The MySQL Check Constraint

The MySQL Check Constraint is a condition that can be applied to a column to ensure that the inserted or updated data in that column meets the specified condition. The database rejects the operation if the condition is not met to maintain data integrity.

Check Constraint with a Trigger

A trigger in MySQL is used to automatically execute a set of SQL statements in response to specific events in the database, such as an INSERT, UPDATE, or DELETE operation.

A check constraint with a trigger allows us to perform actions automatically based on data changes.

Example

Assume we have created a table with name CUSTOMERS in the MySQL database using CREATE TABLE statement as shown below −

CREATE TABLE CUSTOMERS (
   ID INT NOT NULL,
   NAME VARCHAR (20) NOT NULL,
   AGE INT NOT NULL,
   ADDRESS CHAR (25),
   SALARY DECIMAL (18, 2),
   PRIMARY KEY (ID)
);

Following query inserts values into CUSTOMERS table using the INSERT statement −

INSERT INTO CUSTOMERS VALUES
(1, ''Ramesh'', 32, ''Ahmedabad'', 2000.00),
(2, ''Khilan'', 25, ''Delhi'', 1500.00),
(3, ''Kaushik'', 23, ''Kota'', 2000.00),
(4, ''Chaitali'', 25, ''Mumbai'', 6500.00),
(5, ''Hardik'', 27, ''Bhopal'', 8500.00),
(6, ''Komal'', 22, ''Hyderabad'', 4500.00),
(7, ''Muffy'', 24, ''Indore'', 10000.00);

The table obtained is as shown below −

Now, we will add a check constraint to ensure that the age of customers in the CUSTOMERS table should be greater than or equal to 18. Additionally, we will create a trigger that, when an attempt is made to insert a record with an age less than 18, it will raise an error and prevent the insertion −

-- Creating a Trigger
DELIMITER //
CREATE TRIGGER check_age_trigger
BEFORE INSERT ON CUSTOMERS
FOR EACH ROW
BEGIN
   IF NEW.AGE < 18 THEN
      SIGNAL SQLSTATE ''45000''
      SET MESSAGE_TEXT = ''Age must be 18 or older
   END IF;
END;
//
DELIMITER ;

-- Adding a Check Constraint
ALTER TABLE CUSTOMERS
ADD CONSTRAINT check_age_constraint CHECK (AGE >= 18);

Output

We get the output as shown below −

Query OK, 7 rows affected (0.05 sec)
Records: 7  Duplicates: 0  Warnings: 0

Adding Check Constraint on Single Column

We can apply a check constraint on a column by specifying the check constraint after the column name at the time of table creation.

Syntax

Following is the syntax to specify the check constraint on column −

CREATE TABLE table_name (
   column1 datatype(size),
   column datatype(size) constraint constraintName
   CHECK Check(columnName condition value),..., column datatype (size)
);

Example

In this example, we are creating a table named EMPLOYEES and specifying a column-level check constraint on one column −

CREATE TABLE EMPLOYEES(
   EID INT NOT NULL,
   NAME VARCHAR(40),
   AGE INT NOT NULL CHECK(AGE>=20),
   CITY VARCHAR(30),
   C_Phone VARCHAR(12) NOT NULL UNIQUE
);

We can verify if the check constraint is working correctly by inserting a value into the EMPLOYEES table which does not satisfy the condition −

INSERT INTO EMPLOYEES 
VALUES (1, ''John'', 19, ''New York'', ''09182829109'');

Output

The output obtained is as follows −

ERROR 3819 (HY000): Check constraint ''employees_chk_1'' is violated.

Adding Check Constraint on Multiple Columns

We can add check constraint on multiple columns of a table by specifying the constraints for each column after the column name.

Example

In the following example, we are creating a table named STUDENTS and specifying a column-level check constraint on multiple columns (AGE and FEE) −

CREATE TABLE STUDENTS(
   SID INT NOT NULL,
   NAME VARCHAR(20),
   AGE INT NOT NULL CHECK(AGE<=24),
   CITY VARCHAR(30),
   FEE NUMERIC NOT NULL CHECK(FEE>=15000)
);

Now, we can insert records, but if we attempt to insert a record that violates these constraints, the database will reject it.

Here, we are inserting a valid record −

INSERT INTO STUDENTS 
VALUES (001, ''Robert'', 21, ''LA'', 17000);

We can see in the output below that the insertion is successful because the age is within the allowed range, and the fee meets the specified condition −

Query OK, 1 row affected (0.01 sec)

In here, we are attempting to insert a record violating constraints −

INSERT INTO STUDENTS 
VALUES (002, ''James'', 25, ''Barcelona'', 10000);

We can see that the insertion fails since the age exceeds 24, violating the constraint.

ERROR 3819 (HY000): Check constraint ''students_chk_1'' is violated.

Adding Check Constraint on an Existing Table

We can also add a check constraint on an existing table in MySQL by using the ALTER statement. We must ensure that the constraint satisfy for the existing records in the table.

Syntax

ALTER TABLE table_name 
ADD CONSTRAINT ConstraintName 
CHECK(ColumnName condition Value);

Example

In the following example, we are adding a check constraint to the AGE column of the CUSTOMERS table created above −

ALTER TABLE CUSTOMERS 
ADD CONSTRAINT Constraint_Age 
CHECK (AGE >= 21);

Output

Following is the output of the above code −

Query OK, 7 rows affected (0.04 sec)
Records: 7  Duplicates: 0  Warnings: 0

Dropping Check Constraint

We can remove an existing constraint by using the ALTER statement with the DROP statement.

Syntax

Following is the syntax to remove a constraint from the table −

ALTER TABLE table_name 
DROP CONSTRAINT constraint_set;

Example

Following example removes an existing constraint from the AGE column in the CUSTOMERS table created above −

ALTER TABLE CUSTOMERS 
DROP CONSTRAINT Constraint_Age;

Output

After executing the above code, we get the following output −

Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

Check-Constraints Using a Client Program

We can also perform check-constraints using the client program.

Syntax

$sql = "CREATE TABLE EMPLOYEES(EID INT NOT NULL,NAME VARCHAR(40),AGE INT NOT NULL CHECK(AGE>=20),CITY VARCHAR(30),C_Phone VARCHAR(12) NOT NULL UNIQUE)"
$mysqli->query($sql);

sql = "CREATE TABLE EMPLOYEES(EID INT NOT NULL,NAME VARCHAR(40),AGE INT NOT NULL CHECK(AGE>=20),CITY VARCHAR(30),C_Phone VARCHAR(12) NOT NULL UNIQUE)";
con.query(sql);

String sql = "CREATE TABLE EMPLOYEES( EID INT NOT NULL, NAME VARCHAR(40), AGE INT NOT NULL CHECK(AGE>=20), CITY VARCHAR(30), C_Phone VARCHAR(12) NOT NULL UNIQUE)";
statement.execute(sql);

create_table_query = ''CREATE TABLE EMPLOYEES(EID INT NOT NULL, NAME VARCHAR(40), AGE INT NOT NULL CHECK(AGE>=20), CITY VARCHAR(30), C_Phone VARCHAR(12) NOT NULL UNIQUE);
cursorObj.execute(create_table_query)

Example

Following are the programs −

$dbhost = ''localhost
$dbuser = ''root
$dbpass = ''password
$db = ''TUTORIALS
$mysqli = new mysqli($dbhost, $dbuser, $dbpass, $db);
if ($mysqli->connect_errno) {
    printf("Connect failed: %s
", $mysqli->connect_error);
    exit();
}
//printf(''Connected successfully.
'');
$sql = "CREATE TABLE EMPLOYEES(EID INT NOT NULL,NAME VARCHAR(40),AGE INT NOT NULL CHECK(AGE>=20),CITY VARCHAR(30),C_Phone VARCHAR(12) NOT NULL UNIQUE)";
if($mysqli->query($sql)){
    printf("Table created successfully...!n");
}
//let''s insert some records... whose age is greater than 20
$sql = "INSERT INTO EMPLOYEES VALUES(1, ''Jay'', 30, ''Hyderabad'', ''223233'')";
if($mysqli->query($sql)){
    printf("First record(age>20) inserted successfully...!n");
}
$sql = "INSERT INTO EMPLOYEES VALUES(2, ''John'', 35, ''Lucknow'', ''213032'')";
if($mysqli->query($sql)){
    printf("Second record(age>20) inserted successfully...!n");
}
//table record before inserting employee record whose age is less than 20;
$sql = "SELECT * FROM EMPLOYEES";
printf("Table records(before inserting emp record agequery($sql)){
    while($row = mysqli_fetch_array($result)){
        printf("EId: %d, NAME: %s, AGE: %d, CITY %s, C_Phone %d",
        $row[''EID''],
        $row[''NAME''],
        $row[''AGE''],
        $row[''CITY''],
        $row[''C_Phone'']);
        printf("n");
    }
}
//let''s insert some records... whose age is less than 20
$sql = "INSERT INTO EMPLOYEES VALUES(3, ''Vinnet'', 18, ''Hyderabad'', ''228151'')";
if($mysqli->query($sql)){
    printf("Third record(agequery($sql)){
    while($row = mysqli_fetch_array($result)){
        printf("EId: %d, NAME: %s, AGE: %d, CITY %s, C_Phone %d",
        $row[''EID''],
        $row[''NAME''],
        $row[''AGE''],
        $row[''CITY''],
        $row[''C_Phone'']);
        printf("n");
    }
}
if($mysqli->error){
    printf("Error message: ", $mysqli->error);
}
$mysqli->close();

Table created successfully...!
First record(age>20) inserted successfully...!
Second record(age>20) inserted successfully...!
Table records(before inserting emp record age

var mysql = require(''mysql2'');
var con = mysql.createConnection({
host:"localhost",
user:"root",
password:"password"
});

 //Connecting to MySQL
 con.connect(function(err) {
 if (err) throw err;
//   console.log("Connected successfully...!");
//   console.log("--------------------------");
 sql = "USE TUTORIALS";
 con.query(sql);
 //create table with check constraints
 sql = "CREATE TABLE EMPLOYEES(EID INT NOT NULL,NAME VARCHAR(40),AGE INT NOT NULL CHECK(AGE>=20),CITY VARCHAR(30),C_Phone VARCHAR(12) NOT NULL UNIQUE)";
 con.query(sql, function(err, result){
    if (err) throw err;
    console.log("Table created successfully....!");
    });
//now let''s insert some records(age greater than 20)
sql = "INSERT INTO EMPLOYEES VALUES(1, ''Jay'', 30, ''Hyderabad'', ''223233'')";
con.query(sql, function(err, result){
    if (err) throw err;
    console.log("First record inserted successfully...!");
    });
sql = "INSERT INTO EMPLOYEES VALUES(2, ''John'', 35, ''Lucknow'', ''213032'')";
con.query(sql, function(err, result){
    if (err) throw err;
    console.log("Second record inserted successfully...!");
    });
sql = "INSERT INTO EMPLOYEES VALUES(3, ''Vinnet'', 18, ''Hyderabad'', ''228151'')";
    con.query(sql, function(err, result){
        if (err) throw err;
        console.log(result);
        });
sql = "SELECT * FROM EMPLOYEES";
con.query(sql, function(err, result){
    console.log("Table records(with ID auto_increment sequence).");
    if (err) throw err;
    console.log(result);
    });
});    

Table created successfully....!
First record inserted successfully...!
Second record inserted successfully...!
D:test1checkcons.js:34
        if (err) throw err;                 ^
Error: Check constraint ''employees_chk_1'' is violated.    

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
public class CheckConstraints {
    public static void main(String[] args) {
        String url = "jdbc:mysql://localhost:3306/TUTORIALS";
        String user = "root";
        String password = "password";
        ResultSet rs;
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            Connection con = DriverManager.getConnection(url, user, password);
            Statement st = con.createStatement();
            //System.out.println("Database connected successfully...!");
            //create table
            String sql = "CREATE TABLE EMPLOYEES( EID INT NOT NULL, NAME VARCHAR(40), AGE INT NOT NULL CHECK(AGE>=20), CITY VARCHAR(30), C_Phone VARCHAR(12) NOT NULL UNIQUE)";
            st.execute(sql);
            System.out.println("Employees table created successfully...!");
            //let''s insert some records
            String sql1 = "INSERT INTO EMPLOYEES VALUES (1, ''John'', 19, ''New York'', ''09182829109'')";
            st.execute(sql1);
            System.out.println("Record inserted successfully....!");
            //lets print table records
            String sql2 = "SELECT * FROM EMPLOYEES";
            rs = st.executeQuery(sql2);
            while(rs.next()) {
                String id = rs.getString("id");
                String name = rs.getString("name");
                String age = rs.getString("age");
                String city = rs.getString("city");
                String c_phone = rs.getString("C_Phone");
                System.out.println("Id: " + id + ", Name: " + name + ", Age: " + age + ", City: " + city + ", C_phone: " + c_phone);
            }
        }catch(Exception e) {
            e.printStackTrace();
        }
    }
}    

Employees table created successfully...!
java.sql.SQLException: Check constraint ''employees_chk_1'' is violated.    

import mysql.connector
# Establishing the connection
connection = mysql.connector.connect(
    host=''localhost'',
    user=''root'',
    password=''password'',
    database=''tut''
)
# Creating a cursor object
cursorObj = connection.cursor()
# Creating the table 
create_table_query = ''''''
CREATE TABLE EMPLOYEES(
    EID INT NOT NULL,
    NAME VARCHAR(40),
    AGE INT NOT NULL CHECK(AGE>=20),
    CITY VARCHAR(30),
    C_Phone VARCHAR(12) NOT NULL UNIQUE
)
''''''
cursorObj.execute(create_table_query)
print("Table ''EMPLOYEES'' is created successfully!")
# Describing the EMPLOYEES table
describe = "DESC EMPLOYEES"
cursorObj.execute(describe)
print("Table Description:")
for column_info in cursorObj.fetchall():
    print(column_info)
# Inserting the first record
try:
    sql = "INSERT INTO EMPLOYEES VALUES(1, ''Jay'', 30, ''Hyderabad'', ''223233'')"
    cursorObj.execute(sql)
    connection.commit()
    print("First record inserted successfully!")
except mysql.connector.Error as err:
    connection.rollback()
    print(f"Error: {err}")

# Inserting the second record
try:
    sql = "INSERT INTO EMPLOYEES VALUES(2, ''John'', 35, ''Lucknow'', ''213032'')"
    cursorObj.execute(sql)
    connection.commit()
    print("Second record inserted successfully!")
except mysql.connector.Error as err:
    connection.rollback()
    print(f"Error: {err}")

# Inserting the third record with age less than 20 (this will raise an error)
try:
    sql = "INSERT INTO EMPLOYEES VALUES(3, ''Vinnet'', 18, ''Hyderabad'', ''228151'')"
    cursorObj.execute(sql)
    connection.commit()
    print("Third record inserted successfully!")
except mysql.connector.Error as err:
    connection.rollback()
    print(f"Error: {err}")
# Retrieving records
cursorObj.execute("SELECT * FROM person_tbl")
records = cursorObj.fetchall()
# Printing the records
print("Table records.")
for record in records:
    print(record)
# Closing the cursor and connection
cursorObj.close()
connection.close()

Table ''EMPLOYEES'' is created successfully!
Table Description:
(''EID'', b''int'', ''NO'', '''', None, '''')
(''NAME'', b''varchar(40)'', ''YES'', '''', None, '''')
(''AGE'', b''int'', ''NO'', '''', None, '''')
(''CITY'', b''varchar(30)'', ''YES'', '''', None, '''')
(''C_Phone'', b''varchar(12)'', ''NO'', ''PRI'', None, '''')
First record inserted successfully!
Second record inserted successfully!
Error: 3819 (HY000): Check constraint ''employees_chk_1'' is violated.
Table records.
(''Thomas'', ''Jay'', None)
(''Smith'', ''John'', None)