search
HomeDatabaseMysql TutorialMySQL multi-table query case analysis

Multiple table query

Case description

Understanding of Cartesian product

select id,department_name
from employees,departments;#错的

select id,department_id,department_name
from employees CROSS JOIN departments;#错的

Each employee and each department were matched once (number of entries found = Number of ids * number of departments)

Cause of error: missing connection conditions

Solution to Cartesian product

Write connection conditions: Table 1. Column = Table 2. Column (if To connect multiple tables, at least n-1 connection conditions must be used)

select id,employees.name,department_name 
from employees,departments
WHERE employees.name = departments.name;

Note: If the column to be displayed has the same name in the table to be queried, it must be indicated which table it comes from, eg: employees .name

It is recommended to indicate which table information is displayed when querying multiple tables (optimization)

Optimization: You can use the alias of the table after FROM, but once the alias is used, subsequent Be sure to use aliases

Classification of multi-table queries

Equivalent joins and non-equivalent joins

  • Equivalent joins: The above ones with =

  • ##Non-equijoin: No=

  • select t1.id,t1.name,t2.grade
    from employees t1,departments t2
    WHERE ti.salary BETWEEN t2.lowest_salary AND t2.highest_salary ;#非等值
Self-join and non-self-join

  • Non-self-link: Table 1 and Table 2 are connected

  • Self-link: Table 1 and itself are connected

  • #显示员工(t1)和其管理者(t2)的基本信息
    select t1.id,t1.name,t2.id,t2.name
    from employees t1,employees t2#一个表看作两个表
    WHERE t1.manage_id = t2.id ;#自连接
Inner connection and outer connection Connection

  • Inner join: merge tables containing the same column, the result does not include rows that do not match one table with another table

  • Except Join: Merge tables containing the same column. In addition to the results of the inner join, unmatched rows are also queried

Classification of outer joins: left outer join (more left tables, supplement the right), Right outer join (more right tables, fill in the left), full outer join

SQL92: Use ( ) to create a connection

Inner join: see above

Outer join: left table If there is data mismatch, add ( ) to the right table; otherwise, add ( ) to the left table, but MySQL does not support

WHERE t1.department_id = t2.department_id(+)#左连接

SQL99: Use JOIN...ON method

inner join

select t1.id,t1.name,t2.department_name,t3.environment
from employees t1 JOIN departments t2
ON t1.department_id = t2.department_id
JOIN locations t3#加入第二个人表
ON t2.department_location = t3.department_location;

OUTER JOIN

Use OUTER JOIN...ON...

  • Left outer join: LEFT OUTER JOIN

  • RIGHT OUTER JOIN

  • Full OUTER JOIN: FULL OUTER JOIN (MySQL does not support)

  • select t1.name,t2.department_name#左外连接
    from employees t1 LEFT OUTER(可省略) JOIN departments t2
    ON t1.department_id = t2.department_id;
The use of UMION

Merge query results

SELECT colum... FROM table1
UNION (ALL)
SELECT colum... FROM table2

  • UNION operator

The union of two query results, deduplication (Low efficiency)

  • UNION ALL operator (recommended)

The union of two query results without deduplication (high efficiency)

Implementation of 7 kinds of SQL JOINS

MySQL multi-table query case analysis

Middle picture (inner join):

select t1.name,t2.department_name
from employees t1 JOIN departments t2
ON t1.department_id = t2.department_id;

Upper left picture (left outer join):

select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id;

Top right picture (right outer join):

select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id;

Middle left picture:

select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t2.department_id IS NULL;

Middle right picture:

select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t1.department_id IS NULL;

Bottom left picture (full outer join) :

#方式一:左上图 UNION ALL 右中图
select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id
UNION ALL 
select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t1.department_id IS NULL;

#方式二:左中图 UNION ALL 右上图
select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t2.department_id IS NULL
UNION ALL
select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id;

Bottom right picture:

#左中图 UNION ALL 右中图
select t1.name,t2.department_name
from employees t1 LEFT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t2.department_id IS NULL
UNION ALL
select t1.name,t2.department_name
from employees t1 RIGHT JOIN departments t2
ON t1.department_id = t2.department_id
WHERE t1.department_id IS NULL;

New features of SQL syntax

Natural connection

Use keywords: NATURAL JOIN (inflexible), automatic query All the same fields in the table, and then perform an equal value connection

USING connection (not applicable to self-join)

Use the keyword: USING (field with the same name) to automatically join the fields with the same name in the table Equijoining

select t1.name,t2.department_name
from employees t1 JOIN departments t2
ON t1.department_id = t2.department_id;
等价于
select t1.name,t2.department_name
from employees t1 JOIN departments t2
USING(department_id);

The above is the detailed content of MySQL multi-table query case analysis. For more information, please follow other related articles on the PHP Chinese website!

Statement
This article is reproduced at:亿速云. If there is any infringement, please contact admin@php.cn delete
MySQL: BLOB and other no-sql storage, what are the differences?MySQL: BLOB and other no-sql storage, what are the differences?May 13, 2025 am 12:14 AM

MySQL'sBLOBissuitableforstoringbinarydatawithinarelationaldatabase,whileNoSQLoptionslikeMongoDB,Redis,andCassandraofferflexible,scalablesolutionsforunstructureddata.BLOBissimplerbutcanslowdownperformancewithlargedata;NoSQLprovidesbetterscalabilityand

MySQL Add User: Syntax, Options, and Security Best PracticesMySQL Add User: Syntax, Options, and Security Best PracticesMay 13, 2025 am 12:12 AM

ToaddauserinMySQL,use:CREATEUSER'username'@'host'IDENTIFIEDBY'password';Here'showtodoitsecurely:1)Choosethehostcarefullytocontrolaccess.2)SetresourcelimitswithoptionslikeMAX_QUERIES_PER_HOUR.3)Usestrong,uniquepasswords.4)EnforceSSL/TLSconnectionswith

MySQL: How to avoid String Data Types common mistakes?MySQL: How to avoid String Data Types common mistakes?May 13, 2025 am 12:09 AM

ToavoidcommonmistakeswithstringdatatypesinMySQL,understandstringtypenuances,choosetherighttype,andmanageencodingandcollationsettingseffectively.1)UseCHARforfixed-lengthstrings,VARCHARforvariable-length,andTEXT/BLOBforlargerdata.2)Setcorrectcharacters

MySQL: String Data Types and ENUMs?MySQL: String Data Types and ENUMs?May 13, 2025 am 12:05 AM

MySQloffersechar, Varchar, text, Anddenumforstringdata.usecharforfixed-Lengthstrings, VarcharerForvariable-Length, text forlarger text, AndenumforenforcingdataAntegritywithaetofvalues.

MySQL BLOB: how to optimize BLOBs requestsMySQL BLOB: how to optimize BLOBs requestsMay 13, 2025 am 12:03 AM

Optimizing MySQLBLOB requests can be done through the following strategies: 1. Reduce the frequency of BLOB query, use independent requests or delay loading; 2. Select the appropriate BLOB type (such as TINYBLOB); 3. Separate the BLOB data into separate tables; 4. Compress the BLOB data at the application layer; 5. Index the BLOB metadata. These methods can effectively improve performance by combining monitoring, caching and data sharding in actual applications.

Adding Users to MySQL: The Complete TutorialAdding Users to MySQL: The Complete TutorialMay 12, 2025 am 12:14 AM

Mastering the method of adding MySQL users is crucial for database administrators and developers because it ensures the security and access control of the database. 1) Create a new user using the CREATEUSER command, 2) Assign permissions through the GRANT command, 3) Use FLUSHPRIVILEGES to ensure permissions take effect, 4) Regularly audit and clean user accounts to maintain performance and security.

Mastering MySQL String Data Types: VARCHAR vs. TEXT vs. CHARMastering MySQL String Data Types: VARCHAR vs. TEXT vs. CHARMay 12, 2025 am 12:12 AM

ChooseCHARforfixed-lengthdata,VARCHARforvariable-lengthdata,andTEXTforlargetextfields.1)CHARisefficientforconsistent-lengthdatalikecodes.2)VARCHARsuitsvariable-lengthdatalikenames,balancingflexibilityandperformance.3)TEXTisidealforlargetextslikeartic

MySQL: String Data Types and Indexing: Best PracticesMySQL: String Data Types and Indexing: Best PracticesMay 12, 2025 am 12:11 AM

Best practices for handling string data types and indexes in MySQL include: 1) Selecting the appropriate string type, such as CHAR for fixed length, VARCHAR for variable length, and TEXT for large text; 2) Be cautious in indexing, avoid over-indexing, and create indexes for common queries; 3) Use prefix indexes and full-text indexes to optimize long string searches; 4) Regularly monitor and optimize indexes to keep indexes small and efficient. Through these methods, we can balance read and write performance and improve database efficiency.

See all articles

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 Article

Hot Tools

Dreamweaver Mac version

Dreamweaver Mac version

Visual web development tools

SublimeText3 Mac version

SublimeText3 Mac version

God-level code editing software (SublimeText3)

EditPlus Chinese cracked version

EditPlus Chinese cracked version

Small size, syntax highlighting, does not support code prompt function

MinGW - Minimalist GNU for Windows

MinGW - Minimalist GNU for Windows

This project is in the process of being migrated to osdn.net/projects/mingw, you can continue to follow us there. MinGW: A native Windows port of the GNU Compiler Collection (GCC), freely distributable import libraries and header files for building native Windows applications; includes extensions to the MSVC runtime to support C99 functionality. All MinGW software can run on 64-bit Windows platforms.

SecLists

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.