Three table join with joins other than INNER JOIN

Question

I am learning SQL and am trying to learn JOINs this week.

I have gotten to the level where I can do three table joins, similar to a lot of examples I've seen. I'm still trying to figure out the tiny details of how things work. All the examples I've seen of three table joins use INNER JOINS only. What about LEFT and RIGHT JOINs? Do you ever use these in three table joins? What would it mean?

SELECT ~some columns~ FROM ~table name~
LEFT JOIN ~table 2~ ON ~criteria~
INNER JOIN ~table 3~ ON ~criteria~

or

SELECT ~some columns~ FROM ~table name~
INNER JOIN ~table 2~ ON ~criteria~
LEFT JOIN ~table 3~ ON ~criteria~

or

SELECT ~some columns~ FROM ~table name~
LEFT JOIN ~table 2~ ON ~criteria~
LEFT JOIN ~table 3~ ON ~criteria~

or

???

Just trying to explore the space as much as possible

Answer 1

Yes, I do use all three of those JOINs, although I tend to stick to using just LEFT (OUTER) JOINs instead of inter-mixing LEFT and RIGHT JOINs. I also use FULL OUTER JOINs and CROSS JOINs.

In summary, an INNER JOIN restricts the resultset only to those records satisfied by the JOIN condition. Consider the following tables

EDIT: I've renamed the Table names and prefix them with @ so that Table Variables can be used for anyone reading this answer and wanting to experiment.

If you'd also like to experiment with this in the browser, I've set this all up on SQL Fiddle too;

@Table1

id | name
---------
1  | One
2  | Two
3  | Three
4  | Four

@Table2

id | name
---------
1  | Partridge
2  | Turtle Doves
3  | French Hens
5  | Gold Rings

SQL code

DECLARE @Table1 TABLE (id INT PRIMARY KEY CLUSTERED, [name] VARCHAR(25))

INSERT INTO @Table1 VALUES(1, 'One');
INSERT INTO @Table1 VALUES(2, 'Two');
INSERT INTO @Table1 VALUES(3, 'Three');
INSERT INTO @Table1 VALUES(4, 'Four');

DECLARE @Table2 TABLE (id INT PRIMARY KEY CLUSTERED, [name] VARCHAR(25))

INSERT INTO @Table2 VALUES(1, 'Partridge');
INSERT INTO @Table2 VALUES(2, 'Turtle Doves');
INSERT INTO @Table2 VALUES(3, 'French Hens');
INSERT INTO @Table2 VALUES(5, 'Gold Rings');

An INNER JOIN SQL Statement, joined on the id field

SELECT 
    t1.id,
    t1.name,
    t2.name
FROM
    @Table1 t1
INNER JOIN
    @Table2 t2
    ON 
        t1.id = t2.id

Results in

id | name | name
----------------
1  | One  | Partridge
2  | Two  | Turtle Doves
3  | Three| French Hens

A LEFT JOIN will return a resultset with all records from the table on the left hand side of the join (if you were to write out the statement as a one liner, the table that appears first) and fields from the table on the right side of the join that match the join expression and are included in the SELECT clause. Missing details will be populated with NULL

SELECT 
    t1.id,
    t1.name,
    t2.name
FROM
    @Table1 t1
LEFT JOIN
    @Table2 t2
    ON 
        t1.id = t2.id

Results in

id | name | name
----------------
1  | One  | Partridge
2  | Two  | Turtle Doves
3  | Three| French Hens
4  | Four | NULL

A RIGHT JOIN is the same logic as a LEFT JOIN but will return all records from the right-hand side of the join and fields from the left side that match the join expression and are included in the SELECT clause.

SELECT 
    t1.id,
    t1.name,
    t2.name
FROM
    @Table1 t1
RIGHT JOIN
    @Table2 t2
    ON 
        t1.id = t2.id

Results in

id | name | name
----------------
1  | One  | Partridge
2  | Two  | Turtle Doves
3  | Three| French Hens
NULL| NULL| Gold Rings

Of course, there is also the FULL OUTER JOIN, which includes records from both joined tables and populates any missing details with NULL.

SELECT 
    t1.id,
    t1.name,
    t2.name
FROM
    @Table1 t1
FULL OUTER JOIN
    @Table2 t2
    ON 
        t1.id = t2.id

Results in

id | name | name
----------------
1  | One  | Partridge
2  | Two  | Turtle Doves
3  | Three| French Hens
4  | Four | NULL
NULL| NULL| Gold Rings

And a CROSS JOIN (also known as a CARTESIAN PRODUCT), which is simply the product of cross applying fields in the SELECT statement from one table with the fields in the SELECT statement from the other table. Notice that there is no join expression in a CROSS JOIN

SELECT 
    t1.id,
    t1.name,
    t2.name
FROM
    @Table1 t1
CROSS JOIN
    @Table2 t2

Results in

id | name  | name
------------------
1  | One   | Partridge
2  | Two   | Partridge
3  | Three | Partridge
4  | Four  | Partridge
1  | One   | Turtle Doves
2  | Two   | Turtle Doves
3  | Three | Turtle Doves
4  | Four  | Turtle Doves
1  | One   | French Hens
2  | Two   | French Hens
3  | Three | French Hens
4  | Four  | French Hens
1  | One   | Gold Rings
2  | Two   | Gold Rings
3  | Three | Gold Rings
4  | Four  | Gold Rings

EDIT:

Imagine there is now a Table3

@Table3

id | name
---------
2  | Prime 1
3  | Prime 2
5  | Prime 3

The SQL code

DECLARE @Table3 TABLE (id INT PRIMARY KEY CLUSTERED, [name] VARCHAR(25))

INSERT INTO @Table3 VALUES(2, 'Prime 1');
INSERT INTO @Table3 VALUES(3, 'Prime 2');
INSERT INTO @Table3 VALUES(5, 'Prime 3');

Now all three tables joined with INNER JOINS

SELECT 
    t1.id,
    t1.name,
    t2.name,
    t3.name
FROM
    @Table1 t1
INNER JOIN
    @Table2 t2
    ON 
        t1.id = t2.id
INNER JOIN
    @Table3 t3
    ON 
        t1.id = t3.id

Results in

id | name | name         | name
-------------------------------
2  | Two  | Turtle Doves | Prime 1
3  | Three| French Hens  | Prime 2

It might help to understand this result by thinking that records with id 2 and 3 are the only ones common to all 3 tables and are also the field we are joining each table on.

Now all three with LEFT JOINS

SELECT 
    t1.id,
    t1.name,
    t2.name,
    t3.name
FROM
    @Table1 t1
LEFT JOIN
    @Table2 t2
    ON 
        t1.id = t2.id
LEFT JOIN
    @Table3 t3
    ON 
        t1.id = t3.id

Results in

id | name | name         | name
-------------------------------
1  | One  | Partridge    | NULL
2  | Two  | Turtle Doves | Prime 1
3  | Three| French Hens  | Prime 2
4  | Four | NULL         | NULL

Joel's answer is a good explanation for explaining this resultset (Table1 is the base/origin table).

Now with a INNER JOIN and a LEFT JOIN

SELECT 
    t1.id,
    t1.name,
    t2.name,
    t3.name
FROM
    @Table1 t1
INNER JOIN
    @Table2 t2
    ON 
        t1.id = t2.id
LEFT JOIN
    @Table3 t3
    ON 
        t1.id = t3.id

Results in

id | name | name         | name
-------------------------------
1  | One  | Partridge    | NULL
2  | Two  | Turtle Doves | Prime 1
3  | Three| French Hens  | Prime 2

Although we do not know the order in which the query optimiser will perform the operations, we will look at this query from top to bottom to understand the resultset. The INNER JOIN on ids between Table1 and Table2 will restrict the resultset to only those records satisfied by the join condition i.e. the three rows that we saw in the very first example. This temporary resultset will then be LEFT JOINed to Table3 on ids between Table1 and Tables; There are records in Table3 with id 2 and 3, but not id 1, so t3.name field will have details in for 2 and 3 but not 1.