Home Database Mysql Tutorial MySQL学习足迹记录12--使用子查询_MySQL

MySQL学习足迹记录12--使用子查询_MySQL

Jun 01, 2016 pm 01:31 PM
mysql Record

bitsCN.com

MySQL学习足迹记录12--使用子查询

 

1.子查询(subquery):即嵌套在其他查询中的查询

原始数据如下:

 

   mysql> SELECT order_num FROM orderitems;+-----------+| order_num |+-----------+|     20005 ||     20005 ||     20009 ||     20005 ||     20009 ||     20008 ||     20006 ||     20009 ||     20009 ||     20005 ||     20007 |+-----------+11 rows in set (0.01 sec)mysql> SELECT cust_id FROM orders;+---------+| cust_id |+---------+|   10001 ||   10001 ||   10003 ||   10004 ||   10005 |+---------+5 rows in set (0.01 sec)现在先分步查询step1:  mysql> SELECT order_num          -> FROM orderitems          -> WHERE prod_id = 'TNT2';+-----------+| order_num |+-----------+|     20005 ||     20007 |+-----------+2 rows in set (0.00 sec)step2: mysql> SELECT cust_id FROM orders          -> WHERE order_num IN( 20005,20007);+---------+| cust_id |+---------+|   10001 ||   10004 |+---------+2 rows in set (0.00 sec) Step3:  使用子查询把step1,step2组合起来(即把20005,20007换掉)  mysql> SELECT cust_id           -> FROM orders           -> WHERE order_num IN( SELECT order_num           ->                                        FROM orderitems           ->                                         WHERE prod_id = 'TNT2');+---------+| cust_id |+---------+|   10001 ||   10004 |+---------+2 rows in set (0.00 sec)TIPS:  在SELECT语句中,子查询总是从内向外处理的。  子查询可以嵌套多重 step4:  mysql> SELECT cust_name,cust_contact           -> FROM customers           -> WHERE cust_id IN (10001,10004);      #(10001,10004)既是step3查询的结果+----------------+--------------+| cust_name      | cust_contact |+----------------+--------------+| Coyote Inc.    | Y Lee        || Yosemite Place | Y Sam        |+----------------+--------------+2 rows in set (0.01 sec)step5:把step4的IN (10001,10004)换成子查询 mysql> SELECT cust_name,cust_contact          -> FROM customers         -> WHERE cust_id IN (SELECT cust_id         ->                                   FROM orders         ->                                   WHERE order_num IN (SELECT order_num         ->                                                                           FROM orderitems         ->                                                                           WHERE prod_id = 'TNT2'));+----------------+--------------+| cust_name      | cust_contact |+----------------+--------------+| Coyote Inc.    | Y Lee        || Yosemite Place | Y Sam        |+----------------+--------------+2 rows in set (0.00 sec)
Copy after login

2.计算字段使用子查询

原始数据

   mysql> SELECT cust_id FROM orders;+---------+| cust_id |+---------+|   10001 ||   10001 ||   10003 ||   10004 ||   10005 |+---------+5 rows in set (0.01 sec)mysql> SELECT cust_id FROM customers;+---------+| cust_id |+---------+|   10001 ||   10002 ||   10003 ||   10004 ||   10005 |+---------+5 rows in set (0.00 sec)mysql> SELECT cust_id,(SELECT COUNT(*) FROM orders         ->                                WHERE orders.cust_id = customers.cust_id) AS orders        -> FROM customers       -> ORDER BY cust_id;+---------+--------+              | cust_id | orders |+---------+--------+|   10001 |      2 ||   10002 |      0 ||   10003 |      1 ||   10004 |      1 ||   10005 |      1 |+---------+--------+5 rows in set (0.00 sec)
Copy after login

 

 

TIPS:

  子查询最常见的使用是在WHERE子句的IN操作符中,以及用来填充计算列

bitsCN.com
Statement of this Website
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

Video Face Swap

Video Face Swap

Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Tools

Notepad++7.3.1

Notepad++7.3.1

Easy-to-use and free code editor

SublimeText3 Chinese version

SublimeText3 Chinese version

Chinese version, very easy to use

Zend Studio 13.0.1

Zend Studio 13.0.1

Powerful PHP integrated development environment

Dreamweaver CS6

Dreamweaver CS6

Visual web development tools

SublimeText3 Mac version

SublimeText3 Mac version

God-level code editing software (SublimeText3)

MySQL: Simple Concepts for Easy Learning MySQL: Simple Concepts for Easy Learning Apr 10, 2025 am 09:29 AM

MySQL is an open source relational database management system. 1) Create database and tables: Use the CREATEDATABASE and CREATETABLE commands. 2) Basic operations: INSERT, UPDATE, DELETE and SELECT. 3) Advanced operations: JOIN, subquery and transaction processing. 4) Debugging skills: Check syntax, data type and permissions. 5) Optimization suggestions: Use indexes, avoid SELECT* and use transactions.

How to open phpmyadmin How to open phpmyadmin Apr 10, 2025 pm 10:51 PM

You can open phpMyAdmin through the following steps: 1. Log in to the website control panel; 2. Find and click the phpMyAdmin icon; 3. Enter MySQL credentials; 4. Click "Login".

MySQL: An Introduction to the World's Most Popular Database MySQL: An Introduction to the World's Most Popular Database Apr 12, 2025 am 12:18 AM

MySQL is an open source relational database management system, mainly used to store and retrieve data quickly and reliably. Its working principle includes client requests, query resolution, execution of queries and return results. Examples of usage include creating tables, inserting and querying data, and advanced features such as JOIN operations. Common errors involve SQL syntax, data types, and permissions, and optimization suggestions include the use of indexes, optimized queries, and partitioning of tables.

How to use single threaded redis How to use single threaded redis Apr 10, 2025 pm 07:12 PM

Redis uses a single threaded architecture to provide high performance, simplicity, and consistency. It utilizes I/O multiplexing, event loops, non-blocking I/O, and shared memory to improve concurrency, but with limitations of concurrency limitations, single point of failure, and unsuitable for write-intensive workloads.

Why Use MySQL? Benefits and Advantages Why Use MySQL? Benefits and Advantages Apr 12, 2025 am 12:17 AM

MySQL is chosen for its performance, reliability, ease of use, and community support. 1.MySQL provides efficient data storage and retrieval functions, supporting multiple data types and advanced query operations. 2. Adopt client-server architecture and multiple storage engines to support transaction and query optimization. 3. Easy to use, supports a variety of operating systems and programming languages. 4. Have strong community support and provide rich resources and solutions.

MySQL's Place: Databases and Programming MySQL's Place: Databases and Programming Apr 13, 2025 am 12:18 AM

MySQL's position in databases and programming is very important. It is an open source relational database management system that is widely used in various application scenarios. 1) MySQL provides efficient data storage, organization and retrieval functions, supporting Web, mobile and enterprise-level systems. 2) It uses a client-server architecture, supports multiple storage engines and index optimization. 3) Basic usages include creating tables and inserting data, and advanced usages involve multi-table JOINs and complex queries. 4) Frequently asked questions such as SQL syntax errors and performance issues can be debugged through the EXPLAIN command and slow query log. 5) Performance optimization methods include rational use of indexes, optimized query and use of caches. Best practices include using transactions and PreparedStatemen

MySQL and SQL: Essential Skills for Developers MySQL and SQL: Essential Skills for Developers Apr 10, 2025 am 09:30 AM

MySQL and SQL are essential skills for developers. 1.MySQL is an open source relational database management system, and SQL is the standard language used to manage and operate databases. 2.MySQL supports multiple storage engines through efficient data storage and retrieval functions, and SQL completes complex data operations through simple statements. 3. Examples of usage include basic queries and advanced queries, such as filtering and sorting by condition. 4. Common errors include syntax errors and performance issues, which can be optimized by checking SQL statements and using EXPLAIN commands. 5. Performance optimization techniques include using indexes, avoiding full table scanning, optimizing JOIN operations and improving code readability.

Monitor Redis Droplet with Redis Exporter Service Monitor Redis Droplet with Redis Exporter Service Apr 10, 2025 pm 01:36 PM

Effective monitoring of Redis databases is critical to maintaining optimal performance, identifying potential bottlenecks, and ensuring overall system reliability. Redis Exporter Service is a powerful utility designed to monitor Redis databases using Prometheus. This tutorial will guide you through the complete setup and configuration of Redis Exporter Service, ensuring you seamlessly build monitoring solutions. By studying this tutorial, you will achieve fully operational monitoring settings

See all articles