Question

Would it be possible to create a database-level restriction to prevent creating a row that has a column x INT with a value that already exists and is above 0?

Is there a way to use CONSTRAINT for this purpose?

Answer 1

A possible solution is to do the following:

CREATE TABLE test
(
    id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
    myfield INT,
    CONSTRAINT check_myfield UNIQUE ( myfield )
);

Now, the column myfield might be NULL. So when we do the following, there will be a total of 0 errors.

INSERT INTO `test` VALUES ( '', '1' );
INSERT INTO `test` VALUES ( '', '0' );
INSERT INTO `test` VALUES ( '', '5' );
INSERT INTO `test` VALUES ( '', '7' );
etc, you get the point...

Each and every row has a unique value in the column myfield, but there is still the possibility to create rows where the value in this particular column is NULL which is almost exactly what I wanted. I wanted all values above 0 unique, this is all above NULL. The beauty of this solution is that it feels more ‘professional’, no unnecessary logic.