MySQL CHECK Constraint

MySQL CHECK Constraint

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

If you define a CHECK constraint on a column it will allow only certain values for this column.

If you define a CHECK constraint on a table it can limit the values in certain columns based on values in other columns in the row.

CHECK on CREATE TABLE

The following SQL creates a CHECK constraint on the "Age" column when the "Persons" table is created. The constraint ensures that the age of a person must be 18, or older:

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    CHECK (Age>=18)
);

To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax:

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    City varchar(255),
    CONSTRAINT CHK_Person CHECK (Age>=18 AND City='Sandnes')
);

CHECK on ALTER TABLE

To create a CHECK constraint on the "Age" column when the table is already created, use the following SQL:

ALTER TABLE Persons
ADD CHECK (Age>=18);

To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax:

ALTER TABLE Persons
ADD CONSTRAINT CHK_PersonAge CHECK (Age>=18 AND City='Sandnes');

DROP a CHECK Constraint

To drop a CHECK constraint, use the following SQL:

ALTER TABLE Persons
DROP CHECK CHK_PersonAge;

MySQL FOREIGN KEY Constraint

MySQL FOREIGN KEY Constraint

The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.

A FOREIGN KEY is a field (or collection of fields) in one table, that refers to the PRIMARY KEY in another table.

The table with the foreign key is called the child table, and the table with the primary key is called the referenced or parent table.

Look at the following two tables:

Persons Table

Orders Table

Notice that the “PersonID” column in the “Orders” table points to the “PersonID” column in the “Persons” table.

The “PersonID” column in the “Persons” table is the PRIMARY KEY in the "Persons" table.

The “PersonID” column in the “Orders” table is a FOREIGN KEY in the "Orders" table.

The FOREIGN KEY constraint prevents invalid data from being inserted into the foreign key column, because it has to be one of the values contained in the parent table.

FOREIGN KEY on CREATE TABLE

The following SQL creates a FOREIGN KEY on the "PersonID" column when the "Orders" table is created:

CREATE TABLE Orders (
    OrderID int NOT NULL,
    OrderNumber int NOT NULL,
    PersonID int,
    PRIMARY KEY (OrderID),
    FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:

CREATE TABLE Orders (
    OrderID int NOT NULL,
    OrderNumber int NOT NULL,
    PersonID int,
    PRIMARY KEY (OrderID),
    CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID)
    REFERENCES Persons(PersonID)
);

FOREIGN KEY on ALTER TABLE

To create a FOREIGN KEY constraint on the "PersonID" column when the "Orders" table is already created, use the following SQL:

ALTER TABLE Orders
ADD FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);

DROP a FOREIGN KEY Constraint

To drop a FOREIGN KEY constraint, use the following SQL:

ALTER TABLE Orders
DROP FOREIGN KEY FK_PersonOrder;

PRIMARY KEY on ALTER TABLE

 PRIMARY KEY on ALTER TABLE

Continuing the last topic…

The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).

To create a PRIMARY KEY constraint on the "ID" column when the table is already created, use the following SQL:

ALTER TABLE Persons
ADD PRIMARY KEY (ID);

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

ALTER TABLE Persons
ADD CONSTRAINT PK_Person PRIMARY KEY (ID,LastName);

DROP a PRIMARY KEY Constraint

To drop a PRIMARY KEY constraint, use the following SQL:

ALTER TABLE Persons
DROP PRIMARY KEY;

UNIQUE Constraint on ALTER TABLE

 UNIQUE Constraint on ALTER TABLE

Continuation of last blog…

To create a UNIQUE constraint on the "ID" column when the table is already created, use the following SQL:

ALTER TABLE Persons
ADD UNIQUE (ID);

To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:

ALTER TABLE Persons
ADD CONSTRAINT UC_Person UNIQUE (ID,LastName);

DROP a UNIQUE Constraint

To drop a UNIQUE constraint, use the following SQL:

ALTER TABLE Persons
DROP INDEX UC_Person;

MySQL PRIMARY KEY Constraint

MySQL PRIMARY KEY Constraint

The PRIMARY KEY constraint uniquely identifies each record in a table.

Primary keys must contain UNIQUE values, and cannot contain NULL values.

A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).

PRIMARY KEY on CREATE TABLE

The following SQL creates a PRIMARY KEY on the "ID" column when the "Persons" table is created:

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    PRIMARY KEY (ID)
);

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    CONSTRAINT PK_Person PRIMARY KEY (ID,LastName)
);

MySQL NOT NULL Constraint

MySQL NOT NULL Constraint

By default, a column can hold NULL values.

The NOT NULL constraint enforces a column to NOT accept NULL values.

This enforces a field to always contain a value, which means that you cannot insert a new record, or update a record without adding a value to this field.

NOT NULL on CREATE TABLE

The following SQL ensures that the “ID”, “LastName”, and “FirstName” columns will NOT accept NULL values when the “Persons” table is created:

Example

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255) NOT NULL,
    Age int
);

NOT NULL on ALTER TABLE

To create a NOT NULL constraint on the "Age" column when the "Persons" table is already created, use the following SQL:

Example

ALTER TABLE Persons
MODIFY Age int NOT NULL;

MySQL UNIQUE Constraint

The UNIQUE constraint ensures that all values in a column are different.

Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns.

A PRIMARY KEY constraint automatically has a UNIQUE constraint.

However, you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.

UNIQUE Constraint on CREATE TABLE

The following SQL creates a UNIQUE constraint on the "ID" column when the "Persons" table is created:

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    UNIQUE (ID)
);

To name a UNIQUE constraint, and to define a UNIQUE constraint on multiple columns, use the following SQL syntax:

CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    CONSTRAINT UC_Person UNIQUE (ID,LastName)
);

MySQL Constraints

                         MySQL Constraints

The constraint in MySQL is used to specify the rule that allows or restricts what values/data will be stored in the table. They provide a suitable method to ensure data accuracy and integrity inside the table. It also helps to limit the type of data that will be inserted inside the table. If any interruption occurs between the constraint and data action, the action is failed.

Types of MySQL Constraints

Constraints in MySQL is classified into two types:

1. Column Level Constraints: These constraints are applied only to the single column that limits the type of particular column data.
2. Table Level Constraints: These constraints are applied to the entire table that limits the type of data for the whole table.

How to create constraints in MySQL

We can define the constraints during a table created by using the CREATE TABLE statement. MySQL also uses the ALTER TABLE statement to specify the constraints in the case of the existing table schema.

Syntax

The following are the syntax to create a constraints in table:

CREATE TABLE table_name (
    column1 datatype constraint,
    column2 datatype constraint,
    column3 datatype constraint,
    ....
);

Constraints used in MySQL

The following are the most common constraints used in the MySQL:

• NOT NULL
• CHECK
• DEFAULT
• PRIMARY KEY
• AUTO_INCREMENT
• UNIQUE
• INDEX
• ENUM
• FOREIGN KEY

Let us discuss each of these constraints in detail.

In this blog let us discuss only 3 constraints and remaining in next blog

1. Not NULL CONSTRAINT
2. UNIQUE CONSTRAINT
3. CHECK CONSTRAINT

NOT NULL Constraint

This constraint specifies that the column cannot have NULL or empty values. The below statement creates a table with NOT NULL constraint.

mysql> CREATE TABLE Student(Id INTEGER, LastName TEXT NOT NULL, FirstName TEXT NOT NULL, City VARCHAR(35));

Execute the queries listed below to understand how it works:

mysql> INSERT INTO Student VALUES(1, 'Hanks', 'Peter', 'New York');  
  
mysql> INSERT INTO Student VALUES(2, 'peter', 'Amanda', 'Florida');   

UNIQUE Constraint

This constraint ensures that all values inserted into the column will be unique. It means a column cannot stores duplicate values. MySQL allows us to use more than one column with UNIQUE constraint in a table. The below statement creates a table with a UNIQUE constraint:

mysql> CREATE TABLE ShirtBrands(Id INTEGER, BrandName VARCHAR(40) UNIQUE, Size VARCHAR(30));  

Execute the queries listed below to understand how it works:

mysql> INSERT INTO ShirtBrands(Id, BrandName, Size) VALUES(1, 'Pantaloons', 38), (2, 'Cantabil', 40);  
  
mysql> INSERT INTO ShirtBrands(Id, BrandName, Size) VALUES(1, 'Raymond', 38), (2, 'xxx', 40);   

CHECK Constraint

It controls the value in a particular column. It ensures that the inserted value in a column must be satisfied with the given condition. In other words, it determines whether the value associated with the column is valid or not with the given condition.

Syntax:

[CONSTRAINT [symbol]] CHECK (expr) [[NOT] ENFORCED]  

mysql> CREATE TABLE Persons (  
    ID int NOT NULL,  
    Name varchar(45) NOT NULL,  
    Age int CHECK (Age>=18)  
);  

mysql> INSERT INTO Persons(Id, Name, Age)   
VALUES (1,'Robert', 28), (2, 'Joseph', 35), (3, 'Peter', 40);  
  
mysql> INSERT INTO Persons(Id, Name, Age) VALUES (1,'Robert', 15);  

As first statements executes successfully but the second statement does not executes as it violates the syntax.