Check constraint is used to validate values entered into a column. CHECK constraints enforce domain integrity by limiting the values that are accepted by a column. They are similar to FOREIGN KEY constraints in that they control the values that are placed in a column. The difference is in how they determine which values are valid: FOREIGN KEY constraints get the list of valid values from another table.
CHECK constraints determine the valid values from a logical expression that is not based on data in another column. For example, it is possible to limit the range of values for a salary column by creating a CHECK constraint that allows only data that ranges from 15,000 through 500, 00. This prevents salaries from being entered beyond the normal salary range.
You can create a CHECK constraint with any logical (Boolean) expression that returns TRUE or FALSE based on the logical operators.
Syntax:
CONSTRAINT constrain_name CHECK (predicate),
COLUMN LEVEL
SQL> CREATE TABLE Persons
(
P_Id number (6) NOT NULL CHECK (P_Id>0),
LastName varchar2 (15) NOT NULL,
FirstName varchar2 (15),
Address varchar2 (25),
City varchar (15) check (city in ('Ferozepur','Jalandhar','Nabha'))
);
Table created.
TABLE LEVEL
SQL> CREATE TABLE Persons
(
P_Id number (6) NOT NULL,
LastName varchar2 (15) NOT NULL,
FirstName varchar2 (15),
Address varchar2(25),
City varchar2 (13),
CONSTRAINT ck_Person CHECK (P_Id>0 AND City='Ferozepur')
);
TIPS:
• You can use any number of constraints in a table.
• A single column can have any number of check constraints.
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