sql联接查询

SQL Joins combine two or more tables together based on the corresponding common column among them.

SQL Join根据其中的对应公共列将两个或多个表组合在一起。

Note: All the below mentioned queries are executed with reference to MySQL Database.

注意： 下面提到的所有查询都是参照MySQL数据库执行的。

SQL连接的类型 (Types of SQL Joins)

INNER JOIN内部联接
LEFT OUTER JOIN左外连接
RIGHT OUTER JOIN右外连接
FULL OUTER JOIN全外连接
SELF JOIN自我加入

1.内联 (1. INNER JOIN)

SQL INNER JOIN basically serves as the intersection of two or more tables. It returns elements that are common to both of the tables.

SQL INNER JOIN基本上用作两个或多个表的交集。它返回两个表共有的元素。

Let’s use the following tables to serve and understand the purpose of SQL Joins.

让我们使用下表来服务和理解SQL连接的目的。

Table name: Information

表名：信息

Employee Id	Manager Id	Batch
1	1	A
2	2	B
3	5	A

员工ID	经理编号	批量
1个	1个	一个
2	2	乙
3	5	一个

Table name: Manager_Details

表名称：Manager_Details

Manager Id	Name	City	Age
1	Safa Mulani	Pune	52
2	Aman Mulani	Satara	25
3	Divya Trivedi	Mumbai	24

经理编号	名称	市	年龄
1个	萨法·穆拉尼（Safa Mulani）	浦那	52
2	阿曼·穆拉尼（Aman Mulani）	萨塔拉	25
3	迪维亚·特里维迪（Divya Trivedi）	孟买	24

Syntax:

句法：

SELECT Column_Names
FROM Table1
INNER JOIN Table2
ON Table1.Column_Name = Table2.Column_Name;

Example:

例：


SELECT Information.Employee_Id, Manager_details.Manager_Id, Manager_details.Name
FROM Information
INNER JOIN Manager_details ON Information.Manager_Id = Manager_details.Manager_Id;

Output:

输出：

1   1   Safa Mulani
2   2   Aman Mulani

2.左外连接 (2. LEFT OUTER JOIN)

SQL LEFT OUTER JOIN returns all the elements from the left table(i.e. Table1) and the corresponding matched elements from the right table(i.e. Table2).

SQL LEFT OUTER JOIN返回左表（即Table1）中的所有元素和右表（即Table2）中的对应匹配元素。

If the appropriate match isn’t found, it returns NULL values for the unmatched right table column values.

如果找不到合适的匹配项，则它为不匹配的右表列值返回NULL值。

Syntax:

句法：

SELECT Column_Names
FROM Table1
LEFT JOIN Table2
ON Table1.Column_Name = Table2.Column_Name;

Example:

例：


SELECT Information.Employee_Id, Manager_details.Manager_Id, Manager_details.Name
FROM Information
LEFT JOIN Manager_details ON Information.Manager_Id = Manager_details.Manager_Id;

Output:

输出：

1 1   Safa Mulani
2   2   Aman Mulani
3   NULL    NULL

3.右外连接 (3. RIGHT OUTER JOIN)

SQL RIGHT OUTER JOIN returns all the elements from the right table(i.e. Table2) and the corresponding matched elements from the left table(i.e. Table1).

SQL RIGHT OUTER JOIN返回右表（即Table2）中的所有元素和左表（即Table1）中的对应匹配元素。

If the appropriate match isn’t found, it returns NULL values for the unmatched left table column values.

如果找不到合适的匹配项，则它为不匹配的左表列值返回NULL值。

Syntax:

句法：

SELECT Column_Names
FROM Table1
RIGHT JOIN Table2
ON Table1.Column_Name = Table2.Column_Name;

Example:

例：


SELECT Information.Employee_Id, Manager_details.Manager_Id, Manager_details.Name
FROM Information
RIGHT JOIN Manager_details ON Information.Manager_Id = Manager_details.Manager_Id;

Output:

输出：

1   1   Safa Mulani
2   2   Aman Mulani
NULL    3   Divya Trivedi

4.完整的外部连接 (4. FULL OUTER JOIN)

SQL FULL JOIN basically represents all the elements which either matches the elements from the left table or the right table.

SQL FULL JOIN基本上表示与左表或右表中的元素匹配的所有元素。

If the rows of either of the tables do not match each other, still the elements will be represented with NULL values attached to it.

如果两个表中的任何一个表的行都不匹配，则仍将使用NULL值来表示元素。

Syntax:

句法：

SELECT Column_Names
FROM Table1
FULL OUTER JOIN Table2
ON Table1.Column_Name = Table2.Column_Name;

Most of the Databases such as SQLite, MySQL, etc do not support FULL OUTER Joins.

大多数数据库，例如SQLite ， MySQL等，都不支持FULL OUTER Joins。

In order to serve the purpose, we need to modify the query and make use of UNION clause to provide the functionalities of the FULL OUTER JOIN.

为了达到目的，我们需要修改查询并利用UNION子句来提供FULL OUTER JOIN的功能。

Syntax:

句法：

SELECT Column_Names
FROM Table1
LEFT OUTER JOIN Table2
ON Table1.Column_Name = Table2.Column_Name
UNION
SELECT Column_Names
FROM Table1
RIGHT OUTER JOIN Table2
ON Table1.Column_Name = Table2.Column_Name;

Example:

例：


SELECT Information.Employee_Id, Manager_details.Manager_Id, Manager_details.Name
FROM Information
LEFT OUTER JOIN Manager_details ON Information.Manager_Id = Manager_details.Manager_Id
UNION
SELECT Information.Employee_Id, Manager_details.Manager_Id, Manager_details.Name
FROM Information
RIGHT OUTER JOIN Manager_details ON Information.Manager_Id = Manager_details.Manager_Id;

Output:

输出：

1   1   Safa Mulani
2   2   Aman Mulani
3   NULL    NULL
NULL    3   Divya Trivedi

5.自我加入 (5. SELF JOIN)

SQL SELF JOIN represents a join to itself. i.e it renames the table, gives alias names to the corresponding table.

SQL SELF JOIN表示自身的联接。即，它重命名表，为相应的表提供别名。

Syntax:

句法：

SELECT Column_Name
FROM Table1 A, Table1 B
WHERE condition;

Here, A and B are the alias names given to the same Table1.

此处，A和B是赋予同一表1的别名。

Example:

例：


SELECT  a.Manager_Id, b.Name, a.City
FROM Manager_details a, Manager_details b
WHERE a.Age > 24 AND b.Age > 24;

Output:

输出：

1   Safa Mulani Pune
2   Safa Mulani Satara
1   Aman Mulani Pune
2   Aman Mulani Satara

结论 (Conclusion)

Thus, in this article, we have understood the functionalities of SQL Joins.

因此，在本文中，我们了解了SQL Joins的功能。

参考 (Reference)

MySQL DocumentationMySQL文档

翻译自: https://www.journaldev.com/34084/sql-joins

sql联接查询