SQL CONSTRAINTS(NOT NULL,CHECK,UNIQUE)

CONSTRAINTS
When you bind a business rule to a column in the table then those rules are called the Constraints. Constraints are defined while creating the table. Say for example, you cannot have an employee who does not have a name, then employee name column in employee table should be a NOT NULL column. The NOT NULL is a constraint.
The constraint types and short descriptions are given below:
Constraint Types And Description
NOT NULL:-
you must provide the value in that column. you cannot leave that column blank
PRIMARY KEY:-
No duplicate values allowed, for example Empno in Employee table should be unique
CHECK:-
checks the value and controls the inserting and updating values.
DEFAULT:-
Assigns a default value if no value is given.
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NOT NULL:-
If we have a business rule saying that all customers should have a name, we cannot have any customer with out a name. So to implement that business rule we can create customer table and specify customer name column as NOT NULL (constraint)
Example
CREATE TABLE EMPLOYEE (EMPNO NUMBER(4) PRIMARY KEY, ENAME VARCHAR(4) NOT NULL);
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CHECK
Check constraint is used where we define a condition on a column. Check constraint consists of the keyword
col_name datatype CHECK (col_name in(value1, value2))
Example
If you have a business rule saying that all employees in the organization should get atleast $500 then we can use CHECK constraint while creating table.
CREATE TABLE EMPLOYEE ( EMPNO NUMBER(4) PRIMARY KEY, ENAME VARCHAR(4) NOT NULL, SALARY NUMBER(7,2) CHECK (SALARY > 500) );
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DEFAULT
While inserting a row into a table without giving values for every column, SQL must insert a default value to fill in the excluded columns, or the command will be rejected. The most common default value is NULL. This can be used with columns not defined with a NOT NULL.
Default value assigned to a column while creating the table using
CREATE TABLE operation.
Example
CREATE TABLE ITEM (ITEM_ID NUMBER(4) PRIMARY KEY, ITEM_NAME VARCHAR(15),ITEM_DESC VARCHAR(100), QOH NUMBER(4) DEFAULT 100);
Assigning a default value 0 for numeric columns makes the computation.
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UNIQUE:-
The values entered into a column are unique ie no duplicate values exists.This constraint ensures business that there is no duplicates allowed.