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!

ACID attributes include atomicity, consistency, isolation and durability, and are the cornerstone of database design. 1. Atomicity ensures that the transaction is either completely successful or completely failed. 2. Consistency ensures that the database remains consistent before and after a transaction. 3. Isolation ensures that transactions do not interfere with each other. 4. Persistence ensures that data is permanently saved after transaction submission.

MySQL is not only a database management system (DBMS) but also closely related to programming languages. 1) As a DBMS, MySQL is used to store, organize and retrieve data, and optimizing indexes can improve query performance. 2) Combining SQL with programming languages, embedded in Python, using ORM tools such as SQLAlchemy can simplify operations. 3) Performance optimization includes indexing, querying, caching, library and table division and transaction management.

MySQL uses SQL commands to manage data. 1. Basic commands include SELECT, INSERT, UPDATE and DELETE. 2. Advanced usage involves JOIN, subquery and aggregate functions. 3. Common errors include syntax, logic and performance issues. 4. Optimization tips include using indexes, avoiding SELECT* and using LIMIT.

MySQL is an efficient relational database management system suitable for storing and managing data. Its advantages include high-performance queries, flexible transaction processing and rich data types. In practical applications, MySQL is often used in e-commerce platforms, social networks and content management systems, but attention should be paid to performance optimization, data security and scalability.

The relationship between SQL and MySQL is the relationship between standard languages and specific implementations. 1.SQL is a standard language used to manage and operate relational databases, allowing data addition, deletion, modification and query. 2.MySQL is a specific database management system that uses SQL as its operating language and provides efficient data storage and management.

InnoDB uses redologs and undologs to ensure data consistency and reliability. 1.redologs record data page modification to ensure crash recovery and transaction persistence. 2.undologs records the original data value and supports transaction rollback and MVCC.

Key metrics for EXPLAIN commands include type, key, rows, and Extra. 1) The type reflects the access type of the query. The higher the value, the higher the efficiency, such as const is better than ALL. 2) The key displays the index used, and NULL indicates no index. 3) rows estimates the number of scanned rows, affecting query performance. 4) Extra provides additional information, such as Usingfilesort prompts that it needs to be optimized.

Usingtemporary indicates that the need to create temporary tables in MySQL queries, which are commonly found in ORDERBY using DISTINCT, GROUPBY, or non-indexed columns. You can avoid the occurrence of indexes and rewrite queries and improve query performance. Specifically, when Usingtemporary appears in EXPLAIN output, it means that MySQL needs to create temporary tables to handle queries. This usually occurs when: 1) deduplication or grouping when using DISTINCT or GROUPBY; 2) sort when ORDERBY contains non-index columns; 3) use complex subquery or join operations. Optimization methods include: 1) ORDERBY and GROUPB


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

Dreamweaver Mac version
Visual web development tools

PhpStorm Mac version
The latest (2018.2.1) professional PHP integrated development tool

SublimeText3 English version
Recommended: Win version, supports code prompts!

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

mPDF
mPDF is a PHP library that can generate PDF files from UTF-8 encoded HTML. The original author, Ian Back, wrote mPDF to output PDF files "on the fly" from his website and handle different languages. It is slower than original scripts like HTML2FPDF and produces larger files when using Unicode fonts, but supports CSS styles etc. and has a lot of enhancements. Supports almost all languages, including RTL (Arabic and Hebrew) and CJK (Chinese, Japanese and Korean). Supports nested block-level elements (such as P, DIV),