    col1 int not null,
    col2 int not null,
    primary key (col1,col2)
);
DELIMITER $$
CREATE TRIGGER ali_bi BEFORE INSERT ON ali FOR EACH ROW 
BEGIN 
    DECLARE found_count,newcol1,newcol2,dummy INT;
    SET newcol1 = NEW.col1;
    SET newcol2 = NEW.col2;
    SELECT COUNT(1) INTO found_count FROM ali
    WHERE col1 = newcol2 AND col2 = newcol1;
    IF found_count = 1 THEN
        SELECT 1 INTO dummy FROM information_schema.tables;
    END IF;
END; $$ 
DELIMITER ;
INSERT INTO ali VALUES (1,2);
INSERT INTO ali VALUES (3,4);
INSERT INTO ali VALUES (2,1);
INSERT INTO ali VALUES (4,3);
SELECT * FROM ali;

The trigger is designed to break on purpose when FOUND_COUNT is 1.

Here is the sample executed:

mysql> use test
Database changed
mysql> drop table if exists ali;
Query OK, 0 rows affected (0.03 sec)

mysql> create table ali
    -> (
    ->     col1 int not null,
    ->     col2 int not null,
    ->     primary key (col1,col2)
    -> );
Query OK, 0 rows affected (0.06 sec)

mysql> DELIMITER $$
mysql> CREATE TRIGGER ali_bi BEFORE INSERT ON ali FOR EACH ROW
    -> BEGIN
    ->     DECLARE found_count,newcol1,newcol2,dummy INT;
    ->     SET newcol1 = NEW.col1;
    ->     SET newcol2 = NEW.col2;
    ->     SELECT COUNT(1) INTO found_count FROM ali
    ->     WHERE col1 = newcol2 AND col2 = newcol1;
    ->     IF found_count = 1 THEN
    ->         SELECT 1 INTO dummy FROM information_schema.tables;
    ->     END IF;
    -> END; $$
Query OK, 0 rows affected (0.07 sec)

mysql> DELIMITER ;
mysql> INSERT INTO ali VALUES (1,2);
Query OK, 1 row affected (0.07 sec)

mysql> INSERT INTO ali VALUES (3,4);
Query OK, 1 row affected (0.06 sec)

mysql> INSERT INTO ali VALUES (2,1);
ERROR 1172 (42000): Result consisted of more than one row
mysql> INSERT INTO ali VALUES (4,3);
ERROR 1172 (42000): Result consisted of more than one row
mysql> SELECT * FROM ali;
+------+------+
| col1 | col2 |
+------+------+
|    1 |    2 |
|    3 |    4 |
+------+------+
2 rows in set (0.00 sec)

mysql>

Give it a Try !!!

CAVEAT This does not work on bulk INSERTs. Only when inserting one row at a time.

I have used this technique and suggested it in other DBA StackExchange Questions

• check constraint does not work?
• Trigger in MySQL to prevent insertion

UPDATE 2012-02-29 11:46 EDT

I tried to INSERT the same four rows again.

INSERT INTO ali VALUES (1,2);
INSERT INTO ali VALUES (3,4);
INSERT INTO ali VALUES (2,1);
INSERT INTO ali VALUES (4,3);
SELECT * FROM ali;

Here is what I get

mysql> INSERT INTO ali VALUES (1,2);
ERROR 1062 (23000): Duplicate entry '1-2' for key 'PRIMARY'
mysql> INSERT INTO ali VALUES (3,4);
ERROR 1062 (23000): Duplicate entry '3-4' for key 'PRIMARY'
mysql> INSERT INTO ali VALUES (2,1);
ERROR 1172 (42000): Result consisted of more than one row
mysql> INSERT INTO ali VALUES (4,3);
ERROR 1172 (42000): Result consisted of more than one row
mysql> SELECT * FROM ali;
+------+------+
| col1 | col2 |
+------+------+
|    1 |    2 |
|    3 |    4 |
+------+------+
2 rows in set (0.00 sec)

mysql>

This trigger approach work well despite the quirky message.

I have modified ypercube's solution a little bit, so that both source and destination are not null, as is guaranteed by the original design. My CHECK constraints are commented out, as they apparently will not work in MySQL. I kept them as comments because they document my intent, and they will work on other RDBMS.

CREATE TABLE PointType
( PointTypeID tinyint unsigned NOT NULL,
  TypeDescription CHAR(20) NOT NULL,
  PRIMARY KEY (PointTypeID),
  UNIQUE (TypeDescription)
) ;

INSERT INTO PointType 
  (PointTypeID, TypeDescription)
VALUES
  (1, 'Source'), 
  (2, 'Destination') ;

CREATE TABLE PointUsageQuota
( RouteID int unsigned NOT NULL,
  PointTypeID tinyint unsigned NOT NULL,
  PointID smallint(5) unsigned NOT NULL,
  PRIMARY KEY (PointID),     -- and this is what all the fuss is about
  UNIQUE (RouteID , PointID, PointTypeID), -- target for the foreign keys
  FOREIGN KEY (PointTypeID)
    REFERENCES PointType (PointTypeID)
) ;

CREATE TABLE Route
( RouteID int unsigned NOT NULL,
  SourcePointID smallint(5) unsigned NOT NULL,
  SourceTypeID tinyint unsigned NOT NULL, 
      -- CHECK(SourceTypeID = 1),
  FOREIGN KEY (RouteID , SourcePointID, SourceTypeID) 
    REFERENCES PointUsageQuota (RouteID , PointID, PointTypeID) ,
  DestinationPointID smallint(5) unsigned NOT NULL,
  DestinationTypeID tinyint unsigned NOT NULL, 
      -- CHECK(DestinationTypeID = 2),
  FOREIGN KEY (RouteID , DestinationPointID, DestinationTypeID) 
    REFERENCES PointUsageQuota (RouteID , PointID, PointTypeID) ,
  --  other fields
  PRIMARY KEY (RouteID)
) ;

Testing:

insert into PointUsageQuota
values(1,1,666),(1,2,354);

INSERT INTO Route VALUES (1, 666, 1, 354, 2);
-- this fails:
INSERT INTO Route VALUES (2, 666, 1, 354, 2);
-- this fails too:
INSERT INTO Route VALUES (2, 354, 1, 666, 2);