What is CHECK constraint in MySQL?

The CHECK constraint specifies a condition that must be true when manipulating the database table.

Following example shows how a CHECK constraint may be implemented:

CREATE TABLE Emp3 (EmpNo NUMBER(4) NOT NULL CHECK(EmpNo>0) PRIMARY KEY,

   DeptNo NUMBER(7,2) NOT NULL,

   Ename VARCHAR2(9) NOT NULL,

   CONSTRAINT fk_Emp_DeptNo FOREIGN KEY (DeptNo) REFERENCES Dept(DeptNo)

   );

To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, this is done:

CREATE TABLE Emp3 (EmpNo NUMBER(4) NOT NULL PRIMARY KEY,

   DeptNo NUMBER(7,2) NOT NULL,

   Ename VARCHAR2(9) NOT NULL,

   CONSTRAINT fk_Emp_DeptNo FOREIGN KEY (DeptNo) REFERENCES Dept(DeptNo),

   CONSTRAINT chk_Emp CHECK (EmpNo>0 AND DeptNo='20')

   );