SQL constraints in Database

I am looking into setting up the constraints with an sql query instead of visually in programs.when I am going to insert some data in table and need to investigate that particular data is valid for table or not ? So how would I do that.If I am inserting some character and willing to show it into particular format,it can be done and how.Can you introduce me about different constraint in Structure Query Language.

The constraint is like a method in sql which works as a filter ,Every insert,update and delete data is filtered here when it need to send for operation.

The Constraint can be specified after and before creation of table. Before creation of table,the statement takes the form like this "Create table.." and after creation "Alter table ...".

The different types of constraint are as follows:
* NOT NULL
* UNIQUE
* PRIMARY KEY
* FOREIGN KEY
* CHECK
* DEFAULT

NOT NULL:

A NOT NULL constraint is a type of constraint which enforces the user to enter the value in the particular column for which NOT NULL constraint is specified.

CREATE TABLE Family
(
Mem_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int(4)
)

In the given example Mem_id and LastName is declared with the NOT NULL constraint,both columns are mandatory to enter the value.

Unique constraint :

The unique constraint uniquely identifies the every tuple in the table. It automatically created when we create the primary key for a table and primary key is also a database constraint.

The declaration of Unique constraint would be as follows :

CREATE TABLE Family
(
Mem_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int(4),
CONSTRAINT uc_Member_ID UNIQUE (Mem_Id,LastName)
)

In the above example the "uc_Member_ID" is the name of constraint and the values of Mem_id and LastName will be unique in the whole table.