Database Concepts

CONSTRAINTS: These are set of rules that we need to follow when entering a data into an table. The constraints are specified when creating a Table or altering a table.

1.) NOT NULL : All values in column are non-empty.

CREATE TABLE STUDENT ( ID int NOT NULL, STUDENT_NAME VARCHAR(250) NOT NULL );

2.) UNIQUE : All values in column are different and unique.

CREATE TABLE STUDENT ( ID int NOT NULL, STUDENT_NAME VARCHAR(250) NOT NULL , CONSTRAINT ID_C UNIQUE(ID));

3.) PRIMARY KEY : Unique and Not Null. A table can have ONE Primary Key, but it can have MANY Unique constraints.

4.) FOREIGN KEY: To maintain integrity of data