数据库中的联接：了解在另外一个表格中的数据 (在另外一个表格的数据库)
Database Joins: Understanding Data in Another Table
In the world of databases, relating and linking data is an essential aspect. Databases store large amounts of data, typically across multiple tables or even across multiple databases. This is why database joins are crucial in making meaning out of raw data. Without them, the level of ysis that we can perform is limited. In this article, we will seek to understand what database joins are, the types of joins and how to use them effectively.
What is a Database Join?
In simple terms, a join is a method of combining data from two or more tables in a database to obtn a comprehensive set of results. One of the unique aspects of a join is that it allows for the linking of tables based on relationships that exist between them. Therefore, when we merge data from various tables using joins, we primarily do so based on a shared column between the various tables.
For example, if we have two tables in which one table contns customer information and the second table contns product information, we can use the customer ID in each table to join the tables. By doing so, we are creating a dataset that links a customer with the product they have purchased.
Types of Database Joins
1. Inner Join
An inner join is a common type of join used in databases. Inner joins return only the records that exist in both tables being linked. In essence, inner joins are used to extract records that have a common value in both tables.
2. Left Join
Unlike Inner joins, left joins consider all records in the left table and the records in the right table that have a match with the left. If there are records in the left table that have no corresponding records in the right table, they will still be displayed, with NULL values displayed in the columns from the right table.
3. Right Join
Right joins are similar to Left joins, but tables will be swapped. All records on the right table will be displayed, and the corresponding records from the left table, if there are any, will also be displayed. If there are no matching records in the left table, there will be NULL values in the columns coming from the left table.
4. Full Outer Join
A Full Outer Join returns all records from both the left and right tables. Where there are no matching records, NULL values are displayed.
Benefits of Using Joins in Databases
1. Reduction of Data Duplication
When we use joins to extract data from a relational database system, we can reduce duplication of the same data. Instead, we can store the data once and retrieve that data using joins from other tables as required.
2. Data Integrity
Joining tables based on unique identifiers ensures that data is only represented once in the database. This helps to ensure data accuracy and integrity since duplicate entries or conflicting records can be eliminated.
3. Easier Analysis
As we mentioned earlier, when we combine data from different tables using joins, we are creating a dataset that links records from various tables. This dataset can make data ysis easier since data is already linked based on relationships between different records.
In conclusion, database joins play a vital role in the extraction of data from large databases. They help to reduce data duplication, ensure data accuracy and integrity, as well as making data ysis easier. With the four types of joins (Inner, Left, Right, Full Outer), data ysts and programmers can link tables effectively based on relationship, value or unique identifiers.’ By doing so, they can extract meaningful insights that can drive decisions for the organization.
相关问题拓展阅读：
MySQL中如何把一个数据库中的表数据，导到另一个数据库的表中
把双方的mysql服务都停掉，直接把导出方的数据表文件(就是和数据库表名对应的.frm.myd.myi文件）拷到导入方（注意一个表有三个文件)，然后改一下数据表文件名成你要导入的表名，然后启动mysql服者滚务
，燃嫌磨如果导入方原皮斗来的表有数据，可以把原来的数据用正常方式导出，然后在工具里面导入合并就可以了，
再有一个可行的方法就是自写代码一行一行的转移数据了
数据库中怎么在一个表中链接另一个表
不同服务器数据库之间的数据操作
–创建链接服务器
exec sp_addlinkedserver ‘ITSV’ , ” , ‘SQLOLEDB’ , ‘远程服务器名或ip地址’
exec sp_addlinkedsrvlogin ‘ITSV’ , ‘false’ , null , ‘用户名’ , ‘密码’
–查询示例
select * from ITSV.数据库名.dbo.表名
–导入示例
select * into 表 from ITSV.数据库名.dbo.表名
–以后不再使用时删除链接服务器
exec sp_dropserver ‘ITSV’ , ‘droplogins’
–连接远程/局域网数据(openrowset/openquery/opendatasource)
–1、openrowset
–查询示例
select * from openrowset(‘SQLOLEDB’ , ‘sql服务器名’ ; ‘用户名’ ; ‘密码’ , 数据库名.dbo.表名)
–生成本地表
select * into 表 from openrowset(‘SQLOLEDB’ , ‘sql服务器名’ ; ‘用户名’ ; ‘密码’ , 数据库名.dbo.表名)
–把本地表导入激埋晌远程表
insert openrowset( ‘SQLOLEDB’ , ‘sql服务器名’ ; ‘用户名’ ; ‘密液握码’ , 数据库名.dbo.表名)
select *from 本地表
–更新本地表
update b
set b.列A=a.列A
from openrowset(‘SQLOLEDB’ , ‘sql服务器名’ ; ‘用户名’ ; ‘密码’ , 数据库名.dbo.表名) as a inner join 本地表 b
on a.column1 = b.column1
–openquery用法需要创建一个连接
–首先创建一个连接创建链接服务器
exec sp_addlinkedserver ‘ITSV’ , ” , ‘SQLOLEDB’ , ‘远程服务器名或ip地址’
–查询
select *
FROM openquery(ITSV , ‘SELECT * FROM 数据库.dbo.表名’)
–把本地表明锋导入远程表
insert openquery(ITSV , ‘SELECT * FROM 数据库.dbo.表名’)
select * from 本地表
–更新本地表
update b
set b.列B=a.列B
FROM openquery(ITSV , ‘SELECT * FROM 数据库.dbo.表名’) as a
inner join 本地表 b on a.列A=b.列A
–3、opendatasource/openrowset
SELECT *
FROM opendatasource(‘SQLOLEDB’ , ‘Data Source=ip/ServerName ; User ID=登陆名 ; Password=密码’).test.dbo.roy_ta
–把本地表导入远程表
insert opendatasource(‘SQLOLEDB’ , ‘Data Source=ip/ServerName ; User ID=登陆名 ; Password=密码’).数据库.dbo.表名
select * from 本地表
关于在另外一个表格的数据库的介绍到此就结束了，不知道你从中找到你需要的信息了吗 ？如果你还想了解更多这方面的信息，记得收藏关注本站。
