SQL Tip: Get the first N rows of data by group
This article introduces how to efficiently extract the first N rows of data for each group from a SQL database. We will use window functions such as ROW_NUMBER(), RANK(), and DENSE_RANK() to achieve this. These functions generate unique identifiers within a specified partition and are ideal for grouping and row selection.
Use ROW_NUMBER()
ROW_NUMBER() function assigns consecutive serial numbers starting from 1 to the rows in each partition.
SELECT *, ROW_NUMBER() OVER (PARTITION BY id ORDER BY rate DESC) AS rn FROM h WHERE year BETWEEN 2000 AND 2009
In this example, ROW_NUMBER() creates a new column named "rn" to assign a serial number to the rows in each id
group.
Use RANK() and DENSE_RANK()
RANK() and DENSE_RANK() functions provide another way to number rows within a group. RANK() assigns distinct values to rows with the same sorting, while DENSE_RANK() assigns consecutive values.
SELECT *, RANK() OVER (PARTITION BY id ORDER BY rate DESC) AS rk, DENSE_RANK() OVER (PARTITION BY id ORDER BY rate DESC) AS dr FROM h WHERE year BETWEEN 2000 AND 2009
RANK() and DENSE_RANK() create the "rk" and "dr" columns respectively, representing each row's rank and dense ranking within its id
grouping.
Combined with LIMIT or TOP to limit the results
After assigning a unique identifier to each grouping, you can use the LIMIT or TOP modifier to retrieve only the specified number of rows per grouping.
SELECT * FROM h WHERE year BETWEEN 2000 AND 2009 ORDER BY id, rate DESC LIMIT 5
In this example, the LIMIT modifier retrieves the first 5 rows of each id
group. You can adjust the numbers as needed.
The above is the detailed content of How to Get the Top N Rows per Group in SQL?. For more information, please follow other related articles on the PHP Chinese website!

This article explores optimizing MySQL memory usage in Docker. It discusses monitoring techniques (Docker stats, Performance Schema, external tools) and configuration strategies. These include Docker memory limits, swapping, and cgroups, alongside

This article addresses MySQL's "unable to open shared library" error. The issue stems from MySQL's inability to locate necessary shared libraries (.so/.dll files). Solutions involve verifying library installation via the system's package m

The article discusses using MySQL's ALTER TABLE statement to modify tables, including adding/dropping columns, renaming tables/columns, and changing column data types.

This article compares installing MySQL on Linux directly versus using Podman containers, with/without phpMyAdmin. It details installation steps for each method, emphasizing Podman's advantages in isolation, portability, and reproducibility, but also

This article provides a comprehensive overview of SQLite, a self-contained, serverless relational database. It details SQLite's advantages (simplicity, portability, ease of use) and disadvantages (concurrency limitations, scalability challenges). C

This guide demonstrates installing and managing multiple MySQL versions on macOS using Homebrew. It emphasizes using Homebrew to isolate installations, preventing conflicts. The article details installation, starting/stopping services, and best pra

Article discusses configuring SSL/TLS encryption for MySQL, including certificate generation and verification. Main issue is using self-signed certificates' security implications.[Character count: 159]

Article discusses popular MySQL GUI tools like MySQL Workbench and phpMyAdmin, comparing their features and suitability for beginners and advanced users.[159 characters]


Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

AI Hentai Generator
Generate AI Hentai for free.

Hot Article

Hot Tools

SublimeText3 Mac version
God-level code editing software (SublimeText3)

DVWA
Damn Vulnerable Web App (DVWA) is a PHP/MySQL web application that is very vulnerable. Its main goals are to be an aid for security professionals to test their skills and tools in a legal environment, to help web developers better understand the process of securing web applications, and to help teachers/students teach/learn in a classroom environment Web application security. The goal of DVWA is to practice some of the most common web vulnerabilities through a simple and straightforward interface, with varying degrees of difficulty. Please note that this software

SecLists
SecLists is the ultimate security tester's companion. It is a collection of various types of lists that are frequently used during security assessments, all in one place. SecLists helps make security testing more efficient and productive by conveniently providing all the lists a security tester might need. List types include usernames, passwords, URLs, fuzzing payloads, sensitive data patterns, web shells, and more. The tester can simply pull this repository onto a new test machine and he will have access to every type of list he needs.

Atom editor mac version download
The most popular open source editor

ZendStudio 13.5.1 Mac
Powerful PHP integrated development environment
