Posted By: Anonymous
What is a SQL
JOIN and what are different types?
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
LEFT, etc) is an
INNER JOIN. In other words,
a Syntactic sugar for
INNER JOIN (see: Difference between JOIN and INNER JOIN).
2. OUTER JOIN :
OUTER JOIN retrieves
the matched rows from one table and all rows in the other table
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
2.2 RIGHT OUTER JOIN or RIGHT JOIN
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
2.3 FULL OUTER JOIN or FULL JOIN
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 :
JOINis made on all the columns with the same name for equality.
- 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
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
- Equi JOIN
- Theta JOIN
1. Equi JOIN :
JOIN type (
OUTER, etc), if we use ONLY the equality operator (=), then we say that
JOIN is an
2. Theta JOIN :
This is same as
EQUI JOIN but it allows all other operators like >, <, >= etc.
Many consider both
EQUI JOINand Theta
JOINs. But I strongly believe that its a mistake and makes the
ideas vague. Because
OUTER JOINetc are all connected with
the tables and their data whereas
THETA JOINare only
connected with the operators we use in the former.
Again, there are many who consider
NATURAL JOINas some sort of
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.
etc could be an