Constraint:
Constraints are used to apply rules or restrictions on database objects like tables.
We can apply constraints while creating the table or after creating the table.

1.Null Constraint:

•  By default every column in table will accept null values.
•  We no need to specify null constraint while creating a table.

Syntax: create table <table_name>(column name datatype null);

2.Not Null Constraint:
  Not Null constraint will not allow null values.

  Syntax:  create table <table_name>(column_name datatype not null);
   Ex:
  
  Syntax: After Creating table
     Alter table table_name alter column column_name datatype not null;

    
3.Unique Key Constraint:

•   It Will not allow duplicate values.
•   We can have more than one unique key in a table.
•   It allow only one null value in a table.because it consider the null value also a value.  
•   But it not accept 2 null values because the other null value is the duplicate value.

  Syntax: 
           Create table <table_name>(column name datatype unique);

           Here i applied not null and unique constraints to student_id because unique constraint will allow one null value.
      
  Syntax: After Creating table

             Alter table table_name add unique (column_name );

4.Check Constraint:
  Check constraint is used to validate the data before inserting into the table.
  Syntax:
      Create table <table_name>( column_name datatype check(condition))

Inserting the value against the specified condition

Output:


 Syntax: After Creating table
   alter table table_name add Check( column_name condition)


5.Primary Key Constraint:

• Primary key Uniquely identifies each record.
• We can have one primary key in a table which can be applied on single column or combination of columns.
• Whenever we apply primary key on a table then not null and unique key will be applied on the columns.
• It will not allow null and duplicate values.

Syntax: While Creating the table.

    Create table <table_name>( column_name datatype primary key.)

Output:

Inserting the duplicate values into the table.

Output:

6.Foreign Key Constraint:

• Foreign will establish a relationship between tables.
• We can have more than one foreign key in a table.
• It allows null and duplicate values. 

 Syntax: While Creating the table.

  create table table_name(column_name datatype, parent_Column_name datatype foreign key references     Parent_table_name(parent_Column_name))

Syntax: After creating the table

7.Default constraint:

• Default constraint is used to provide default value to a column.
•    If the value is specified then the value will be insert into the table.

Syntax: Create table <table_name>( column_name datatype default 'value'.)

Output:

8.Auto Increment:

• This key will generate a unique value automatically when ever the new record is inserted.
•  The generated value can not be changed.

Syntax: create table table_name(column _name datatype identity or identity(starting value , increment value))

9.Composite key:

• Applying primary key for the combinations of columns is called as composite key.
• It will not allow null values.
• It will not allow duplicate values.

Syntax:

create table table_name(column 1 _name datatype,column 2 _name datatype, primary key( column 1 _name,column 2)

inserting the value which is already inserted.

Output: