Unique constraints and indexes (SQLite)

Overview

This page explains how to configure a unique constraint / index in your SQLite database. Constraints and indexes are very similar in SQLite: When adding a unique constraint to one or more columns, SQLite will always create a corresponding unique index.

In this guide, you'll always configure unique constraints (which will automatically configure unique indexes as well).

Since SQLite does not support ALTER TABLE statements with the ADD CONSTRAINT operation, constraints can only be added when you create the table initially (using CREATE TABLE). You can configure unique constraints either on a single column or on multiple columns. This guide covers both of the supported 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:

• the sqlite3 command line client for SQLite
• Node.js installed on your machine

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-demo
cd unique-demo

Afterwards, execute the following command in your terminal to create a new database called UniqueDemo:

sqlite3 UniqueDemo.db

You can validate that the database was created by running the .tables command which lists all tables (relations) in your database (right now there are none):

sqlite3 UniqueDemo.db '.tables'

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 SQLite 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 "User" (
  "id" INTEGER PRIMARY KEY,
  "email" TEXT UNIQUE
);

Now run the SQL statement against your database to create a new table called User:

sqlite3 UniqueDemo.db < 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 index. SQLite also automatically added a corresponding unique index (do not run this code):

CREATE UNIQUE INDEX "sqlite_autoindex_User_1" ON "User"("email");

CREATE TABLE "User" (
  "id" INTEGER PRIMARY KEY,
  "email" TEXT,
  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 "AnotherUser" (
  "id" INTEGER PRIMARY KEY,
  "firstName" TEXT,
  "lastName" TEXT,
  UNIQUE ("firstName", "lastName")
);

Now run the SQL statement against your database to create a new table called AnotherUser:

sqlite3 UniqueDemo.db < 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. SQLite also automatically added a corresponding unique index (do not run this code):

CREATE UNIQUE INDEX "sqlite_autoindex_AnotherUser_1" ON "AnotherUser"("firstName", "lastName");

4. Introspect your database with Prisma

In the previous sections, you created two tables with unique constraints:

• The table User has a singe-column unique constraint and index on the email column
• The table AnotherUser has a multi-column unique constraint and index on the firstName and lastName 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 -y
npm 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 = "sqlite"
  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=file:UniqueDemo.db

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 = "sqlite"
  url      = env("DATABASE_URL")
}
model User {
  email String? @unique
  id    Int     @default(autoincrement()) @id
}
model AnotherUser {
  firstName String?
  id        Int     @default(autoincrement()) @id
  lastName  String?
  @@unique([firstName, lastName], name: "sqlite_autoindex_AnotherUser_1")
}

5. 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.

6. 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:14072) UnhandledPromiseRejectionWarning: Error:
Invalid `newUser2 = await prisma.user.create()` invocation in
/Users/janedoe/unique-demo/index.js:12:38
   8     email: "alice@prisma.io"
   9   }
  10 })
  11 console.log(newUser1)
→ 12 const newUser2 = await prisma.user.create(Unique constraint failed on the fields: (`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:14273) UnhandledPromiseRejectionWarning: Error:
Invalid `newUser2 = await prisma.anotherUser.create()` invocation in
/Users/janedoe/unique-demo/index.js:13:45
   9     lastName: "Smith"
  10   }
  11 })
  12 console.log(newUser1)
→ 13 const newUser2 = await prisma.anotherUser.create(Unique constraint failed on the fields: (`firstName`,`lastName`)

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.