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 NULLwhat is primary key
PRIMARY KEY -- unique identifier for the entire row of record in a table
-- can not be null and must be uniqueprimary key
CREATE TABLE Persons (
Rollno int NOT NULL,
FirstName varchar(255),
LastName varchar(255),
Age int,
PRIMARY KEY (ID)
);primary key
Primary Key :
It is unique column in every table in a database
It can ONLY accept;
- nonduplicate values
- cannot be NULLprimary key
CONSTRAINT pk_purchase_orders PRIMARY KEY(po_nr)
Code language: SQL (Structured Query Language) (sql)Copyright © 2021 Codeinu
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