The structure of stored data.
A TABLE
Holds one kind of thing. A posts table holds posts; a users table holds users.
Each table has a defined set of columns.
A COLUMN
One piece of information, with a type: a number, text, a date.
The type determines what can be stored. A column defined for short text rejects a long value.
A ROW
One record. One post, one user, one order.
THE PRIMARY KEY
A column, usually called id, uniquely identifying each row. Almost always a number that increments automatically.
Other tables reference rows by this id rather than duplicating the data.
HOW TABLES RELATE
An orders table stores a customer id rather than the customer's full details.
That is why deleting a row can break things elsewhere: something may still reference it.
WHY THIS MATTERS PRACTICALLY
When you edit data directly, you are editing rows. Understanding that a row may be referenced elsewhere is what prevents breaking the site.
