SQL Join: Are selects between more than 2 tables still joins? - mysql

Question

When I read about inner or outer joins in SQL, all examples and descriptions are about 2 tables being joined. What if there are more than 2 tables in the query? Is that still considered a join?
I think inner join still makes sense even if it is between multiple tables; but I'm not sure outer joins makes sense between more than 2 table.
Can someone please clarify this issue?

Answer 1

Inner joins and outer joins are perfectly reasonable to use with more than 2 tables.
Inner joins force the result to display only data that has whatever row you joined on, whereas outer joins display all data no matter what.
Let us say you wanted to join 4 tables together...
select * from testtable
inner join testable2 on col1 = othercolumn
inner join testable3 on col2 = othercolumn
leftjoin testable4 on col3 = othercolumn
In this case, it would return only results that existed in the inner joins, but the result would not have to exist in the outside/left join. You are forcing testtables 2 & 3 to have a value on what you are joining on.. it cannot be null.
The left join does not care if the value is null, and will show results anyway.
I hope this helps some... Basically.. if you inner join on a value, and it can possibly be null, then the entire query will show blank. This is the scenario you would use an outter join.. you are not forcing the value to exist.

Answer 2

Most examples of joins will include two tables. However, joins can be done on any number of tables.
You can read more about joins all over the interwebs, but you might want to start with:
http://www.w3schools.com/sql/sql_join.asp
http://blog.codinghorror.com/a-visual-explanation-of-sql-joins/
The w3schools article first thing stated is:
SQL joins are used to combine rows from two or more tables.
This isn't entirely true, as you can even join tables on themselvees!
Consider:
Employees
-----
EmployeeId
ManagerId
EmployeeName
if you want to find out the employees of a specific manager, that could be written as:
select manager.EmployeeName, subordinates.*
from employees manager
inner join employees subordinates on manager.employeeId = subordinates.managerId
For multiple table joins consider:
Employees
----
EmployeeId
ManagerId
EmployeeName
Departments
----
DepartmentId
DepartmentName
EmployeeDepartments
----
DepartmentId
EmployeeId
In this case, if you wanted to find out all department names that employee 5 belonged too, you could do:
select d.DepartmentName
from employees e
inner join employeeDepartments ed on e.employeeId = ed.employeeId
inner join departments d on ed.departmentId = d.departmentId
where e.employeeId = 5
TLDR; - yes including more than 2 tables is still considered join(s)

Answer 3

Every join clause is (logically) between two virtual tables but the virtual tables themselves can be defined as joins on further tables.
So in the following example
SELECT Foo
FROM A
INNER JOIN B
ON A.X = B.X
INNER JOIN C
ON C.Y = A.Y
AND C.Z = B.Z
it can be considered that logically A joins to B then the virtual table (A x B) is joined to C. Columns from all three of those tables are thus available in the final ON clause.
You can control the virtual tables that are evaluated by the placement of the ON clause.
The following example creates a virtual table (A x B) and a virtual table (C x D) and then joins these two together.
SELECT Foo
FROM A
INNER JOIN B
ON A.X = B.X
INNER JOIN C
INNER JOIN D
ON C.Y = D.Y /*Only C and D in scope here*/
ON A.Z = D.Z /*All tables back in scope*/
The query optimiser is free to actually implement the joins in any way that maintains the semantics however. As inner joins are commutative and associative the tables in the above example can be freely re-arranged. For outer joins re-arranging them could change the semantics.