Topic: SQL Command CONSTRAINT Help

[$Three3]

Hi everyone, I am new to SQL and I have been reading a book called "Learning SQL" and I am stuck on this one part of the book. It has to do with the CONSTRAINT command and I am not sure what this command does or means. The book never even explains its functionality. Here is the code from the book:

Code: [Select]

CREATE TABLE person (person_id SMALLINT UNSIGNED,fname VARCHAR(20), lname VARCHAR(20), gender ENUM('M','F'), birth_date DATE, street VARCHAR(30), city VARCHAR(20), state VARCHAR(20), country VARCHAR(20), postal_code VARCHAR(20), CONSTRAINT pk_person PRIMARY KEY (person_id));
And the second table that goes along with the code above is:

Code: [Select]

CREATE TABLE favorite_food (person_id SMALLINT UNSIGNED,food VARCHAR(20), CONSTRAINT pk_favorite_food PRIMARY KEY (person_id, food), CONSTRAINT fk_fav_food_person_id FOREIGN KEY (person_id)REFERENCES person (person_id));

[thorpe]

It constrains the field in question into being of a certain data.

eg; A primary key constraint means that each field must contain a unique number.

[ksrao]

Hi

See what is constraint ? Constraint is basically used to impose some limitation in Data base to get the uniformity in Data Base.

Commonly used constraints
1. Primary key  - Which uniquely identifies row in a table.
2. Foreign Key  - Which is used to refers to the other table.
3. Check           - To have a validation checks on input/output
4. Not Null        - It will not allow null values.
5. Unique.       - Repetition of the values are not allowed.

To clear your basics in RDBMS go through some sql server interview questions at http://www.wiziq.com/online-class/292863-sql-server-interview-questions

Thanks
Regards
Kolla Sanjeeva Rao
OCP Oracle.