Answers for "wht is a primary key"

what is primary key

Primary Key :
It is unique column in every table in a database
It can ONLY accept;
    - nonduplicate values
    - cannot be NULL



Foreign Key: 
It is a column that comes from a different table and
using Foreign key tables are related each other
It is the primary key of another table
It can be duplicate or null for another table




Unique Key:
Only unique value and also can contain NULL

primary key

CONSTRAINT pk_purchase_orders PRIMARY KEY(po_nr)
Code language: SQL (Structured Query Language) (sql)