SQL CHECK Constraint
A SQL CHECK
constraint is used to ensure that values in a column meet a specific condition or set of conditions. It helps enforce data integrity by restricting the range or type of data that can be entered into a table.
Syntax
CREATE TABLE table_name (
column_name data_type CHECK (condition)
);
Example
Let's create a table called Employees
where we want to enforce that the salary of each employee must be greater than 0.
CREATE TABLE Employees (
EmployeeID INT PRIMARY KEY,
Name VARCHAR(50),
Salary DECIMAL(10, 2) CHECK (Salary > 0)
);
Explanation
EmployeeID
is the primary key.Name
is a column for the employee's name.Salary
is a column for the employee's salary, which must be greater than 0 due to theCHECK
constraint.
Inserting Data
If we try to insert valid and invalid data:
-- Valid insert
INSERT INTO Employees (EmployeeID, Name, Salary)
VALUES (1, 'John Doe', 50000.00);
-- Invalid insert (will fail)
INSERT INTO Employees (EmployeeID, Name, Salary)
VALUES (2, 'Jane Smith', -3000.00);
Output
For the valid insert, the data is added to the table:
EmployeeID | Name | Salary |
---|---|---|
1 | John Doe | 50000.00 |
For the invalid insert, you will receive an error message like:
ERROR: CHECK constraint failed: Employees
This error indicates that the Salary
value does not satisfy the CHECK
constraint condition.
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