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Learn about various joins in SQL and their differences

2022-07-02 06:21:00 【Jiang Xia】

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This article altogether 2215 word , Expected reading 610 minute ‍

Preface

We're writing sql At the time of statement , The most painful is the joint query involving multiple tables , Especially various links of various tables , what inner join、left join、right join、union、union all, You may feel confused when you see this .

Today's article is about studying and sorting out , These various join What exactly do they mean , What's the difference between .

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First, suppose we have two tables .Table user It's the watch on the left .Table t_user It's the watch on the right . Each has a certain record , There are two records username It's the same , As shown below ： Let's see the difference JOIN What is the difference in the use of ：

user surface ：

t_user surface ：

1.INNER JOIN

First look at it. inner join, This is also used more , Let's execute the following statement , Use the left table and the right table inner join Joint query ：

SELECT * FROM user INNER JOIN t_user ON user.username = t_user.username

The result data of the query is as follows ：

It can be found that the result of the query is the same intersection of the associated fields of the left table and the right table . That is, the query result is the data of the overlapped part in the figure below .

2.LEFT JOIN

left join It is also one of the more common joint query methods used in daily development . Again, let's look at the examples first and then discuss the results , Execute the following statement ：

SELECT * FROM user LEFT JOIN t_user ON user.username = t_user.username

The results of the query are as follows ：

At this time, if you change the left table to the right table, what is the result ？

SELECT * FROM t_user LEFT JOIN user ON user.username = t_user.username

At this time, you can summarize and use left join The result of the query is based on the left table , Query all the data in the left table , The data in the right table only shows the matching data , The mismatched data is null. Here is left .username= Right .username, So the data in the right table shown here is in the right table username And in the left table username Equal data , The right table of unequal parts is empty . It can be shown in the figure below , The result of the query is the shaded part of the red mark , That is, all the data in the left table and the data in the overlapping part of the right table and the left table .

3.RIGHT JOIN

Actually right join and left join It's like , The result of the query is the opposite .

Execute the following statement ：

SELECT * FROM user RIGHT JOIN t_user ON user.username = t_user.username

That is, the query results here are based on the data in the right table , All the data in the right table are displayed , The left table shows the data of the intersection with the right table . It can be shown in the figure below , All values in the right table of the query results , If there is a match in the left table, there is a value , If it doesn't match null Express , and left join contrary ！

4.UNION And UNION ALL

UNION The operator is used to merge two or more SELECT The result set of the statement . however ,UNION Inside SELECT Statements must have the same number of columns . Columns must also have similar data types . meanwhile , Every one of them SELECT The order of the columns in the statement must be the same .UNION Select only records , and UNION ALL All records will be listed .

First look at union：

SELECT username FROM user UNION SELECT username FROM t_user

The query results are as follows ：

Look again union all：

SELECT username FROM user UNION all SELECT username FROM t_user

The query results are as follows ：

You can find union and union all comparison ,union Two pieces of data are missing , Namely user Table and t_user Table duplicate data , therefore union The selected records will be filtered , Delete duplicate data , and union all Then all data will be selected .

union It can be expressed as follows ：

union all Shown by the following ：

5.CROSS JOIN

cross join Cross connect , Also known as Cartesian connection (cartersian join) Or cross multiplication (product), If A and B It's two sets , Their cross connections are marked as :A*B, For example, execute the following statement ：

SELECT * FROM user a CROSS JOIN t_user b

Some of the results are as follows ：

You can see that the resulting result set is the product of the respective numbers of the two tables , Usually, this kind of operation should be avoided in actual operation .

summary

That's all sql Some join queries in the statement and the differences between them , There's another one full join Indicates full connection , however mysql China does not support it. , It can be represented by the combination of left connection and right connection .full join It represents the union of left table and right table , But for records that do not match , Will take null Express .

Finally, there is a picture on the Internet that can well show the relationship between these connections , Here's the picture ：