Question

Two tables share a unique identifier ‘id’. Both tables are meant to be joined by using ‘id’.
Defining ‘id’ as an auto incrementing primary key in both tables may risk update inconsistencies.

Is there some general pattern to avoid such a situation or do I have to deal with updating table1 first and table2 by utilizing the last inserted id after (therefore not declaring id as auto inc in table2)?

Answer 1

First, if you use InnoDB table engine in MySQL you could use both transactions and foreign keys for data consistency.

Second, after the insert in the first table, you could get the last insert id (depending on the way you access the db) and use it as foreign key.

Eg
Table 1: Users: user_id, username
Table 2: User_Profiles: user_id, name, phone

In User_Profiles you don’t need to define user_id as auto increment, but first insert a record in Users table and use the user_id for the User_Profiles record. If you do this in transaction, the Users record won’t be seen outside of the transaction connection until it’s completed, this way you guarantee that even if something bad happens after you insert the user, but before you have inserted the profile – there won’t be messed up data.

You could also define that the user_id column in User_Profiles table is foreign key of Users table thus if someone deletes a record from the Users table, the database would automatically delete the one in User_Profiles. There are many other options – read more about that.