Keys

• Primary key (PK)
• A field (column) or group of fields that is used to identify the row uniquely
• PK is UNIQUE and NOT NULL
• Foreign key (FK)
• A field that uniquely identifies a record in another table
• Refers to the PK of the other table
• Table with the FK: referencing table / child table
• Table with the PK: referenced table / parent table
• A table can have multiple foreign keys
• FK and the referenced PK must MATCH!
• By defining a field as FK, each record in the column must have a reference in the parent table

• PK can be NULL!

• Field constraints

• NOT NULL
• UNIQUE
• PRIMARY KEY (not null + unique)
• CHECK: check a condition when you insert or update a value
• REFERENCES: values of the column must exist in another column in another table. Used for FK constraints