Use of join in SQL Query

SQL QUERY

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, andOUTER 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

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:

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SELECT <select_list>

FROM Table_A A

INNER JOIN Table_B B

ON A.Key = B.Key

Left JOIN

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:

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SELECT <select_list>

FROM Table_A A

LEFT JOIN Table_B B

ON A.Key = B.Key

Right JOIN

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:

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SELECT <select_list>

FROM Table_A A

RIGHT JOIN Table_B B

ON A.Key = B.Key

Outer JOIN

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:

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SELECT <select_list>

FROM Table_A A

FULL OUTER JOIN Table_B B

ON A.Key = B.Key

Left Excluding JOIN

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:

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SELECT <select_list>

FROM Table_A A

LEFT JOIN Table_B B

ON A.Key = B.Key

WHERE B.Key IS NULL

Right Excluding JOIN

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:

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SELECT <select_list>

FROM Table_A A

RIGHT JOIN Table_B B

ON A.Key = B.Key

WHERE A.Key IS NULL

Outer Excluding JOIN

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:

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SELECT <select_list>

FROM Table_A A

FULL OUTER JOIN Table_B B

ON A.Key = B.Key

WHERE A.Key IS NULL OR B.Key IS NULL

Examples

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

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TABLE_A

  PK Value

---- ----------

   1 FOX

   2 COP

   3 TAXI

   6 WASHINGTON

   7 DELL

   5 ARIZONA

   4 LINCOLN

  10 LUCENT

TABLE_B

  PK Value

---- ----------

   1 TROT

   2 CAR

   3 CAB

   6 MONUMENT

   7 PC

   8 MICROSOFT

   9 APPLE

  11 SCOTCH

The results of the seven Joins are shown below:

INNER JOIN : Inner join is work as Intersection(ᴒ) , that select only unique(same)  value from both tables.

SELECT A.PK AS A_PK, A.Value AS A_Value,

       B.Value AS B_Value, B.PK AS B_PK

FROM Table_A A

INNER JOIN Table_B B

ON A.PK = B.PK

                                            

 

A_PK A_Value    B_Value    B_PK

---- ---------- ---------- ----

   1 FOX        TROT          1

   2 COP        CAR           2

   3 TAXI       CAB           3

   6 WASHINGTON MONUMENT      6

   7 DELL       PC            7

(5 row(s) affected)                     

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LEFT JOIN : Left join is work as Intersection(ᴒ) of A and B and Union of A , that select only A Table all value like as : A U (A ᴒ B).

SELECT A.PK AS A_PK, A.Value AS A_Value,

B.Value AS B_Value, B.PK AS B_PK

FROM Table_A A

LEFT JOIN Table_B B                                          

ON A.PK = B.PK

                                                 

A_PK A_Value    B_Value    B_PK

---- ---------- ---------- ----

   1 FOX        TROT          1

   2 COP        CAR           2

   3 TAXI       CAB           3

   4 LINCOLN    NULL       NULL

   5 ARIZONA    NULL       NULL

   6 WASHINGTON MONUMENT      6

   7 DELL       PC            7

  10 LUCENT     NULL       NULL

(8 row(s) affected)

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RIGHT JOIN : Right join is work as Intersection(ᴒ) of A and B and Union of B , that select only B Table all value like as : B U (A ᴒ B).

SELECT A.PK AS A_PK, A.Value AS A_Value,

B.Value AS B_Value, B.PK AS B_PK

FROM Table_A A

RIGHT JOIN Table_B B

ON A.PK = B.PK

                                           

A_PK A_Value    B_Value    B_PK

---- ---------- ---------- ----

   1 FOX        TROT          1

   2 COP        CAR           2

   3 TAXI       CAB           3

   6 WASHINGTON MONUMENT      6

   7 DELL       PC            7

NULL NULL       MICROSOFT     8

NULL NULL       APPLE         9

NULL NULL       SCOTCH       11

(8 row(s) affected)

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OUTER JOIN : Outerjoin is work as Union of A and B like as : (A U B).

SELECT A.PK AS A_PK, A.Value AS A_Value,

B.Value AS B_Value, B.PK AS B_PK

FROM Table_A A

FULL OUTER JOIN Table_B B

ON A.PK = B.PK

                                           

A_PK A_Value    B_Value    B_PK

---- ---------- ---------- ----

   1 FOX        TROT          1

   2 COP        CAR           2

   3 TAXI       CAB           3

   6 WASHINGTON MONUMENT      6

   7 DELL       PC            7

NULL NULL       MICROSOFT     8

NULL NULL       APPLE         9

NULL NULL       SCOTCH       11

   5 ARIZONA    NULL       NULL

   4 LINCOLN    NULL       NULL

  10 LUCENT     NULL       NULL

(11 row(s) affected)

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LEFT EXCLUDING JOIN : It is work as (A – B) , Only select left table(A) data that not present in B table.

SELECT A.PK AS A_PK, A.Value AS A_Value,

B.Value AS B_Value, B.PK AS B_PK

FROM Table_A A

LEFT JOIN Table_B B

ON A.PK = B.PK

WHERE B.PK IS NULL

                                         

A_PK A_Value    B_Value    B_PK

---- ---------- ---------- ----

   4 LINCOLN    NULL       NULL

   5 ARIZONA    NULL       NULL

  10 LUCENT     NULL       NULL

(3 row(s) affected)

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RIGHT EXCLUDING JOIN : It is work as (B - A) , Only select right table(B) data that not present in A table.

SELECT A.PK AS A_PK, A.Value AS A_Value,

B.Value AS B_Value, B.PK AS B_PK

FROM Table_A A

RIGHT JOIN Table_B B

ON A.PK = B.PK

WHERE A.PK IS NULL

                                       

A_PK A_Value    B_Value    B_PK

---- ---------- ---------- ----

NULL NULL       MICROSOFT     8

NULL NULL       APPLE         9

NULL NULL       SCOTCH       11

(3 row(s) affected)

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OUTER EXCLUDING JOIN : It is work as (A U B)- (A ᴒ B), select both tables data that not same.

SELECT A.PK AS A_PK, A.Value AS A_Value,

B.Value AS B_Value, B.PK AS B_PK

FROM Table_A A

FULL OUTER JOIN Table_B B

ON A.PK = B.PK

WHERE A.PK IS NULL

OR B.PK IS NULL

                                                

   

A_PK A_Value    B_Value    B_PK

---- ---------- ---------- ----

NULL NULL       MICROSOFT     8

NULL NULL       APPLE         9

NULL NULL       SCOTCH       11

   5 ARIZONA    NULL       NULL

   4 LINCOLN    NULL       NULL

  10 LUCENT     NULL       NULL

(6 row(s) affected)

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.

JOINS

·                     Left join

·                     right join

·                     INNER join

·                     FULL OUTER JOIN

 

 

 

 

 

INNER JOIN

SELECT * FROM  Table1 INNER JOIN  Table2 ON Table1.name = Table2.name

 

FULL OUTER JOIN

SELECT * FROM Table1  FULL OUTER JOIN Table2 ON Table1.name = Table2.name

LEFT OUTER JOIN

SELECT * FROM Table1 LEFT OUTER JOIN Table2 ON Table1.name = Table2.name

RIGHT OUTER JOIN

SELECT * FROM Table1 RIGHT OUTER JOIN Table2 ON Table1.name = Table2.name

LEFT OUTER JOIN WITH EXCLUDE OF RIGHT SIDE

SELECT * FROM Table1 LEFT OUTER JOIN Table2 ON Table1.name = Table2.name WHERE Table2.id IS null

ULL OUTER JOIN WITH EXCLUDE OF BOTH SIDES

SELECT * FROM TableA FULL OUTER JOIN TableB ON TableA.name = TableB.name WHERE TableA.id IS null OR TableB.id IS null