Automatically update MySQL association table that has an auto increment foreign key

Question

I have an application where a particular event updates my database with INSERT s to two tables A and B . There is an additional association/link table AB with foreign keys id_A , id_B to A and B that maps their many-to-many relationship. My question is how to best update AB when one of the foreign keys isn't known to the application logic (in this case because it is an AUTO_INCREMENT ID).

Additional details:

• Only id_B is set to AUTO_INCREMENT , so its value is not known before the insert.
• B is updated with a multi-row INSERT ; A is a single row INSERT .
• id_B is a unique key but there are possible duplicates which are handled by an ON DUPLICATE KEY UPDATE which replaces the old values with new ones.
• A and B are updated in the same transaction in response to the same event.
• MySQL 5.7 using InnoDB.

My idea was to include all the insert statements in a stored procedure where the final INSERT to AB would get the values for id_B from LAST_INSERT_ID() but apparently this will only return the value for the first row in a multi-row insert: https://dev.mysql.com/doc/refman/5.7/en/information-functions.html#function_last-insert-id

I suppose a bonus would be if the logic to update AB could be separated out into a trigger or a different statement from what is being used to update the parent tables.

Edit: response to comments.

Answer 1

Before the statement(s) used to insert your data into A and B place the following statement...

SET @before = ( SELECT MAX( id_B ) AS maxIDB
                FROM B );

This will find the largest value of id_B in B at this moment. All subsequent insertions into B will have a larger value than this. We can use this fact to our advantage.

After the above mentioned insert statement place the following statement...

INSERT INTO AB ( id_A,
                 id_B )
SELECT id_A_value,
       B.id_B
FROM B
WHERE B.id_B > @maxIDB;

Note : By your above question I am assuming that id_A is known. I am representing this value with id_A_value .

This statement will insert into AB rows containing the known value of id_A and all of the freshly generated values of id_B .

If you have any questions or comments, then please feel free to post a Comment accordingly.