Task04: set operation - addition and subtraction of tables, join, etc. - learning notes of Tianchi Longzhu project SQL training camp

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Preface

This chapter is about the Tianchi dragon ball project SQL Training camp Task04 Lesson notes , About SQL Set operations - Addition and subtraction of tables join And so on .

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One 、 The addition and subtraction of tables

1. What is set operation

Sets represent... In the field of mathematics “ The sum of all kinds of things ”, Represents a collection of records in the database domain . say concretely , surface 、 The execution results of views and queries are collections of records , The elements are each row in the table or query results .
In standard SQL in , Use... For the search results respectively UNION, INTERSECT, EXCEPT To search the results and , Intersection and difference operations , image UNION,INTERSECT, EXCEPT This kind of operator used for set operation is called set operator .

Two 、 Link (JOIN)

1. Inner link (INNER JOIN)

The syntax format of the inner link is :
– Inner link
FROM <tb_1> INNER JOIN <tb_2> ON <condition(s)>

2. External links (OUTER JOIN)

— The inner link will discard the unsatisfied in the two tables ON The conditions are right , The opposite of the inner link is the outer link . The outer link will selectively keep the unmatched rows according to the type of outer link .
According to which table the reserved rows are located , There are three forms of external links : Left link , Right link and all outer link .
The left link will be saved and cannot be followed in the left table ON Clause matches to the line , At this time, the rows corresponding to the right table are all missing values ; The right link will save the right table, which cannot be followed ON Clause matches to the line , At this time, the rows corresponding to the left table are all missing values ; The total external join will save two tables at the same time ON Clause matches to the line , The corresponding row in another table is filled with missing values .
The corresponding grammars of the three external links are :
– Left link
FROM <tb_1> LEFT OUTER JOIN <tb_2> ON <condition(s)>
– Right link
FROM <tb_1> RIGHT OUTER JOIN <tb_2> ON <condition(s)>
– All external links
FROM <tb_1> FULL OUTER JOIN <tb_2> ON <condition(s)>

summary

That's what we're going to talk about today , This article only briefly introduces the set operation 、 Definition of intra table connection and outer table connection .
author ： Yu xiaoxiaoxiaoyu

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