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Database Joins: Understanding Data in Another Table
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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.
相關(guān)問題拓展閱讀：
- MySQL中如何把一個數(shù)據(jù)庫中的表數(shù)據(jù)，導(dǎo)到另一個數(shù)據(jù)庫的表中
- 數(shù)據(jù)庫中怎么在一個表中鏈接另一個表
MySQL中如何把一個數(shù)據(jù)庫中的表數(shù)據(jù)，導(dǎo)到另一個數(shù)據(jù)庫的表中
把雙方的mysql服務(wù)都停掉，直接把導(dǎo)出方的數(shù)據(jù)表文件(就是和數(shù)據(jù)庫表名對應(yīng)的.frm.myd.myi文件）拷到導(dǎo)入方（注意一個表有三個文件)，然后改一下數(shù)據(jù)表文件名成你要導(dǎo)入的表名，然后啟動mysql服者滾務(wù)
，燃嫌磨如果導(dǎo)入方原皮斗來的表有數(shù)據(jù)，可以把原來的數(shù)據(jù)用正常方式導(dǎo)出，然后在工具里面導(dǎo)入合并就可以了，
再有一個可行的方法就是自寫代碼一行一行的轉(zhuǎn)移數(shù)據(jù)了
數(shù)據(jù)庫中怎么在一個表中鏈接另一個表
不同服務(wù)器數(shù)據(jù)庫之間的數(shù)據(jù)操作
–創(chuàng)建鏈接服務(wù)器
exec sp_addlinkedserver ‘ITSV’ , ” , ‘SQLOLEDB’ , ‘遠程服務(wù)器名或ip地址’
exec sp_addlinkedsrvlogin ‘ITSV’ , ‘false’ , null , ‘用戶名’ , ‘密碼’
–查詢示例
select * from ITSV.數(shù)據(jù)庫名.dbo.表名
–導(dǎo)入示例
select * into 表 from ITSV.數(shù)據(jù)庫名.dbo.表名
–以后不再使用時刪除鏈接服務(wù)器
exec sp_dropserver ‘ITSV’ , ‘droplogins’
–連接遠程/局域網(wǎng)數(shù)據(jù)(openrowset/openquery/opendatasource)
–1、openrowset
–查詢示例
select * from openrowset(‘SQLOLEDB’ , ‘sql服務(wù)器名’ ; ‘用戶名’ ; ‘密碼’ , 數(shù)據(jù)庫名.dbo.表名)
–生成本地表
select * into 表 from openrowset(‘SQLOLEDB’ , ‘sql服務(wù)器名’ ; ‘用戶名’ ; ‘密碼’ , 數(shù)據(jù)庫名.dbo.表名)
–把本地表導(dǎo)入激埋晌遠程表
insert openrowset( ‘SQLOLEDB’ , ‘sql服務(wù)器名’ ; ‘用戶名’ ; ‘密液握碼’ , 數(shù)據(jù)庫名.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服務(wù)器名’ ; ‘用戶名’ ; ‘密碼’ , 數(shù)據(jù)庫名.dbo.表名) as a inner join 本地表 b
on a.column1 = b.column1
–openquery用法需要創(chuàng)建一個連接
–首先創(chuàng)建一個連接創(chuàng)建鏈接服務(wù)器
exec sp_addlinkedserver ‘ITSV’ , ” , ‘SQLOLEDB’ , ‘遠程服務(wù)器名或ip地址’
–查詢
select *
FROM openquery(ITSV , ‘SELECT * FROM 數(shù)據(jù)庫.dbo.表名’)
–把本地表明鋒導(dǎo)入遠程表
insert openquery(ITSV , ‘SELECT * FROM 數(shù)據(jù)庫.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 數(shù)據(jù)庫.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=登陸名 ; Password=密碼’).test.dbo.roy_ta
–把本地表導(dǎo)入遠程表
insert opendatasource(‘SQLOLEDB’ , ‘Data Source=ip/ServerName ; User ID=登陸名 ; Password=密碼’).數(shù)據(jù)庫.dbo.表名
select * from 本地表
關(guān)于在另外一個表格的數(shù)據(jù)庫的介紹到此就結(jié)束了，不知道你從中找到你需要的信息了嗎 ？如果你還想了解更多這方面的信息，記得收藏關(guān)注本站。
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