معرفی انواع Join در SQL

Introduction

This is just a simple article visually explaining SQL JOINs.

Background

I’m a pretty visual person. Things seem to make more sense as a picture. I looked all over the Internet for a good graphical representation of SQL JOINs, but I couldn’t find any to my liking. Some had good diagrams but lacked completeness (they didn’t have all the possible JOINs), and some were just plain terrible. So, I decided to create my own and write an article about it.

Using the code

I am going to discuss seven different ways you can return data from two relational tables. I will be excluding cross Joins and self referencing Joins. The seven Joins I will discuss are shown below:

  1. INNER JOIN
  2. LEFT JOIN
  3. RIGHT JOIN
  4. OUTER JOIN
  5. LEFT JOIN EXCLUDING INNER JOIN
  6. RIGHT JOIN EXCLUDING INNER JOIN
  7. OUTER JOIN EXCLUDING INNER JOIN

For the sake of this article, I’ll refer to 5, 6, and 7 as LEFT EXCLUDING JOIN, RIGHT EXCLUDING JOIN, and OUTER EXCLUDING JOIN, respectively. Some may argue that 5, 6, and 7 are not really joining the two tables, but for simplicity, I will still refer to these as Joins because you use a SQL Join in each of these queries (but exclude some records with a WHERE clause).

Inner JOIN

INNER_JOIN.png

This is the simplest, most understood Join and is the most common. This query will return all of the records in the left table (table A) that have a matching record in the right table (table B). This Join is written as follows:

Left JOIN

LEFT_JOIN.png

This query will return all of the records in the left table (table A) regardless if any of those records have a match in the right table (table B). It will also return any matching records from the right table. This Join is written as follows:

Right JOIN

RIGHT_JOIN.png

This query will return all of the records in the right table (table B) regardless if any of those records have a match in the left table (table A). It will also return any matching records from the left table. This Join is written as follows:

Outer JOIN

FULL_OUTER_JOIN.png

This Join can also be referred to as a FULL OUTER JOIN or a FULL JOIN. This query will return all of the records from both tables, joining records from the left table (table A) that match records from the right table (table B). This Join is written as follows:

Left Excluding JOIN

LEFT_EXCLUDING_JOIN.png

This query will return all of the records in the left table (table A) that do not match any records in the right table (table B). This Join is written as follows:

Right Excluding JOIN

RIGHT_EXCLUDING_JOIN.png

This query will return all of the records in the right table (table B) that do not match any records in the left table (table A). This Join is written as follows:

Outer Excluding JOIN

OUTER_EXCLUDING_JOIN.png

This query will return all of the records in the left table (table A) and all of the records in the right table (table B) that do not match. I have yet to have a need for using this type of Join, but all of the others, I use quite frequently. This Join is written as follows:

Examples

Suppose we have two tables, Table_A and Table_B. The data in these tables are shown below:

The results of the seven Joins are shown below:

Note on the OUTER JOIN that the inner joined records are returned first, followed by the right joined records, and then finally the left joined records (at least, that’s how my Microsoft SQL Server did it; this, of course, is without using any ORDER BY statement).

You can visit the Wikipedia article for more info here (however, the entry is not graphical).

I’ve also created a cheat sheet that you can print out if needed. If you right click on the image below and select “Save Target As…”, you will download the full size image.

یک پاسخ

  1. motoapk thanks می‌گه:

    Fantastic article post.Really looking forward to read more. Much obliged.

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