Summary: in this tutorial, you will learn how to use the SQLite UNIQUE constraint to ensure all values in a column or a group of columns are unique.
Introduction to SQLite UNIQUE constraint
A UNIQUE constraint ensures all values in a column or a group of columns are distinct from one another or unique.
To define a UNIQUE constraint, you use the UNIQUE keyword followed by one or more columns.
You can define a UNIQUE constraint at the column or the table level. Only at the table level, you can define a UNIQUE constraint across multiple columns.
The following shows how to define a UNIQUE constraint for a column at the column level:
1 2 3 4 5 | CREATE TABLE table_name( ..., column_name type UNIQUE, ... ); |
Or at the table level:
1 2 3 4 | CREATE TABLE table_name( ..., UNIQUE(column_name) ); |
The following illustrates how to define a UNIQUE constraint for multiple columns:
1 2 3 4 | CREATE TABLE table_name( ..., UNIQUE(column_name1,column_name2,...) ); |
Once a UNIQUE constraint is defined, if you attempt to insert or update a value that already exists in the column, SQLite will issue an error and abort the operation.
SQLite UNIQUE constraint examples
Let’s take some examples of using the UNIQUE constraint.
Defining a UNIQUE constraint for one column example
The following statement creates a new table named contacts with a UNIQUE constraint defined for the email column:
1 2 3 4 5 6 | CREATE TABLE contacts( contact_id INTEGER PRIMARY KEY, first_name TEXT, last_name TEXT, email TEXT NOT NULL UNIQUE ); |
The following example inserts a new row into the contacts table:
1 2 | INSERT INTO contacts(first_name,last_name,email) VALUES ('John','Doe','john.doe@gmail.com'); |
If you attempt to insert a new contact with the same email, you will get an error message:
1 2 | INSERT INTO contacts(first_name,last_name,email) VALUES ('Johnny','Doe','john.doe@gmail.com'); |
Here is the error message:
1 | Error while executing SQL query on database 'chinook': UNIQUE constraint failed: contacts.email |
Defining a UNIQUE constraint for multiple columns example
The following statement creates the shapes table with a UNIQUE constraint defined for the background_color and foreground_color columns:
1 2 3 4 5 6 | CREATE TABLE shapes( shape_id INTEGER PRIMARY KEY, background_color TEXT, foreground_color TEXT, UNIQUE(background_color,foreground_color) ); |
The following statement inserts a new row into the shapes table:
1 2 | INSERT INTO shapes(background_color,foreground_color) VALUES('red','green'); |
The following statement works because of no duplication violation in both background_color and foreground_color columns:
1 2 | INSERT INTO shapes(background_color,foreground_color) VALUES('red','blue'); |
However, the following statement causes an error due to the duplicates in both background_color and foreground_color columns:
1 2 | INSERT INTO shapes(background_color,foreground_color) VALUES('red','green'); |
Here is the error:
1 | Error while executing SQL query on database 'chinook': `UNIQUE` constraint failed: shapes.background_color, shapes.foreground_color |
SQLite UNIQUE constraint and NULL
SQLite treats all NULL values are different, therefore, a column with a UNIQUE constraint can have multiple NULL values.
The following statement creates a new table named lists whose email column has a UNIQUE constraint:
1 2 3 4 | CREATE TABLE lists( list_id INTEGER PRIMARY KEY, email TEXT UNIQUE ); |
The following statement inserts multiple NULL values into the email column of the lists table:
1 2 | INSERT INTO lists(email) VALUES(NULL),(NULL); |
Let’s query data from the lists table:
1 | SELECT * FROM lists; |
Here is the output:
As you can see, even though the email column has a UNIQUE constraint, it can accept multiple NULL values.
In this tutorial, you have learned how to use the SQLite UNIQUE constraint to ensure all values in a column or a group of columns are unique.