Adding Constraints in SQL

NOT NULL Constraint :

• Ensures that null values are not permitted for the column
• CREATE TABLE emp(employee_id numeric(6)CONSTRAINT emp_emp_id_pk PRIMARY KEY, first_name varchar(20) NOT NULL,...);

UNIQUE Constraint :

• Unique constraint allow only unique values to be inserted in the column.
• No duplicates allowed e.g emailIDs
• CREATE TABLE emp(employee_id numeric(6) CONSTRAINT emp_emp_id_pk PRIMARY KEY, first_name varchar(20) NOT NULL, Email_id varchar(20) UNIQUE,...);

PRIMARY KEY Constraint :

• Don’t allow duplicate values
  
• Don’t allow null values
• Only one primary key can be created per table
• CREATE TABLE emp(employee_id numeric(6) CONSTRAINT emp_emp_id_pk RIMARY KEY, first_name varchar(20),...);

FOREIGN KEY Constraint :

• Defined at either the table level or the column level : continued in the next post