SQL CHECK Constraint

A verification or check  constraint is a type of integrity constraint in SQL that specifies a requirement that must be met standard each line in a database table. Coercion must be a predicate. It can refer to a single column, or several columns of the table. The result of the predicate may be true, false or unknown, depending on the presence of NULL. If the predicate evaluates unknown, then the constraint is not violated and the line can be inserted or updated into the scene. This is contrary to the predicates set out in the WHERE conditions in the SELECT or UPDATE statements.

• The CHECK constraint is used to limit the value range that can be placed in a column. 

• If you set a CHECK constraint on a single column, it only allows certain values for that column.

• If you set a CHECK constraint on a table, it can limit the values of certain columns based on values in other columns in the line.

The CHECK constraint allows a condition to verify the value entered in a record. If the condition is false, the recording violates the coercion and has not entered the table. CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint  CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint  CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint  CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint  CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint  CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint  CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint  CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint SQL CHECK Constraint

SQL CHECK on CREATE TABLE  

Syntax 

CREATE TABLE table_name (

    column_Name_01 datatype constraint,
    column_Name_02 datatype constraint,
    column_Name_03 datatype check(Condition),
    ....
);

Example 01: 

CREATE TABLE PERSON(

    NAME varchar2(20) UNIQUE,
    Age  Number CHECK(AGE >= 20),
    Hobbies Varchar2(20) NOT NULL UNIQUE,

    TEL varchar2(20),

);

Example 02:

CREATE TABLE PERSON(

    NAME varchar2(20) UNIQUE,
    Age  Number ,
    tel Varchar2(20) NOT NULL UNIQUE,

    profession varchar2(20),

constraint check_person check(Age >= 20 and                Profession= "Director" );

);

DROP a CHECK Constraint

Syntax 

ALTER TABLE Table_Name DROP CONSTRAINT Constraint_Name;

Example :

ALTER TABLE PERSON DROP CONSTRAINT Check_person;

Constraint CHECK on ALTER TABLE

Syntax 

ALTER TABLE Table_Name ADD CHECK column+Condition ;

Example 01:

To make a CHECK requirement on the "Age" section when the table is made, utilize the accompanying SQL:

ALTER TABLE PERSON ADD CHECK AGE <= 50;

Example 02:

To permit naming of a CHECK imperative, and for characterizing a CHECK requirement on different segments, utilize the accompanying SQL grammar:

ALTER TABLE PERSON ADD CONSTRAINT check_person CHECK(AGE < 50 and profession='director');

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