Page 2 - DDL and DML Commands

UNIQUE constraint
Can be applied at table or column level.
If applied for a column values have to be unique for the column.
create table STUDENT
(admNo integer NOT NULL UNIQUE,
name varchar(50)) UNIQUE;
Or
create table STUDENT
(admNo integer NOT NULL,
firstName varchar(20),
lastName varchar(20),
UNIQUE(firstName, lastName); (unique entries for firstName + lastName, togther)

Primary Key constraint
Can be applied at column or table level.
A column specified as primary key contain unique and no null values.
create table STUDENT
(admNo integer PRIMARY KEY,
stream varchar(20),
name varchar(50));
Or
create table STUDENT
(admNo integer,
stream varchar(20),
name varchar(50)
PRIMARY KEY(admNo, stream);

Foreign Key constraint

Can be applied at column or table level.
Values entered in the field, must be a valid value in the primary key field of the other specified table
create table STUDENT
(admNo integer primary key,
name varchar(50),
subjectCode char(2) REFERENCES SUBJECT(subCode));
OR
create table STUDENT
(admNo integer primary key,
name varchar(50),
subjectCode char(2),
FOREIGN KEY(subjectCode) REFERENCES SUBJECT(subCode));

Check constraint

Can be applied at column or table level.
Values entered in the field, must be a valid value in the primary key field of the other specified table