Question

Possible Duplicate:
Should I use foreign keys?

Ok, let’s assume we have two tables, users and comments. In comments we have a column comment_made_by_user_id and it means which user typed that particular comment. Why do we need to specify it as a foreign key? If we don’t do that, it will still work. We specify primary keys, because it makes queries faster as far as I know (we need to search only for one row while when we don’t have a primary key/index, we have to go through all rows). Is it just a good coding practise?

Answer 1

When you use foreign keys you get:

1. Data integrity
2. faster queries.

users:
user id:

• 1
• 2
• 3

Comments:
user:

• 1
• 2
• 4 XXX invalid as 4 isn’t in the users table.

Read Wikipedia please for more details about Data integrity