学习的时候了解了一下join操作，包含left join，right join，inner join和out join，每种操作之后生成的表的空间大小不同；假设A和B表做join操作，如果是left join，则以A表为基准，然后和B表进行匹配，表的大小至少A表的大小（乘以B表中和A表中连接的最大覆盖率，及A表的一条记录所对应B表中记录的最大条数）；则right join则以B表为基准；inner join，则为A和B intersection之后的交集；outer intersection，则为A和B union之后的结果。

（下面图解以及链接非常清楚，请参考http://www.codeproject.com/Articles/33052/Visual-Representation-of-SQL-Joins）

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

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 AINNER JOIN Table_B BON 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 ALEFT JOIN Table_B BON 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 ARIGHT JOIN Table_B BON 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 AFULL OUTER JOIN Table_B BON 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 ALEFT JOIN Table_B BON A.Key = B.KeyWHERE 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 ARIGHT JOIN Table_B BON A.Key = B.KeyWHERE 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 AFULL OUTER JOIN Table_B BON A.Key = B.KeyWHERE 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 LUCENTTABLE_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:

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-- INNER JOINSELECT A.PK AS A_PK, A.Value AS A_Value,       B.Value AS B_Value, B.PK AS B_PKFROM Table_A AINNER JOIN Table_B BON A.PK = B.PKA_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 JOINSELECT A.PK AS A_PK, A.Value AS A_Value,B.Value AS B_Value, B.PK AS B_PKFROM Table_A ALEFT JOIN Table_B BON A.PK = B.PKA_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 JOINSELECT A.PK AS A_PK, A.Value AS A_Value,B.Value AS B_Value, B.PK AS B_PKFROM Table_A ARIGHT JOIN Table_B BON A.PK = B.PKA_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            7NULL NULL       MICROSOFT     8NULL NULL       APPLE         9NULL NULL       SCOTCH       11(8 row(s) affected)

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-- OUTER JOINSELECT A.PK AS A_PK, A.Value AS A_Value,B.Value AS B_Value, B.PK AS B_PKFROM Table_A AFULL OUTER JOIN Table_B BON A.PK = B.PKA_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            7NULL NULL       MICROSOFT     8NULL NULL       APPLE         9NULL 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 JOINSELECT A.PK AS A_PK, A.Value AS A_Value,B.Value AS B_Value, B.PK AS B_PKFROM Table_A ALEFT JOIN Table_B BON A.PK = B.PKWHERE B.PK IS NULLA_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 JOINSELECT A.PK AS A_PK, A.Value AS A_Value,B.Value AS B_Value, B.PK AS B_PKFROM Table_A ARIGHT JOIN Table_B BON A.PK = B.PKWHERE A.PK IS NULLA_PK A_Value    B_Value    B_PK---- ---------- ---------- ----NULL NULL       MICROSOFT     8NULL NULL       APPLE         9NULL NULL       SCOTCH       11(3 row(s) affected)

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-- OUTER EXCLUDING JOINSELECT A.PK AS A_PK, A.Value AS A_Value,B.Value AS B_Value, B.PK AS B_PKFROM Table_A AFULL OUTER JOIN Table_B BON A.PK = B.PKWHERE A.PK IS NULLOR B.PK IS NULLA_PK A_Value    B_Value    B_PK---- ---------- ---------- ----NULL NULL       MICROSOFT     8NULL NULL       APPLE         9NULL 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.