MySql: Why foreign key is null? - mysql

Question

I have 2 tables. users and post. I want to use users(id) column (which is pk) as foreign key in post(user_id) column. I used:
ALTER TABLE post ADD FOREIGN KEY (user_id) REFERENCES users(id);
and that action succeeded without any errors but I see only null in (user_id) column. Do I miss something or what? Shouldn't it copy the id values from users table?

Answer 1

Q: Why foreign key is null?
A: Whether or not a column can contain a NULL value is determined by the presence or absence of a NOT NULL constraint. This is entirely independent of whether the column is referenced in a foreign key constraint.
The value in the column is NULL because that's the value that was assigned when the row was inserted. The value was assigned, whether it was explicitly set, or whether it was derived from the default value for the column. (If the column was added to an existing table, then the values in the new column was the default value for the column.)
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Q: Do I miss something or what?
A: The behavior and results you observe are exactly as we expect.
Q: Shouldn't it copy the id values from users table?
A: If you're asking if MySQL should automatically populate the user_id column in the post table, the answer to that question is no, it shouldn't.
I think maybe you've clued in on a key idea:
The "relationship" between a row in one table to a row in another table is represented in the relational database by storing a common value.
But the database doesn't know which row is related to which row. You have to tell it. You have to provide that information.
When you insert a row into the post table, you can provide a value for the user_id column. You would provide a value that's equal to the id value of some row in user.
The idea with a FOREIGN KEY constraint is that it's restriction. It only allows valid values. It prevents an invalid value from being stored. (That's true with InnoDB if FOREIGN_KEY_CHECKS=1, that's not true for MyISAM, because MyISAM doesn't enforce foreign key constraints.)
The foreign key is saying that you want to "constrain" the values that can be stored. It's saying that it's not going to allow rows in post to have have user_id values that point to a "missing" row in the users table.
It's perfectly acceptable to store a NULL in a foreign key column. When a NULL value is stored, that's saying that the row is not related to a row in the users table.
Disallowing NULL values in a column is done with a different kind of constraint, a NOT NULL constraint.
It's possible to define both a foreign key constraint and a NOT NULL constraint on the same column. That's a design decision, whether you want to allow NULL values or not. In some cases, we may want to disallow NULL values in a foreign key. For example, if we were to add a NOT NULL constraint on the user_id column of post, that would effectively be saying that a row cannot exist in post if it's not related to a row in users. And that's a very common pattern.

Answer 2

Adding foreign key to any column in MySQL does not copy any values.