Just a Theory

By David E. Wheeler

Enforce Foreign Key Integrity in SQLite with Triggers

After some some Googling and experimentation, I’ve figured out how to enforce foreign key constraints in SQLite. I got most of the code from Cody Pisto’s sqlite_fk utility. I couldn’t get it to work, but the essential code for the triggers was in its fk.c file, so I just borrowed from that (public domain) code to figure it out.

Since I couldn’t find documentation for this elsewhere on the Net (though I’m sure it exists somewhere), I decided to just put an example here. Interested? Read on!

Say you have these two table declarations:

create table foo (
    id INTEGER NOT NULL PRIMARY KEY
);

CREATE TABLE bar (
    id INTEGER NOT NULL PRIMARY KEY,
    foo_id INTEGER NOT NULL
            CONSTRAINT fk_foo_id REFERENCES a(id) ON DELETE CASCADE
);

Table bar has a foreign key reference to the primary key column in the foo table. Although SQLite supports this syntax (as well as named foreign key constraints), it ignores them. So if you want the references enforced, you need to create triggers to do the job. Triggers were added to SQLite version 2.5, so most users can take advantage of this feature. Each constraint must have three triggers: one for INSERTs, one for UPDATESs, and one for DELETESs. The INSERT trigger looks like this:

CREATE TRIGGER fki_bar_foo_id
BEFORE INSERT ON bar
FOR EACH ROW BEGIN 
    SELECT CASE
        WHEN ((SELECT id FROM foo WHERE id = NEW.foo_id) IS NULL)
        THEN RAISE(ABORT, 'insert on table "bar" violates foreign key '
                || 'constraint "fk_foo_id"')
    END;
END;

(You can put the RAISE error string all on one line; I’ve concatenated two lines to keep line lengths reasonable here.) If your foreign key column is not NOT NULL, the trigger’s SELECT CASE clause needs to an extra case:

CREATE TRIGGER fki_bar_foo_id
BEFORE INSERT ON bar
FOR EACH ROW BEGIN 
    SELECT CASE
        WHEN ((new.foo_id IS NOT NULL)
            AND ((SELECT id FROM foo WHERE id = new.foo_id) IS NULL))
        THEN RAISE(ABORT, 'insert on table "bar" violates foreign key '
                || 'constraint "fk_foo_id"')
    END;
END;

The UPDATE statements are almost identical; if your foreign key column is NOT NULL, then do this:

CREATE TRIGGER fku_bar_foo_id
BEFORE UPDATE ON bar
FOR EACH ROW BEGIN 
    SELECT CASE
        WHEN ((SELECT id FROM foo WHERE id = new.foo_id) IS NULL))
        THEN RAISE(ABORT, 'update on table "bar" violates foreign key '
                || 'constraint "fk_foo_id"')
    END;
END;

And if NULLs are allowed, do this:

CREATE TRIGGER fku_bar_foo_id
BEFORE UPDATE ON bar
FOR EACH ROW BEGIN 
    SELECT CASE
        WHEN ((new.foo_id IS NOT NULL)
            AND ((SELECT id FROM foo WHERE id = new.foo_id) IS NULL))
        THEN RAISE(ABORT, 'update on table "bar" violates foreign key '
                || 'constraint "fk_foo_id"')
    END;
END;

The DELETE trigger is, of course, the reverse of the INSERT and UPDATE triggers, in that it applies to the primary key table, rather than the foreign key table. To whit, in our example, it watches for DELETEs on the foo table:

CREATE TRIGGER fkd_bar_foo_id
BEFORE DELETE ON foo
FOR EACH ROW BEGIN 
    SELECT CASE
    WHEN ((SELECT foo_id FROM bar WHERE foo_id = OLD.id) IS NOT NULL)
    THEN RAISE(ABORT, 'delete on table "foo" violates foreign key '
                || ' constraint "fk_foo_id"')
    END;
END;

This trigger will prevent DELETEs on the foo table when there are existing foreign key references in the bar table. This is generally the default behavior in databases with referential integrity enforcement, sometimes specified explicitly as ON DELETE RESTRICT. But sometimes you want the deletes in the primary key table to “cascade” to the foreign key tables. Such is what our example declaration above specifies, and this is the trigger to to the job:

CREATE TRIGGER fkd_bar_foo_id
BEFORE DELETE ON foo
FOR EACH ROW BEGIN 
    DELETE from bar WHERE foo_id = OLD.id;
END;

Pretty simple, eh? The trigger support in SQLite is great for building your own referential integrity checks. Hopefully, these examples will get you started down the path of creating your own.

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