A detailed introduction to the join operation in Mysql
This article mainly introduces the relevant information of Mysql join operation. Friends who need it can refer to
Types of join
1. Inner join: A join that forms a record set by combining those records whose fields in the two tables have a join relationship and that match the join relationship.
2. Outer join: divided into outer left join and outer right join.
Case background
create table java (name varchar(255)); insert into java values ('java1'),('java2'),('blue'); create table mysql (name varchar(255)); insert into mysql values ('mysql1'),('mysql2'),('blue');
Inner join
select * from java,mysql where java.name=mysql.name; SELECT * FROM java JOIN mysql ON java.name=mysql.name; SELECT * FROM java INNER JOIN mysql ON java.name=mysql.name; SELECT * FROM java CROSS JOIN mysql ON java.name=mysql.name; SELECT * FROM java STRAIGHT_JOIN mysql ON java.name=mysql.name;
These four statements are all inner joins, and the returned results are all
+------+------+ | name | name | +------+------+ | blue | blue | +------+------+
Each comma in the table_reference entry is regarded as equivalent to an internal union
The default JOINs are INNER JOIN
CROSS JOIN is grammatically equivalent to INNER JOIN
STRAIGHT_JOIN is the same as JOIN. Except for one difference, the left table will be read before the right table. STRAIGH_JOIN can be used in situations where the join optimizer sorts the tables in the wrong order.
The syntax of inner join is as follows:
join_table: table_reference [INNER | CROSS] JOIN table_factor [join_condition] | table_reference STRAIGHT_JOIN table_factor | table_reference STRAIGHT_JOIN table_factor ON condition
Outer join
Left join
SELECT * FROM java LEFT JOIN mysql ON java.name=mysql.name;
The result is
+-------+------+ | name | name | +-------+------+ | java1 | NULL | | java2 | NULL | | blue | blue | +-------+------+
So from the above results, it can be seen that because the names of the java1 and java2 records in the Java table do not have corresponding names in the MySQL table, they are empty, but all the columns of java There are still java1 and java2 records, and all columns of the mysql table are NULL. The remaining blue record is the result of the internal connection between the java table and the mysql table .
If there are no matching records for the right table in the ON or USING section of the LEFT JOIN, a row with all columns set to NULL is used for the right table. If a table has no counterpart inother tables, you can use this method to find records in such tables:
SELECT * FROM java LEFT JOIN mysql ON java.name=mysql.name WHERE mysql.name IS NULL;
Right join
SELECT * FROM java RIGHT JOIN mysql ON java.name=mysql.name;
+------+--------+ | name | name | +------+--------+ | NULL | mysql1 | | NULL | mysql2 | | blue | blue | +------+--------+
Syntax of outer join
join_table:| table_reference LEFT [OUTER] JOIN table_reference join_condition | table_reference NATURAL [LEFT [OUTER]] JOIN table_factor | table_reference RIGHT [OUTER] JOIN table_reference join_condition | table_reference NATURAL [RIGHT [OUTER]] JOIN table_factor
USING(column_list) clause
Used to name a series of columns, which must exist in both tables at the same timeSELECT java.*,mysql.* FROM java LEFT JOIN mysql USING (name);
+-------+------+ | name | name | +-------+------+ | java1 | NULL | | java2 | NULL | | blue | blue | +-------+------+
Operation order of join
SELECT * FROM t1 LEFT JOIN (t2, t3, t4) ON (t2.a=t1.a AND t3.b=t1.b AND t4.c=t1.c); --相当于 SELECT * FROM t1 LEFT JOIN (t2 CROSS JOIN t3 CROSS JOIN t4) ON (t2.a=t1.a AND t3.b=t1.b AND t4.c=t1.c)
SELECT t1.id,t2.id,t3.id FROM t1,t2 LEFT JOIN t3 ON (t3.id=t1.id) WHERE t1.id=t2.id; --实际上这么执行 SELECT t1.id,t2.id,t3.id FROM t1,( t2 LEFT JOIN t3 ON (t3.id=t1.id) ) WHERE t1.id=t2.id; --应该这么写 SELECT t1.id,t2.id,t3.id FROM (t1,t2) LEFT JOIN t3 ON (t3.id=t1.id) WHERE t1.id=t2.id;
queries Don’t forget to write a few more parentheses, at least this way you can avoid a lot of mistakes
The above is the detailed content of A detailed introduction to the join operation in Mysql. For more information, please follow other related articles on the PHP Chinese website!

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

Notepad++7.3.1
Easy-to-use and free code editor

SublimeText3 Chinese version
Chinese version, very easy to use

Zend Studio 13.0.1
Powerful PHP integrated development environment

Dreamweaver CS6
Visual web development tools

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

Hot Topics



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.

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 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.

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.

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.

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.

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

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
