SQL Constraints
In this tutorial you will learn how to use SQL constraints.
What is Constraint?
A constraint is simply a restriction placed on one or more columns of a table to limit the type of values that can be stored in that column. Constraints provide a standard mechanism to maintain the accuracy and integrity of the data inside a database table.
There are several different types of constraints in SQL, including:
NOT NULL
PRIMARY KEY
UNIQUE
DEFAULT
FOREIGN KEY
CHECK
Now, let’s discuss each of these constraints in detail.
NOT NULL Constraint
The NOT NULL
constraint specifies that the column does not accept NULL
values.
This means if NOT NULL
constraint is applied on a column then you cannot insert a new row in the table without adding a non-NULL value for that column.
The following SQL statement creates a table named persons with four columns, out of which three columns, id, name and phone do not accept NULL values.
Example
CREATE TABLE persons (
id INT NOT NULL,
name VARCHAR(30) NOT NULL,
birth_date DATE,
phone VARCHAR(15) NOT NULL
);
Note: A null value or
NULL
is different from zero (0
), blank, or a zero-length character string such as''
.NULL
means that no entry has been made.
PRIMARY KEY Constraint
The PRIMARY KEY
constraint identify the column or set of columns that have values that uniquely identify a row in a table. No two rows in a table can have the same primary key value. Also, you cannot enter NULL
value in a primary key column.
The following SQL statement creates a table named persons and specifies the id column as the primary key. That means this field does not allow NULL
or duplicate values
.
Example
CREATE TABLE persons (
id INT NOT NULL PRIMARY KEY,
name VARCHAR(30) NOT NULL,
birth_date DATE,
phone VARCHAR(15) NOT NULL
);
Tip: The primary key typically consists of one column in a table, however more than one column can comprise the primary key, e.g. either the employee’s email address or assigned identification number is the logical primary key for an employee table.
UNIQUE Constraint
The UNIQUE
constraint restricts one or more columns to contain unique values within a table.
Although both a UNIQUE
constraint and a PRIMARY KEY
constraint enforce uniqueness, use a UNIQUE
constraint instead of a PRIMARY KEY
constraint when you want to enforce the uniqueness of a column, or combination of columns, that is not the primary key.
The following SQL statement creates a table named persons and specifies the phone column as unique. That means this field does not allow duplicate values.
Example
CREATE TABLE persons (
id INT NOT NULL PRIMARY KEY,
name VARCHAR(30) NOT NULL,
birth_date DATE,
phone VARCHAR(15) NOT NULL UNIQUE
);
Note: Multiple
UNIQUE
constraints can be defined on a table, whereas only onePRIMARY KEY
constraint can be defined on a table. Also, unlikePRIMARY KEY
constraints, theUNIQUE
constraints allowNULL
values.
DEFAULT Constraint
The DEFAULT
constraint specifies the default value for the columns.
A column default is some value that will be inserted in the column by the database engine when an INSERT
statement doesn’t explicitly assign a particular value.
The following SQL statement creates a default for the country column.
Example
CREATE TABLE persons (
id INT NOT NULL PRIMARY KEY,
name VARCHAR(30) NOT NULL,
birth_date DATE,
phone VARCHAR(15) NOT NULL UNIQUE,
country VARCHAR(30) NOT NULL DEFAULT 'India'
);
Note: If you define a table column as
NOT NULL
, but assign the column a default value, then in theINSERT
statement you don’t need to explicitly assign a value for that column in order to insert a new row in the table.
FOREIGN KEY Constraint
A foreign key (FK) is a column or combination of columns that is used to establish and enforce a relationship between the data in two tables.
Here’s a sample diagram showing the relationship between the employees and departments table. If you look at it carefully, you will notice that the dept_id column of the employees table matches the primary key column of the departments table. Therefore, the dept_id column of the employees table is the foreign key to the departments table.
![Foreign Key Relationship Diagram](C:\Users\The_MAK\OneDrive\Desktop\Lectures\Docs\MySQL\Level 00\foreign-key-relationship-diagram.png)
In MySQL you can create a foreign key by defining a FOREIGN KEY
constraint when you create a table as follow. The following statement establishes a foreign key on the dept_id column of the employees table that references the dept_id column of the departments table.
Example
-- Create the departments Table
CREATE TABLE departments (
dept_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
dept_name VARCHAR(100) NOT NULL
);
-- Create the employees_fk Table
CREATE TABLE employees_fk (
emp_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
emp_name VARCHAR(55) NOT NULL,
hire_date DATE NOT NULL,
salary INT,
dept_id INT,
FOREIGN KEY (dept_id) REFERENCES departments(dept_id)
);
-- Insert Data into departments
INSERT INTO departments (dept_name) VALUES ('Sales');
INSERT INTO departments (dept_name) VALUES ('Marketing');
INSERT INTO departments (dept_name) VALUES ('Engineering');
-- Insert Data into employees_fk
INSERT INTO employees_fk (emp_name, hire_date, salary, dept_id) VALUES
('Alice Johnson', '2024-01-15', 60000, 1),
('Bob Smith', '2024-02-20', 55000, 2),
('Carol White', '2024-03-25', 70000, 3);
CHECK Constraint
The CHECK
constraint is used to restrict the values that can be placed in a column.
For example, the range of values for a salary column can be limited by creating a CHECK
constraint that allows values only from 3,000 to 10,000. This prevents salaries from being entered beyond the regular salary range. Here’s an example:
Example
CREATE TABLE employees (
emp_id INT NOT NULL PRIMARY KEY,
emp_name VARCHAR(55) NOT NULL,
hire_date DATE NOT NULL,
salary INT NOT NULL CHECK (salary >= 3000 AND salary <= 10000),
dept_id INT,
FOREIGN KEY (dept_id) REFERENCES departments(dept_id)
);