What Kind of Join Do I Need

How do I decide when to use right joins/left joins or inner joins Or how to determine which table is on which side?

Yes, it depends on the situation you are in.

Why use SQL JOIN?

Answer: Use the SQL JOIN whenever multiple tables must be accessed through an SQL SELECT statement and no results should be returned if there is not a match between the JOINed tables.

Reading this original article on The Code Project will help you a lot: Visual Representation of SQL Joins.

Also check this post: SQL SERVER – Better Performance – LEFT JOIN or NOT IN?.

Find original one at: Difference between JOIN and OUTER JOIN in MySQL.

What kind of join do I need?

You would use an INNER join to establish the relationship between the common gameid field;

select
  votes.userid,
  games.title
from games
  inner join votes on (votes.gameid = game.gameid)
where
  votes.userid = 'a'

What's the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN and FULL JOIN?

Reading this original article on The Code Project will help you a lot: Visual Representation of SQL Joins.

Also check this post: SQL SERVER – Better Performance – LEFT JOIN or NOT IN?.

Find original one at: Difference between JOIN and OUTER JOIN in MySQL.

SQL JOIN and different types of JOINs

What is SQL JOIN ?

SQL JOIN is a method to retrieve data from two or more database tables.

What are the different SQL JOINs ?

There are a total of five JOINs. They are :

  1. JOIN or INNER JOIN
  2. OUTER JOIN

     2.1 LEFT OUTER JOIN or LEFT JOIN
     2.2 RIGHT OUTER JOIN or RIGHT JOIN
     2.3 FULL OUTER JOIN or FULL JOIN

  3. NATURAL JOIN
  4. CROSS JOIN
  5. SELF JOIN

1. JOIN or INNER JOIN :

In this kind of a JOIN, we get all records that match the condition in both tables, and records in both tables that do not match are not reported.

In other words, INNER JOIN is based on the single fact that: ONLY the matching entries in BOTH the tables SHOULD be listed.

Note that a JOIN without any other JOIN keywords (like INNER, OUTER, LEFT, etc) is an INNER JOIN. In other words, JOIN is
a Syntactic sugar for INNER JOIN (see: Difference between JOIN and INNER JOIN).

2. OUTER JOIN :

OUTER JOIN retrieves

Either,
the matched rows from one table and all rows in the other table
Or,
all rows in all tables (it doesn't matter whether or not there is a match).

There are three kinds of Outer Join :

2.1 LEFT OUTER JOIN or LEFT JOIN

This join returns all the rows from the left table in conjunction with the matching rows from the
right table. If there are no columns matching in the right table, it returns NULL values.

2.2 RIGHT OUTER JOIN or RIGHT JOIN

This JOIN returns all the rows from the right table in conjunction with the matching rows from the
left table. If there are no columns matching in the left table, it returns NULL values.

2.3 FULL OUTER JOIN or FULL JOIN

This JOIN combines LEFT OUTER JOIN and RIGHT OUTER JOIN. It returns rows from either table when the conditions are met and returns NULL value when there is no match.

In other words, OUTER JOIN is based on the fact that: ONLY the matching entries in ONE OF the tables (RIGHT or LEFT) or BOTH of the tables(FULL) SHOULD be listed.

Note that `OUTER JOIN` is a loosened form of `INNER JOIN`.

3. NATURAL JOIN :

It is based on the two conditions :

1. the JOIN is made on all the columns with the same name for equality.
2. Removes duplicate columns from the result.

This seems to be more of theoretical in nature and as a result (probably) most DBMS
don't even bother supporting this.

4. CROSS JOIN :

It is the Cartesian product of the two tables involved. The result of a CROSS JOIN will not make sense
in most of the situations. Moreover, we won't need this at all (or needs the least, to be precise).

5. SELF JOIN :

It is not a different form of JOIN, rather it is a JOIN (INNER, OUTER, etc) of a table to itself.

JOINs based on Operators

Depending on the operator used for a JOIN clause, there can be two types of JOINs. They are

1. Equi JOIN
2. Theta JOIN

1. Equi JOIN :

For whatever JOIN type (INNER, OUTER, etc), if we use ONLY the equality operator (=), then we say that
the JOIN is an EQUI JOIN.

2. Theta JOIN :

This is same as EQUI JOIN but it allows all other operators like >, <, >= etc.

Many consider both EQUI JOIN and Theta JOIN similar to INNER, OUTER
etc JOINs. But I strongly believe that its a mistake and makes the
ideas vague. Because INNER JOIN, OUTER JOIN etc are all connected with
the tables and their data whereas EQUI JOIN and THETA JOIN are only
connected with the operators we use in the former.

Again, there are many who consider NATURAL JOIN as some sort of
"peculiar" EQUI JOIN. In fact, it is true, because of the first
condition I mentioned for NATURAL JOIN. However, we don't have to
restrict that simply to NATURAL JOINs alone. INNER JOINs, OUTER JOINs
etc could be an EQUI JOIN too.

Difference between JOIN and INNER JOIN

They are functionally equivalent, but INNER JOIN can be a bit clearer to read, especially if the query has other join types (i.e. LEFT or RIGHT or CROSS) included in it.

What kind of JOIN do I need to perform this type of query?

You can cross join to safety tests from cars - then left join to TestResults - the cross join ensures you get the 10 x 20 matrix, the left join gives you the results

Use ISNULL or COALESCE to replace nulls in the result set with zeroes

e.g.

SELECT car.name, testresults.testname, isnull(testresults.result, 0) FROM 
cars CROSS JOIN
safetytests
LEFT JOIN testresults on safetytests.testid = testresults.testid AND car.id = testresults.carid

Did a sqlfiddle as someone suggested :) real good that site

http://sqlfiddle.com/#!3/2bc73/2

What is the difference between INNER JOIN and OUTER JOIN ?

Assuming you're joining on columns with no duplicates, which is a very common case:

• An inner join of A and B gives the result of A intersect B, i.e. the inner part of a Venn diagram intersection.

• An outer join of A and B gives the results of A union B, i.e. the outer parts of a Venn diagram union.

Examples

Suppose you have two tables, with a single column each, and data as follows:

A    B
-    -
1    3
2    4
3    5
4    6

Note that (1,2) are unique to A, (3,4) are common, and (5,6) are unique to B.

Inner join

An inner join using either of the equivalent queries gives the intersection of the two tables, i.e. the two rows they have in common.

select * from a INNER JOIN b on a.a = b.b;
select a.*, b.*  from a,b where a.a = b.b;

a | b
--+--
3 | 3
4 | 4

Left outer join

A left outer join will give all rows in A, plus any common rows in B.

select * from a LEFT OUTER JOIN b on a.a = b.b;
select a.*, b.*  from a,b where a.a = b.b(+);

a |  b
--+-----
1 | null
2 | null
3 |    3
4 |    4

Right outer join

A right outer join will give all rows in B, plus any common rows in A.

select * from a RIGHT OUTER JOIN b on a.a = b.b;
select a.*, b.*  from a,b where a.a(+) = b.b;

a    |  b
-----+----
3    |  3
4    |  4
null |  5
null |  6

Full outer join

A full outer join will give you the union of A and B, i.e. all the rows in A and all the rows in B. If something in A doesn't have a corresponding datum in B, then the B portion is null, and vice versa.

select * from a FULL OUTER JOIN b on a.a = b.b;

 a   |  b
-----+-----
   1 | null
   2 | null
   3 |    3
   4 |    4
null |    6
null |    5

Which kind of join do I need to use here?

Looks like a cross join:

select y.field1, x.field2
from x cross join
     y;

What kind of JOIN or WHERE clause do I need?

Did you try this:

SELECT * FROM Messages m 
INNER JOIN Employee e 
INNER JOIN Departmnet d 
ON ((m.sender=e.employeeId || m.receiver=e.employeeId) d.departmentId=e.departmentId) 
WHERE m.sender = '5' OR m.receiver = '5'

What kind of join, if any, is appropriate here?

Try this Query.

select 
S.userName,
S.firstName,
S.lastName,
A.actionDetails,
A.actionDate
from Departments D 
join Staff S on d.departmentID=s.departmentID
join Actions A on s.staffID=A.staffID
WHERE D.departmentID = YourDeparmentID;

Types Of JOIN in SQl

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