- 1. API with NestJS #1. Controllers, routing and the module structure
- 2. API with NestJS #2. Setting up a PostgreSQL database with TypeORM
- 3. API with NestJS #3. Authenticating users with bcrypt, Passport, JWT, and cookies
- 4. API with NestJS #4. Error handling and data validation
- 5. API with NestJS #5. Serializing the response with interceptors
- 6. API with NestJS #6. Looking into dependency injection and modules
- 7. API with NestJS #7. Creating relationships with Postgres and TypeORM
- 8. API with NestJS #8. Writing unit tests
- 9. API with NestJS #9. Testing services and controllers with integration tests
- 10. API with NestJS #10. Uploading public files to Amazon S3
- 11. API with NestJS #11. Managing private files with Amazon S3
- 12. API with NestJS #12. Introduction to Elasticsearch
- 13. API with NestJS #13. Implementing refresh tokens using JWT
- 14. API with NestJS #14. Improving performance of our Postgres database with indexes
- 15. API with NestJS #15. Defining transactions with PostgreSQL and TypeORM
- 16. API with NestJS #16. Using the array data type with PostgreSQL and TypeORM
- 17. API with NestJS #17. Offset and keyset pagination with PostgreSQL and TypeORM
- 18. API with NestJS #18. Exploring the idea of microservices
- 19. API with NestJS #19. Using RabbitMQ to communicate with microservices
- 20. API with NestJS #20. Communicating with microservices using the gRPC framework
- 21. API with NestJS #21. An introduction to CQRS
- 22. API with NestJS #22. Storing JSON with PostgreSQL and TypeORM
- 23. API with NestJS #23. Implementing in-memory cache to increase the performance
- 24. API with NestJS #24. Cache with Redis. Running the app in a Node.js cluster
- 25. API with NestJS #25. Sending scheduled emails with cron and Nodemailer
- 26. API with NestJS #26. Real-time chat with WebSockets
- 27. API with NestJS #27. Introduction to GraphQL. Queries, mutations, and authentication
- 28. API with NestJS #28. Dealing in the N + 1 problem in GraphQL
- 29. API with NestJS #29. Real-time updates with GraphQL subscriptions
- 30. API with NestJS #30. Scalar types in GraphQL
- 31. API with NestJS #31. Two-factor authentication
- 32. API with NestJS #32. Introduction to Prisma with PostgreSQL
- 33. API with NestJS #33. Managing PostgreSQL relationships with Prisma
- 34. API with NestJS #34. Handling CPU-intensive tasks with queues
- 35. API with NestJS #35. Using server-side sessions instead of JSON Web Tokens
- 36. API with NestJS #36. Introduction to Stripe with React
- 37. API with NestJS #37. Using Stripe to save credit cards for future use
- 38. API with NestJS #38. Setting up recurring payments via subscriptions with Stripe
- 39. API with NestJS #39. Reacting to Stripe events with webhooks
- 40. API with NestJS #40. Confirming the email address
- 41. API with NestJS #41. Verifying phone numbers and sending SMS messages with Twilio
- 42. API with NestJS #42. Authenticating users with Google
- 43. API with NestJS #43. Introduction to MongoDB
- 44. API with NestJS #44. Implementing relationships with MongoDB
- 45. API with NestJS #45. Virtual properties with MongoDB and Mongoose
- 46. API with NestJS #46. Managing transactions with MongoDB and Mongoose
- 47. API with NestJS #47. Implementing pagination with MongoDB and Mongoose
- 48. API with NestJS #48. Definining indexes with MongoDB and Mongoose
- 49. API with NestJS #49. Updating with PUT and PATCH with MongoDB and Mongoose
- 50. API with NestJS #50. Introduction to logging with the built-in logger and TypeORM
- 51. API with NestJS #51. Health checks with Terminus and Datadog
- 52. API with NestJS #52. Generating documentation with Compodoc and JSDoc
- 53. API with NestJS #53. Implementing soft deletes with PostgreSQL and TypeORM
- 54. API with NestJS #54. Storing files inside a PostgreSQL database
- 55. API with NestJS #55. Uploading files to the server
- 56. API with NestJS #56. Authorization with roles and claims
- 57. API with NestJS #57. Composing classes with the mixin pattern
- 58. API with NestJS #58. Using ETag to implement cache and save bandwidth
- 59. API with NestJS #59. Introduction to a monorepo with Lerna and Yarn workspaces
- 60. API with NestJS #60. The OpenAPI specification and Swagger
- 61. API with NestJS #61. Dealing with circular dependencies
- 62. API with NestJS #62. Introduction to MikroORM with PostgreSQL
- 63. API with NestJS #63. Relationships with PostgreSQL and MikroORM
- 64. API with NestJS #64. Transactions with PostgreSQL and MikroORM
- 65. API with NestJS #65. Implementing soft deletes using MikroORM and filters
- 66. API with NestJS #66. Improving PostgreSQL performance with indexes using MikroORM
- 67. API with NestJS #67. Migrating to TypeORM 0.3
- 68. API with NestJS #68. Interacting with the application through REPL
- 69. API with NestJS #69. Database migrations with TypeORM
- 70. API with NestJS #70. Defining dynamic modules
- 71. API with NestJS #71. Introduction to feature flags
- 72. API with NestJS #72. Working with PostgreSQL using raw SQL queries
- 73. API with NestJS #73. One-to-one relationships with raw SQL queries
- 74. API with NestJS #74. Designing many-to-one relationships using raw SQL queries
- 75. API with NestJS #75. Many-to-many relationships using raw SQL queries
- 76. API with NestJS #76. Working with transactions using raw SQL queries
- 77. API with NestJS #77. Offset and keyset pagination with raw SQL queries
- 78. API with NestJS #78. Generating statistics using aggregate functions in raw SQL
- 79. API with NestJS #79. Implementing searching with pattern matching and raw SQL
- 80. API with NestJS #80. Updating entities with PUT and PATCH using raw SQL queries
- 81. API with NestJS #81. Soft deletes with raw SQL queries
- 82. API with NestJS #82. Introduction to indexes with raw SQL queries
- 83. API with NestJS #83. Text search with tsvector and raw SQL
- 84. API with NestJS #84. Implementing filtering using subqueries with raw SQL
- 85. API with NestJS #85. Defining constraints with raw SQL
- 86. API with NestJS #86. Logging with the built-in logger when using raw SQL
- 87. API with NestJS #87. Writing unit tests in a project with raw SQL
- 88. API with NestJS #88. Testing a project with raw SQL using integration tests
- 89. API with NestJS #89. Replacing Express with Fastify
- 90. API with NestJS #90. Using various types of SQL joins
- 91. API with NestJS #91. Dockerizing a NestJS API with Docker Compose
- 92. API with NestJS #92. Increasing the developer experience with Docker Compose
- 93. API with NestJS #93. Deploying a NestJS app with Amazon ECS and RDS
- 94. API with NestJS #94. Deploying multiple instances on AWS with a load balancer
- 95. API with NestJS #95. CI/CD with Amazon ECS and GitHub Actions
- 96. API with NestJS #96. Running unit tests with CI/CD and GitHub Actions
- 97. API with NestJS #97. Introduction to managing logs with Amazon CloudWatch
- 98. API with NestJS #98. Health checks with Terminus and Amazon ECS
- 99. API with NestJS #99. Scaling the number of application instances with Amazon ECS
- 100. API with NestJS #100. The HTTPS protocol with Route 53 and AWS Certificate Manager
- 101. API with NestJS #101. Managing sensitive data using the AWS Secrets Manager
- 102. API with NestJS #102. Writing unit tests with Prisma
- 103. API with NestJS #103. Integration tests with Prisma
- 104. API with NestJS #104. Writing transactions with Prisma
- 105. API with NestJS #105. Implementing soft deletes with Prisma and middleware
- 106. API with NestJS #106. Improving performance through indexes with Prisma
- 107. API with NestJS #107. Offset and keyset pagination with Prisma
- 108. API with NestJS #108. Date and time with Prisma and PostgreSQL
- 109. API with NestJS #109. Arrays with PostgreSQL and Prisma
- 110. API with NestJS #110. Managing JSON data with PostgreSQL and Prisma
- 111. API with NestJS #111. Constraints with PostgreSQL and Prisma
- 112. API with NestJS #112. Serializing the response with Prisma
- 113. API with NestJS #113. Logging with Prisma
- 114. API with NestJS #114. Modifying data using PUT and PATCH methods with Prisma
- 115. API with NestJS #115. Database migrations with Prisma
- 116. API with NestJS #116. REST API versioning
- 117. API with NestJS #117. CORS – Cross-Origin Resource Sharing
- 118. API with NestJS #118. Uploading and streaming videos
- 119. API with NestJS #119. Type-safe SQL queries with Kysely and PostgreSQL
- 120. API with NestJS #120. One-to-one relationships with the Kysely query builder
- 121. API with NestJS #121. Many-to-one relationships with PostgreSQL and Kysely
- 122. API with NestJS #122. Many-to-many relationships with Kysely and PostgreSQL
- 123. API with NestJS #123. SQL transactions with Kysely
- 124. API with NestJS #124. Handling SQL constraints with Kysely
- 125. API with NestJS #125. Offset and keyset pagination with Kysely
- 126. API with NestJS #126. Improving the database performance with indexes and Kysely
- 127. API with NestJS #127. Arrays with PostgreSQL and Kysely
- 128. API with NestJS #128. Managing JSON data with PostgreSQL and Kysely
- 129. API with NestJS #129. Implementing soft deletes with SQL and Kysely
- 130. API with NestJS #130. Avoiding storing sensitive information in API logs
- 131. API with NestJS #131. Unit tests with PostgreSQL and Kysely
- 132. API with NestJS #132. Handling date and time in PostgreSQL with Kysely
- 133. API with NestJS #133. Introducing database normalization with PostgreSQL and Prisma
- 134. API with NestJS #134. Aggregating statistics with PostgreSQL and Prisma
- 135. API with NestJS #135. Referential actions and foreign keys in PostgreSQL with Prisma
- 136. API with NestJS #136. Raw SQL queries with Prisma and PostgreSQL range types
- 137. API with NestJS #137. Recursive relationships with Prisma and PostgreSQL
- 138. API with NestJS #138. Filtering records with Prisma
- 139. API with NestJS #139. Using UUID as primary keys with Prisma and PostgreSQL
- 140. API with NestJS #140. Using multiple PostgreSQL schemas with Prisma
- 141. API with NestJS #141. Getting distinct records with Prisma and PostgreSQL
- 142. API with NestJS #142. A video chat with WebRTC and React
- 143. API with NestJS #143. Optimizing queries with views using PostgreSQL and Kysely
- 144. API with NestJS #144. Creating CLI applications with the Nest Commander
- 145. API with NestJS #145. Securing applications with Helmet
- 146. API with NestJS #146. Polymorphic associations with PostgreSQL and Prisma
- 147. API with NestJS #147. The data types to store money with PostgreSQL and Prisma
- 148. API with NestJS #148. Understanding the injection scopes
- 149. API with NestJS #149. Introduction to the Drizzle ORM with PostgreSQL
- 150. API with NestJS #150. One-to-one relationships with the Drizzle ORM
- 151. API with NestJS #151. Implementing many-to-one relationships with Drizzle ORM
- 152. API with NestJS #152. SQL constraints with the Drizzle ORM
- 153. API with NestJS #153. SQL transactions with the Drizzle ORM
- 154. API with NestJS #154. Many-to-many relationships with Drizzle ORM and PostgreSQL
- 155. API with NestJS #155. Offset and keyset pagination with the Drizzle ORM
- 156. API with NestJS #156. Arrays with PostgreSQL and the Drizzle ORM
- 157. API with NestJS #157. Handling JSON data with PostgreSQL and the Drizzle ORM
- 158. API with NestJS #158. Soft deletes with the Drizzle ORM
- 159. API with NestJS #159. Date and time with PostgreSQL and the Drizzle ORM
- 160. API with NestJS #160. Using views with the Drizzle ORM and PostgreSQL
- 161. API with NestJS #161. Generated columns with the Drizzle ORM and PostgreSQL
- 162. API with NestJS #162. Identity columns with the Drizzle ORM and PostgreSQL
- 163. API with NestJS #163. Full-text search with the Drizzle ORM and PostgreSQL
- 164. API with NestJS #164. Improving the performance with indexes using Drizzle ORM
- 165. API with NestJS #165. Time intervals with the Drizzle ORM and PostgreSQL
- 166. API with NestJS #166. Logging with the Drizzle ORM
- 167. API with NestJS #167. Unit tests with the Drizzle ORM
- 168. API with NestJS #168. Integration tests with the Drizzle ORM
- 169. API with NestJS #169. Unique IDs with UUIDs using Drizzle ORM and PostgreSQL
- 170. API with NestJS #170. Polymorphic associations with PostgreSQL and Drizzle ORM
- 171. API with NestJS #171. Recursive relationships with Drizzle ORM and PostgreSQL
- 172. API with NestJS #172. Database normalization with Drizzle ORM and PostgreSQL
- 173. API with NestJS #173. Storing money with Drizzle ORM and PostgreSQL
- 174. API with NestJS #174. Multiple PostgreSQL schemas with Drizzle ORM
- 175. API with NestJS #175. PUT and PATCH requests with PostgreSQL and Drizzle ORM
- 176. API with NestJS #176. Database migrations with the Drizzle ORM
- 177. API with NestJS #177. Response serialization with the Drizzle ORM
PostgreSQL allows us to filter a query’s results and ensure we don’t get duplicate rows. This can be helpful when your table has many rows where the data in the columns is the same. In this article, we explore two ways PostgreSQL helps us solve this problem. We also learn how to do that with Prisma and configure it to use the native database features instead of filtering the duplicates in memory.
The DISTINCT keyword
Let’s say we have the following model created with Prisma.
schema.prisma
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model Address { id Int @id @default(autoincrement()) street String city String country String user User? } model User { id Int @id @default(autoincrement()) email String @unique name String password String address Address? @relation(fields: [addressId], references: [id]) addressId Int? @unique } |
Above, we define a relationship between the users and the addresses. If you want to know more, check out API with NestJS #33. Managing PostgreSQL relationships with Prisma
Let’s say that we have the following addresses in our table:
id | street | city | country |
---|---|---|---|
1 | 350 5th Ave | New York | United States |
2 | 290 Bremner Blvd | Toronto | Canada |
3 | Westminster | London | United Kingdom |
4 | Great Russell St | London | United Kingdom |
6 | 1600 Pennsylvania Ave NW | Washington, D.C. | United States |
7 | 1000 5th Ave | New York | United States |
8 | Hill Rise | Richmond | United Kingdom |
9 | East Main Street | Richmond | United States |
First, let’s create a query to get a list of all city names from our database.
1 |
SELECT city FROM "Address"; |
city |
---|
New York |
Toronto |
London |
London |
Washington, D.C. |
New York |
Richmond |
Richmond |
We can modify the above query using the DISTINCT keyword to get a list of unique cities.
1 |
SELECT DISTINCT city FROM "Address"; |
city |
---|
New York |
Washington, D.C. |
London |
Toronto |
Richmond |
Using multiple columns
When we look closer at the list of addresses, we can see a city called Richmond, both in the UK and the United States. Fortunately, we can use the DISTINCT keyword to get a unique combination of the city and country.
1 |
SELECT DISTINCT city, country FROM "Address"; |
city | country |
---|---|
New York | United States |
Toronto | Canada |
London | United Kingdom |
Washington, D.C. | United States |
Richmond | United Kingdom |
Richmond | United States |
The DISTINCT ON keyword
With PostgreSQL, we can access another handy tool called the DISTINCT ON. It is similar to DISTINCT but allows us to retain other columns from the row.
1 |
SELECT DISTINCT ON (city) * FROM "Address"; |
id | street | city | country |
---|---|---|---|
3 | Westminster | London | United Kingdom |
7 | 1000 5th Ave | New York | United States |
9 | East Main Street | Richmond | United States |
2 | 290 Bremner Blvd | Toronto | Canada |
6 | 1600 Pennsylvania Ave NW | Washington, D.C. | United States |
Above, we selected distinct cities, but we also see ids, countries, and streets.
The catch is that DISTINCT ON selected one row per each distinct value, but it is unpredictable. In the case of New York, it removed 350 5th Ave (the Empire State Building) and selected 1000 5th Ave (The Metropolitan Museum of Art).
The DISTINCT ON keyword makes the most sense when combined with the ORDER BY clause. If we order the results before applying the DISTINCT ON filter, we can be sure that PostgreSQL will always choose the first row based on the order we specified. Thanks to that, we can have predictable results.
Let’s use DISTINCT ON to find the first user who signed up with an address from a particular country.
First, we need to join the User and Address table.
1 2 3 |
SELECT "User".id AS userId, "Address".country AS country FROM "User" JOIN "Address" ON "User"."addressId" = "Address".id |
If you want to learn more about joins, check out API with NestJS #90. Using various types of SQL joins
Let’s make sure each country is unique using the DISTINCT ON clause. By ordering the rows by the user ID, we ensure we get the users with the lowest ID possible. Thanks to that, we get the first users who signed up from a particular country.
1 2 3 4 |
SELECT DISTINCT ON(country) "User".id AS "firstRegisteredUserId", "Address".country AS country FROM "User" JOIN "Address" ON "User"."addressId" = "Address".id ORDER BY "Address".country, "firstRegisteredUserId" |
firstRegisteredUserId | country |
---|---|
3 | Canada |
4 | United Kingdom |
2 | United States |
Finding distinct values using Prisma
Prisma allows us to filter duplicate rows when using the findMany query.
1 2 3 4 5 6 |
const distinctCities = await this.prismaService.address.findMany({ distinct: ['city'], select: { city: true } }) |
We can take it further and replicate the example we wrote before using the DISTINCT ON keyword.
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const countriesWithFirstUser = await this.prismaService.address.findMany({ include: { user: true, }, distinct: ['country'], orderBy: { user: { id: 'asc', }, }, }); |
How it works under the hood
Let’s configure Prisma to log all SQL queries to the console.
prisma.service.ts
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import { Injectable, OnModuleInit } from '@nestjs/common'; import { PrismaClient, Prisma } from '@prisma/client'; @Injectable() export class PrismaService extends PrismaClient<Prisma.PrismaClientOptions, Prisma.LogLevel> implements OnModuleInit { constructor() { super({ log: [ { emit: 'event', level: 'query', }, ], }); } async onModuleInit() { await this.$connect(); this.$on('query', (event) => { console.log(`Query: ${event.query}`); }); } } |
If you want to know more about logging with Prisma and NestJS, take a look at API with NestJS #113. Logging with Prisma
Let’s run our findMany query that finds all distinct cities and inspect the SQL query that Prisma makes under the hood:
1 |
SELECT "public"."Address"."id", "public"."Address"."city" FROM "public"."Address" WHERE 1=1 OFFSET $1 |
Unfortunately, there is a catch. By default, Prisma makes separate SELECT queries and processes the data in memory when we use distinct. This can result in a performance that is not as good as a native SQL query that uses the DISTINCT keyword. Since Prisma 5.7.0, we can affect that by using a preview feature.
schema.prisma
1 2 3 4 |
generator client { provider = "prisma-client-js" previewFeatures = ["nativeDistinct"] } |
When we add the nativeDistinct preview feature and run npx prisma generate, Prisma starts using the DISTINCT ON clause.
1 |
SELECT DISTINCT ON ("public"."Address"."city") "public"."Address"."id", "public"."Address"."city" FROM "public"."Address" WHERE 1=1 OFFSET $1 |
Unfortunately, it only works with unordered queries right now. If we run our findMany query that finds the first users who signed up from a particular country, Prisma still parses the data in memory instead of using DISTINCT ON:
1 2 |
SELECT "public"."Address"."id", "public"."Address"."street", "public"."Address"."city", "public"."Address"."country" FROM "public"."Address" LEFT JOIN "public"."User" AS "orderby_1" ON ("orderby_1"."addressId") = ("public"."Address"."id") WHERE 1=1 ORDER BY "orderby_1"."id" ASC OFFSET $1; SELECT "public"."User"."id", "public"."User"."email", "public"."User"."name", "public"."User"."password", "public"."User"."addressId" FROM "public"."User" WHERE "public"."User"."addressId" IN ($1,$2,$3) OFFSET $4; |
Hopefully, Prisma will improve its distinct implementation soon and use the native DISTINCT ON in more cases. We can track the progress of this issue on GitHub.
Summary
PostgreSQL offers efficient methods to filter out duplicate rows in queries, which helps handle large datasets with repetitive data. In this article, we explored the DISTINCT and DISTINCT ON keywords in PostgreSQL and compared them using various examples.
We also learned how to filter out duplicate values through Prisma. Moreover, we’ve looked under the hood and learned that Prisma filters out the data in memory by default. Finally, we’ve learned how to change the default behavior and configure Prisma to rely more on the native features built into PostgreSQL.