- 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
So far, we’ve been mostly writing SQL queries that either store or retrieve the data from the database. Besides that, we can rely on PostgreSQL to process the data and get the computed results. By doing that, we can learn more about the rows in our tables. In this article, we look into how we can use aggregate functions to generate statistics about our data.
For the code from this article check out this repository.
The purpose of aggregate functions
The job of an aggregate function is to compute a single result from multiple input rows. One of the most popular aggregate functions is count(). When used with an asterisk, it measures the total number of rows in the table.
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SELECT count(*) FROM users |
When we provide the count function with a column name, it counts the number of rows with a non-NULL value for that column.
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SELECT count(address_id) AS number_of_users_with_address FROM users |
The count() function was handy in the previous article when we implemented pagination.
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import { Injectable } from '@nestjs/common'; import DatabaseService from '../database/database.service'; import PostModel from './post.model'; @Injectable() class PostsRepository { constructor(private readonly databaseService: DatabaseService) {} async get(offset = 0, limit: number | null = null, idsToSkip = 0) { const databaseResponse = await this.databaseService.runQuery( ` WITH selected_posts AS ( SELECT * FROM posts WHERE id > $3 ORDER BY id ASC OFFSET $1 LIMIT $2 ), total_posts_count_response AS ( SELECT COUNT(*)::int AS total_posts_count FROM posts ) SELECT * FROM selected_posts, total_posts_count_response `, [offset, limit, idsToSkip], ); const items = databaseResponse.rows.map( (databaseRow) => new PostModel(databaseRow), ); const count = databaseResponse.rows[0]?.total_posts_count || 0; return { items, count, }; } // ... } export default PostsRepository; |
Grouping data in the table
Aggregate functions work great when we perform them on groups of data
1 2 |
SELECT author_id, count(*) FROM posts GROUP BY author_id |
When we do the above, PostgreSQL divides the data into groups and runs the aggregate function on each group individually.
We could make our query even more helpful and order our results. By doing that, we can ensure the authors with the highest number of posts are at the top of the list.
1 2 3 |
SELECT author_id, count(*) AS posts_count FROM posts GROUP BY author_id ORDER BY posts_count DESC |
Let’s create a model that can hold the above data.
postAuthorStatistics.model.ts
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export interface PostAuthorStatisticsModelData { author_id: number; posts_count: number; } class PostAuthorStatisticsModel { authorId: number; postsCount: number; constructor(postAuthorStatisticsData: PostAuthorStatisticsModelData) { this.authorId = postAuthorStatisticsData.author_id; this.postsCount = postAuthorStatisticsData.posts_count; } } export default PostAuthorStatisticsModel; |
Let’s create a separate statistics repository to prevent our PostsRepository class from getting too big.
postsStatistics.repository.ts
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import { Injectable, } from '@nestjs/common'; import DatabaseService from '../database/database.service'; import PostAuthorStatisticsModel from './postAuthorStatistics.model'; @Injectable() class PostsStatisticsRepository { constructor(private readonly databaseService: DatabaseService) {} async getPostsAuthorStatistics() { const databaseResponse = await this.databaseService.runQuery( ` SELECT author_id, count(*)::int AS posts_count FROM posts GROUP BY author_id ORDER BY posts_count DESC `, [], ); return databaseResponse.rows.map( (databaseRow) => new PostAuthorStatisticsModel(databaseRow), ); } } export default PostsStatisticsRepository; |
The count() function returns the value using the bigint data type. Because of that, we convert it to a regular integer. If you want to know more, check out the previous article.
We also need to point to our new repository in the PostsService class.
posts.service.ts
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import { Injectable } from '@nestjs/common'; import PostsRepository from './posts.repository'; import PostsStatisticsRepository from './postsStatistics.repository'; @Injectable() export class PostsService { constructor( private readonly postsRepository: PostsRepository, private readonly postsStatisticsRepository: PostsStatisticsRepository, ) {} getPostAuthorStatistics() { return this.postsStatisticsRepository.getPostsAuthorStatistics(); } // ... } |
The last step is to use it in the controller.
posts.controller.ts
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import { ClassSerializerInterceptor, Controller, Get, Query, UseInterceptors, } from '@nestjs/common'; import { PostsService } from './posts.service'; import GetPostsByAuthorQuery from './getPostsByAuthorQuery'; import PaginationParams from '../utils/paginationParams'; @Controller('posts') @UseInterceptors(ClassSerializerInterceptor) export default class PostsController { constructor(private readonly postsService: PostsService) {} @Get() getPosts( @Query() { authorId }: GetPostsByAuthorQuery, @Query() { offset, limit, idsToSkip }: PaginationParams, ) { return this.postsService.getPosts(authorId, offset, limit, idsToSkip); } @Get('statistics') getStatistics() { return this.postsService.getPostAuthorStatistics(); } // ... } |
Other aggregate functions
There are more aggregate functions besides count(). Let’s go through them.
max and min
Using the max() function, we can find the largest value of the selected column. Respectively, the min() function returns the smallest value of the column.
Since we don’t have any numerical columns in our posts, let’s pair the above functions with length(). This way, we can get the longest and shortest posts of a particular author.
The length() function returns the length of a string.
postsStatistics.repository.ts
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import { Injectable } from '@nestjs/common'; import DatabaseService from '../database/database.service'; import PostAuthorStatisticsModel from './postAuthorStatistics.model'; @Injectable() class PostsStatisticsRepository { constructor(private readonly databaseService: DatabaseService) {} async getPostsAuthorStatistics() { const databaseResponse = await this.databaseService.runQuery( ` SELECT author_id, count(*)::int AS posts_count, max(length(post_content)) AS longest_post_length, min(length(post_content)) AS shortest_post_length FROM posts GROUP BY author_id ORDER BY posts_count DESC `, [], ); return databaseResponse.rows.map( (databaseRow) => new PostAuthorStatisticsModel(databaseRow), ); } } export default PostsStatisticsRepository; |
Running max() or min() on a text column returns a string based on the alphabetical order.
sum
With the sum() function, we can return a total sum of a particular column. Since it only works with numerical values, we also need the length() function.
Since the sum() function also returns the value in the bigint format, we transform it to a regular integer. We can do it because we don’t expect values bigger than 2³¹⁻¹.
postsStatistics.repository.ts
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import { Injectable } from '@nestjs/common'; import DatabaseService from '../database/database.service'; import PostAuthorStatisticsModel from './postAuthorStatistics.model'; @Injectable() class PostsStatisticsRepository { constructor(private readonly databaseService: DatabaseService) {} async getPostsAuthorStatistics() { const databaseResponse = await this.databaseService.runQuery( ` SELECT author_id, count(*)::int AS posts_count, max(length(post_content)) AS longest_post_length, min(length(post_content)) AS shortest_post_length, sum(length(post_content))::int AS all_posts_content_sum FROM posts GROUP BY author_id ORDER BY posts_count DESC `, [], ); return databaseResponse.rows.map( (databaseRow) => new PostAuthorStatisticsModel(databaseRow), ); } } export default PostsStatisticsRepository; |
avg
The avg() function calculates the average of the values in a group. Let’s combine it with the length() function to calculate the average length of all posts of a particular author.
postsStatistics.repository.ts
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import { Injectable } from '@nestjs/common'; import DatabaseService from '../database/database.service'; import PostAuthorStatisticsModel from './postAuthorStatistics.model'; @Injectable() class PostsStatisticsRepository { constructor(private readonly databaseService: DatabaseService) {} async getPostsAuthorStatistics() { const databaseResponse = await this.databaseService.runQuery( ` SELECT author_id, count(*)::int AS posts_count, max(length(post_content)) AS longest_post_length, min(length(post_content)) AS shortest_post_length, sum(length(post_content))::int AS all_posts_content_sum, avg(length(post_content))::real AS average_post_content_length FROM posts GROUP BY author_id ORDER BY posts_count DESC `, [], ); return databaseResponse.rows.map( (databaseRow) => new PostAuthorStatisticsModel(databaseRow), ); } } export default PostsStatisticsRepository; |
A significant thing about the avg() function is that it returns the data in the numeric type. It can store many digits and is very useful when exactness is crucial. Parsing this data type to JSON converts it to a string by default. Since we don’t need many digits after the decimal, we convert it to the real data type.
Aggregating data from more than one table
So far, we’ve been grouping and aggregating data in one table. However, a typical case might be when we want to aggregate data using more than one table. In this case, we need to use the JOIN keyword.
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SELECT addresses.country, COUNT(*) AS number_of_users FROM users LEFT JOIN addresses ON users.address_id = addresses.id GROUP BY addresses.country |
Above, we perform an outer join using the LEFT JOIN keyword. If you want to know more, check out API with NestJS #73. One-to-one relationships with raw SQL queries
Filtering using aggregate functions and grouping
So far, to filter the results from the database, we’ve been using the WHERE keyword.
1 2 |
SELECT * FROM posts WHERE id = 1 |
When we want to filter using aggregate functions and grouping, we need to look at the execution order of SQL clauses.
- FROM
- WHERE
- GROUP BY
- HAVING
- DISTINCT
- SELECT
- ORDER BY
- LIMIT
Since PostgreSQL executes the WHERE clause before GROUP BY, we can’t use it with aggregate functions.
1 2 3 |
SELECT author_id, count(*) as posts_count FROM posts WHERE count(*) > 100 GROUP BY author_id |
ERROR: aggregate functions are not allowed in WHERE
LINE 2: WHERE count(*) > 100
Instead, we need to use the HAVING keyword.
1 2 3 |
SELECT author_id, count(*) AS posts_count FROM posts GROUP BY author_id HAVING count(*) > 100 |
Using aliases
An important caveat is that we can’t use column aliases with the HAVING keyword. So, for example, the following code wouldn’t work:
1 2 3 |
SELECT author_id, count(*) AS posts_count FROM posts GROUP BY author_id HAVING posts_count > 100 |
Instead, we need to use the count() function twice and rely on PostgreSQL to optimize it.
Summary
In this article, we’ve gone through how to use aggregate functions together with grouping. When doing so, we’ve implemented an endpoint that returns statistics about a particular table. We also wrote an example that uses an aggregate function and grouping when joining two tables. Finally, we’ve also learned how to filter our data using aggregate functions and why we can’t do that with the WHERE keyword.
There is still more to cover when writing raw SQL with NestJS, so stay tuned!