There are two types of schema - logical and physical. We focus on logical schema, before briefly explaining physical schema.
A logical schema is like an architect's blueprint of our database. It contains all the information required to configure it. In our schema we will include:
- names of tables
- names of the fields within those tables
- relationships between fields in different tables
It is important to carefully plan out your logical schema before creating a database as it is difficult to change once the database is in use.
The physical schema is the structure and layout of the database in practice - how it's actually stored.
A primary key is a number which the database uses to link one table with another. It is a unique, autoincrementing ID which is filled in by the database - in other words it is NEVER NULL. A primary ID number will only ever be issued once; if, for example, you delete the latest record with an ID of 7 in your table FAC, and then make a new record, the new record's ID will become 8. This is really useful! If we need to refer to a record in separate table, we can import this ID as a foreign key, to be sure we are referring to the actual record.
