Question

Can you help me with this problem? I have two tables in a MySQL database:

ServiceProvider(SPID, Name, ... etc.)
hasTag(SPID, TagID)

Each service provider is supposed to have at least one tag, and a maximum of five tags. The max-constraint is not a problem, but the min-constraint refuses to work properly. I first tried to implement this via assertions, but then I found out, that MySQL does not support assertions. Thus, I wrote the following trigger:

delimiter |

CREATE  TRIGGER MinTags BEFORE INSERT
        ON ServiceProvider
        FOR EACH ROW BEGIN
            IF EXISTS (SELECT SPID FROM ServiceProvider
                       WHERE NOT EXISTS (SELECT DISTINCT SPID FROM hasTag))
            THEN INSERT INTO stop_action VALUES(1, 'Assert Failure');
            END IF;
        END;
|

delimiter ;

The insert of 'Assert Failure' into the stop_action table is only to create a constraint violation, so that the DB would abort the action.

Now, normally, when I insert any value into the ServiceProvider table, without inserting anything in to the hasTag table, I should get an error, right? But, somehow it doesn’t work … I can insert anything I want into the ServiceProvider table, without receiving any kind of error. Do you know, what is wrong with my code?

Answer 1

How about denormalising a tad:

ALTER TABLE ServiceProvider
  ADD COLUMN TagID1 BIGINT UNSIGNED NOT NULL,
  ADD COLUMN TagID2 BIGINT UNSIGNED NULL,
  ADD COLUMN TagID3 BIGINT UNSIGNED NULL,
  ADD COLUMN TagID4 BIGINT UNSIGNED NULL,
  ADD COLUMN TagID5 BIGINT UNSIGNED NULL;

Include foreign key constraints, if appropriate.