How should I handle updating data in a one-to-many relationship database schema?

Question

Lets say I have a database similar to the following:

Table: People

id | name | age
---+------+-----
 1 | dave | 78

Table: Likes

id | userid | like
---+--------+-------
 1 |    1   | apples
---+--------+-------
 2 |    1   | oranges
---+--------+-------
 3 |    1   | women

What would be the best way to handle updating daves data? Currently, I do the following:

Update his age
UPDATE People SET age = 79 WHERE id = 1;

Update his likes
DELETE FROM Likes WHERE userid = 1;
INSERT INTO LIKES (userid, like) VALUES (1, 'new like');
INSERT INTO LIKES (userid, like) VALUES (1, 'another like');

I delete all the users data from the table and then readd their new stuff. This seems inefficient. Is there a better way?

Answer 1

It's not clear to me why you are suggesting a link between updating a record in the parent table and its dependents in the child table. The point of having separate tables is precisely that we can modify the non-key columns in People without touching Likes.

When it comes to updating Likes there are two different business transactions. The first is when Dave says, "I didn't mean 'oranges' I meant to say I like flower arranging". Correcting a mistake would use an update:

update likes
set like = 'flower arranging'
where userid = 1
and like = 'oranges'
/

The WHERE clause could use the LIKES.ID column instead.

The other case is where the preferences have actually changed. That is, when Dave says "Now I'm 79 I don't like women any more. I have new tastes.". This might look like this:

delete from likes
where userid = 1
and like = 'women'
/
insert into likes (userid, like) 
values (1, 'dominoes')
/
insert into likes (userid, like) 
values (1, 'Werthers Orignals')
/

The difference between these two statements is primarily one of clarity. We could have implemented the second set of statements as an update and a single insert but that would be misleading. Keeping the distinction between meaningful changes to the data and correcting mistakes is a useful discipline. It is especially helpful when we are keeping historical records and/or auditing changes.

What is definitely a bad idea is deleting all Dave's Likes records and then re-inserting them. Your application should be able to track which records have changed.