InnoDB only insert record if referenced id exists (without FOREIGN KEYS)

Question

Foreign keys may be the best approach for this problem. However, I'm trying to learn about table locking/transactions, and so I'm hoping that we can ignore them for the moment.

Let's pretend that I have two tables in an InnoDB database: categories and jokes; and that I'm using PHP/MySQLi to do the work. The tables look like so:

CATEGORIES
id (int, primary, auto_inc)  |  category_name (varchar[64])
============================================================
1                               knock, knock

JOKES
id (int, primary, auto_inc)  |  category_id (int)  | joke_text (varchar[255])
=============================================================================
empty

Here are two functions, each of which is being called by a different connection, at the same time. The calls are: delete_category(1) and add_joke(1,"Interrupting cow. Interrup-MOOOOOOOO!")

function delete_category($category_id) {

    // only delete the category if there are no jokes in it
    $query = "SELECT id FROM jokes WHERE category_id = '$category_id'";
    $result = $conn->query($query);

    if ( !$result->num_rows ) {
        $query = "DELETE FROM categories WHERE id = '$category_id'";
        $result = $conn->query($query);
        if ( $conn->affected_rows ) {
            return true;
        }
    }

    return false;
}

function add_joke($category_id,$joke_text) {

    $new_id = -1;

    // only add the joke if the category exists
    $query = "SELECT id FROM categories WHERE id = '$category_id'";
    $result = $conn->query($query);

    if ( $result->num_rows ) {

        $query = "INSERT INTO jokes (joke_text) VALUES ('$joke_text')";
        $result = $conn->query($query);

        if ( $conn->affected_rows ) {
            $new_id = $conn->insert_id;
            return $new_id;
        }
    }

    return $new_id;
}

Now, if the SELECT statements from both functions execute at the same time, and proceed from there, delete_category will think it's okay to delete the category, and add_joke will think it's okay to add the joke to the existing category, so I'll get an empty categories table and an entry in the joke table that references a non-existent category_id.

Without using foreign keys, how would you solve this problem?

My best thought so far would be to do the following:

1) "LOCK TABLES categories WRITE, jokes WRITE" at the start of delete_category. However, since I'm using InnoDB, I'm quite keen to avoid locking entire tables (especially main ones that will be used often).

2) Making add_joke a transaction and then doing "SELECT id FROM categories WHERE id = '$category_id'" after inserting the record as well. If it doesn't exist at that point, rollback the transaction. However, since the two SELECT statements in add_joke might return different results, I believe I need to look into transaction isolation levels, which I'm not familiar with.

It seems to me that if I did both of those things, it should work as expected. Nevertheless, I'm keen to hear more informed opinions. Thanks.

Answer 1

You can DELETE a category only if is no matching joke:

DELETE c FROM categories AS c
LEFT OUTER JOIN jokes AS j ON c.id=j.category_id
WHERE c.id = $category_id AND j.category_id IS NULL;

If there are any jokes for the category, the join will find them, and therefore the outer join will return a non-null result. The condition in the WHERE clause eliminates non-null results, so the overall delete will match zero rows.

Likewise, you can INSERT a joke to a category only if the category exists:

INSERT INTO jokes (category_id, joke_text)
SELECT c.id, '$joke_text'
FROM categories AS c WHERE c.id = $category_id;

If there is no such category, the SELECT returns zero rows, and the INSERT is a no-op.

Both of these cases create a shared lock (S-lock) on the categories table.

Demonstration of an S-lock:

In one session I run:

mysql> INSERT INTO bar (i) SELECT SLEEP(600) FROM foo;

In second session I run:

mysql> SHOW ENGINE INNODB STATUS\G
. . .
---TRANSACTION 3849, ACTIVE 1 sec
mysql tables in use 2, locked 2
2 lock struct(s), heap size 376, 1 row lock(s)
MySQL thread id 18, OS thread handle 0x7faefe7d1700, query id 203 192.168.56.1 root User sleep
insert into bar (i) select sleep(600) from foo
TABLE LOCK table `test`.`foo` trx id 3849 lock mode IS
RECORD LOCKS space id 22 page no 3 n bits 72 index `GEN_CLUST_INDEX` of table `test`.`foo` trx id 3849 lock mode S

You can see that this creates an IS-lock on the table foo, and an S-lock on one row of foo, the table I'm reading from.

The same thing happens for any hybrid read/write operations such as SELECT...FOR UPDATE, INSERT...SELECT, CREATE TABLE...SELECT, to block the rows being read from being modified while they are needed as a source for the write operation.

The IS-lock is a table-level lock that prevents DDL operations on the table, so no one issues DROP TABLE or ALTER TABLE while this transaction is depending on some content in the table.