PHP Introduction - Lecture #14

INTRODUCTION TO DATABASE Contd.

Hello All... welcome every one. I'm going to start PHP lectures for Very Beginners, who wants to learn PHP from very beginning. No matter if you do not have any basic programming concept. If you follow my tutorials I hope you will surely get some how good knowledge of PHP.

Today our Topic is

DATABASE- SQL CONSTRAINTS

A constraint is simply a rule, or condition that is applied on a column or multiple columns in a table. Following are some of the important constraint that are briefly discussed.

  ð Null Constraint: By default a column contains NULL values. Null means empty or nothing. Whenever there is Null constraint in a column means that column can hold no value or that column can be left empty.

  ð Not Null Constraint: Not null constraint forces a column not to left empty cell or no any column can have 0 value. Any column with Not Null constraint must contain a value.

  ð Unique Constraint (UC): This constraint enforces that no two rows can have same value in a specified column. Each value that is filled in a rows must be unique.

  ð Primary Key Constraint (PK): More similar to Unique constraint but unique constraint can be applied on multiple columns but Primary key constraint can only be applied on a single column within a table. Primary key cannot contain null values, cannot hold same values. PK uniquely identifies a record and its best approach to have Primary Key in each table so that record can easily be fetched.

  ð Default Constraint (DC): Default value is specified when w user does not enter any value within a column then Default value should be saved by using Default clause.

  ð Foreign Key Constraint (FK): Foreign key is formed when we have a relationship between two tables via a single common column and that common column is usually the Primary Key of one table and that same column is the Foreign Key in other table. Let suppose we have two table of Teachers and Departments and each record of teacher and department is differentiated by Primary Key in each table. What could be the relation between these two that if we want to display those teachers who are in the same department then surely we have relationship between these two tables. In order to get those teachers we will have a column as Primary Key in Department Table and Foreign Key in Teachers table.

How will you apply these constraints to the columns will be told in next lecture for now I think you all got the concept about `Database Constraints`.

Thanks Guys. Any thing missing or any question from this topic you can comment below, I will be trying to solve and answer your question.

Our topic continues to `Database`

Good Luck J