Unique constraints and indexes (MySQL)
Overview
This page explains how to configure a unique constraint / index in your MySQL database. Constraints and indexes are very similar in MySQL (learn more here): When adding a unique constraint to one or more columns, MySQL will always create a corresponding unique index.
In this guide, you'll always configure unique constraints (which will automatically configure unique indexes as well).
You can configure unique constraints either on a single column or on multiple columns. These can be added when you create the table initially (using CREATE TABLE
) or to an already existing table (using ALTER TABLE
). This guide covers all four combinations.
At the end of the guide, you'll introspect your database to reflect the unique constraint in the Prisma schema, then you'll generate Prisma Client and write a simple Node.js script to validate the constraints.
Prerequisites
In order to follow this guide, you need:
- a MySQL database server running
- the
mysql
command line client for MySQL - Node.js installed on your machine
Note: If you are using a version of MySQL where MyISAM is the default engine, you must specify
ENGINE = InnoDB;
when you create a table.
1. Create a new database and project directory
Start by creating a project directory where you can put the files you'll create throughout this guide:
mkdir unique-democd unique-demo
Next, make sure that your MySQL database server is running. Then execute the following command in your terminal to create a new database called UniqueDemo
:
mysql -e 'CREATE DATABASE UniqueDemo;'
Note: When invoking the
mysql
CLI, you might need to authenticate against your MySQL server. To do so, you need to pass two options to the command:
-u __USERNAME__
: The database user-p
: Enable a password prompt for authenticationHere's a sample invocation for a user called
root
using the above command:
mysql -u root -p -e 'CREATE DATABASE UniqueDemo;'
You can validate that the database was created by running the SHOW TABLES
command which lists all tables (relations) in your database (right now there are none):
mysql -e 'SHOW TABLES in UniqueDemo;'
2. Create a table with a single-column unique constraint and index
In this section, you'll create a new table with a single-column unique constraint in the UniqueDemo
database. As mentioned above, this means that MySQL automatically adds a unique index to the same column.
Create a new file called single-column-unique.sql
and add the following code to it:
CREATE TABLE `UniqueDemo`.`User` (`id` INT AUTO_INCREMENT PRIMARY KEY,`email` VARCHAR(350) UNIQUE);
Now run the SQL statement against your database to create a new table called User
:
mysql < single-column-unique.sql
Congratulations, you just created a table called User
in the database. The table has an id
column and a column called email
on which you defined a unique constraint. MySQL also automatically added a corresponding unique index (do not run this code):
CREATE UNIQUE INDEX `email` ON `UniqueDemo`.`User`(`email`);
In the code above, you created the unique constraint as a column constraint. Alternatively, you can define it as a table constraint. There's no practical difference between the two, the alternative is just added for completeness.
To add the unique constraint as a table constraint, you need to adjust your SQL statement to look as follows:
CREATE TABLE `UniqueDemo`.`User` (`id` INT AUTO_INCREMENT PRIMARY KEY,`email` VARCHAR(350),UNIQUE (`email`));
3. Create a table with a multi-column unique constraint and index
Next, you'll create a table with a multi-column unique constraint. This also adds a unique index to the columns with the constraint.
Create a new file called multi-column-unique.sql
and add the following code to it:
CREATE TABLE `UniqueDemo`.`AnotherUser` (`id` INT AUTO_INCREMENT PRIMARY KEY,`firstName` VARCHAR(256),`lastName` VARCHAR(256),UNIQUE (`firstName`, `lastName`));
Now run the SQL statement against your database to create a new table called AnotherUser
:
mysql < multi-column-unique.sql
Congratulations, you just created a table called AnotherUser
in the database. The table has an id
column and two column called firstName
and lastName
on which you defined a unique index. MySQL also automatically added a corresponding unique index (do not run this code):
CREATE UNIQUE INDEX `firstName` ON `UniqueDemo`.`AnotherUser`(`firstName`, `lastName`);
4. Adding a single-column unique constraint to an existing table
In this section, you'll add a single-column unique constraint to a table that already exists in your database. To do so, you first need to create a new table and then alter the table to add the constraint.
Create a new file called add-single-unique-constraint-later.sql
and add the following code:
CREATE TABLE `UniqueDemo`.`OneMoreUser` (`id` INT AUTO_INCREMENT PRIMARY KEY,email VARCHAR(350));ALTER TABLE `UniqueDemo`.`OneMoreUser` ADD CONSTRAINT `OneMoreUser_email_unique_constraint` UNIQUE (`email`);
This code contains two SQL statements:
- Create a new table called
OneMoreUser
- Alter the table to add an unique constraint
Now run the SQL statements against your database to create a new table called OneMoreUser
:
mysql < add-single-unique-constraint-later.sql
Congratulations, you just created a table called OneMoreUser
in the database. The table has an id
column and a column called email
on which you later added a unique constraint in the second SQL statement. MySQL also automatically added a corresponding unique index (do not run this code):
CREATE UNIQUE INDEX `OneMoreUser_email_unique_constraint` ON `UniqueDemo`.`OneMoreUser`(`email`);
5. Adding a multi-column unique constraint to an existing table
In this section, you'll add a multi-column unique constraint to a table that already exists in your database. To do so, you first need to create a new table and then alter the table to add the constraint.
Create a new file called add-multi-unique-constraint-later.sql
and add the following code:
CREATE TABLE `UniqueDemo`.`TheLastUser` (`id` INT AUTO_INCREMENT PRIMARY KEY,`firstName` VARCHAR(256),`lastName` VARCHAR(256));ALTER TABLE `UniqueDemo`.`TheLastUser` ADD CONSTRAINT `TheLastUser_firstName_lastName_unique_constraint` UNIQUE (`firstName`, `lastName`);
This code contains two SQL statements:
- Create a new table called
TheLastUser
- Alter the table to add an unique constraint
Now run the SQL statements against your database to create a new table called OneMoreUser
:
mysql < add-multi-unique-constraint-later.sql
Congratulations, you just created a table called OneMoreUser
in the database. The table has an id
column and two columns called firstName
and lastName
on which you later added a unique constraint in the second SQL statement. MySQL also automatically added a corresponding unique index (do not run this code):
CREATE UNIQUE INDEX `TheLastUser_firstName_lastName_unique_constraint` ON `UniqueDemo`.`TheLastUser`(`firstname`, `lastname`);
6. Introspect your database with Prisma
In the previous sections, you created four tables with unique constraints:
- The table
User
has a singe-column unique constraint and index on theemail
column - The table
AnotherUser
has a multi-column unique constraint and index on thefirstName
andlastName
columns - The table
OneMoreUser
has a singe-column unique constraint and index on theemail
column - The table
TheLastUser
has a multi-column unique constraint and index on thefirstName
andlastName
columns
In this section you'll introspect your database to generate the Prisma models for these tables.
To start, set up a new Node.js project and add the prisma
CLI as a development dependency:
npm init -ynpm install prisma --save-dev
In order to introspect your database, you need to tell Prisma how to connect to it. You do so by configuring a datasource
in your Prisma schema.
Create a new file called schema.prisma
and add the following code to it:
datasource db {provider = "mysql"url = env("DATABASE_URL")}
The database connection URL is set via an environment variable. The Prisma CLI automatically supports the dotenv
format which automatically picks up environment variables defined in a file called .env
.
Create a new file called .env
and set your database connection URL as the DATABASE_URL
environment variable:
DATABASE_URL=mysql://__USER__:__PASSWORD__@__HOST__:__PORT__/UniqueDemo
In the above code snippet, you need to replace the uppercase placeholders with your own connection details. For example, if your database is running locally it could look like this:
DATABASE_URL=mysql://janedoe:mypassword@localhost:3306/UniqueDemo
With both the schema.prisma
and .env
files in place, you can run Prisma's introspection with the following command:
npx prisma introspect
This command introspects your database and for each table adds a Prisma model to the Prisma schema:
datasource db {provider = "mysql"url = env("DATABASE_URL")}model AnotherUser {firstName String?id Int @default(autoincrement()) @idlastName String?@@unique([firstName, lastName], name: "firstName")}model OneMoreUser {email String? @uniqueid Int @default(autoincrement()) @id}model TheLastUser {firstName String?id Int @default(autoincrement()) @idlastName String?@@unique([firstName, lastName], name: "TheLastUser_firstName_lastName_unique_constraint")}model User {email String? @uniqueid Int @default(autoincrement()) @id}
7. Generate Prisma Client
To validate whether the unique constraints work, you'll now generate Prisma Client and send a few sample queries to the database.
First, add a generator
block to your Prisma schema (typically added right below the datasource
block):
generator client {provider = "prisma-client-js"}
Run the following command to install and generate Prisma Client in your project:
npx prisma generate
Now you can use Prisma Client to send database queries in Node.js.
8. Validate the unique constraints in a Node.js script
Create a new file called index.js
and add the following code to it:
const { PrismaClient } = require('@prisma/client');const prisma = new PrismaClient();async function main() {const newUser1 = await prisma.user.create({data: {email: 'alice@prisma.io',},});console.log(newUser1);const newUser2 = await prisma.user.create({data: {email: 'alice@prisma.io',},});console.log(newUser2);}main();
In this code, you're creating two users with the same email
, so you're violating the unique constraint that's configured on the User
table.
Run the code with this command:
node index.js
After newUser1
gets printed to the console succesfully, the script throws an error indicating that the unique constraint on email
is violated:
{ email: 'alice@prisma.io', id: 1 }(node:6883) UnhandledPromiseRejectionWarning: Error:Invalid `newUser2 = await prisma.user.create()` invocation in/Users/janedoe/unique-demo/index.js:12:388 email: "alice@prisma.io"9 }10 })11 console.log(newUser1)→ 12 const newUser2 = await prisma.user.create(Unique constraint failed on the constraint: `email`
To validate the multi-column unique constraint, replace the code in index.js
with the following:
const { PrismaClient } = require('@prisma/client');const prisma = new PrismaClient();async function main() {const newUser1 = await prisma.anotherUser.create({data: {firstName: 'Alice',lastName: 'Smith',},});console.log(newUser1);const newUser2 = await prisma.anotherUser.create({data: {firstName: 'Alice',lastName: 'Smith',},});console.log(newUser2);}main();
Run the script again with this command:
node index.js
This time, you'll see a similar error message indicating the unique constraint on firstName
and lastName
was violated:
{ firstName: 'Alice', id: 1, lastName: 'Smith' }(node:6913) UnhandledPromiseRejectionWarning: Error:Invalid `newUser2 = await prisma.anotherUser.create()` invocation in/Users/janedoe/unique-demo/index.js:13:459 lastName: "Smith"10 }11 })12 console.log(newUser1)→ 13 const newUser2 = await prisma.anotherUser.create(Unique constraint failed on the constraint: `firstName`
Note that you can add NULL
values for these columns without violating the constraints. For example, the following code snippet will not fail:
const { PrismaClient } = require('@prisma/client');const prisma = new PrismaClient();async function main() {const newUser1 = await prisma.user.create({ data: {} });console.log(newUser1);const newUser2 = await prisma.user.create({ data: {} });console.log(newUser2);}main();
It will create two new records where the email
is set to NULL
in the database.