Joins in databases combine records from two or more tables. The main types of joins are natural joins, equijoins, self joins, and outer joins. Natural joins automatically match columns with the same name, while equijoins use equality comparisons in the WHERE clause. Self joins match a table to itself, and outer joins return all records from one or both tables even if there are no matches.

1. JOINS IN DATABASE

2. Definition Of JOINS :-
An SQL join clause combines
records from two or more tables in a
database. It creates a set that can be
saved as a table or used as is. A JOIN
is a means for combining fields from
two tables by using values common
to each.

3. Natural
Join
Self JOIN Equi
Join Join
S
Outer
Join

4.  Natural Join - The NATURAL JOIN is a type of
equi-join and is structured in such a way
that, columns with same name of associate tables
will appear once only.
Eg - Select * FROM table1 NATURAL JOIN
table2;
 EquiJoin - SQL EQUI JOIN performs a JOIN
against equality or matching column(s) values of the
associated tables. An equal sign (=) is used as
comparison operator in the where clause to refer
equality.
Eg - Select column_list FROM
table1, table2 WHERE table1.column_name
=table2.column_name

5.  Self Join - SELF JOIN is used to join a table to
itself, as if the table were two tables, temporarily
renaming at least one table in the SQL statement.
Eg - SELECT a.column_name, b.column_name FROM
table1 a, table1 b WHERE a.common_filed =
b.common_field;
 Outer Join –
 Right Outer Join -
 Left Outer Join

6.  Right Outer Join - The LEFT JOIN keyword returns
all rows from the left table (table_name1), even if there
are no matches in the right table (table_name2).
Eg - SELECT column_name(s) FROM table_name1 LEFT JOIN
table_name2 ON table_name1.column_name=table_name2.column_name
 Left Outer Join - The RIGHT JOIN keyword
returns all the rows from the right table
(table_name2), even if there are no matches in the
left table (table_name1).
Eg -SELECT column_name(s) FROM table_name1 RIGHT JOIN
table_name2 ON
table_name1.column_name=table_name2.column_name

7.  A SQL nested query is a SELECT query that is
nested inside a SELECT, UPDATE, INSERT, or
DELETE SQL query. Here is a simple example
of SQL nested query:
Eg - SELECT Model FROM Product WHERE
ManufacturerID IN (SELECT ManufacturerID
FROM Manufacturer WHERE Manufacturer =
'Dell')

8.  UNIONALL - Combines the results of two SELECT
statements into one result set.
 SELECTCUST_NBR, NAME FROM CUSTOMER WHERE
REGION_ID = 5 UNION ALL SELECT
C.CUST_NBR, C.NAME FROM CUSTOMER C WHERE
C.CUST_NBR IN (SELECT O.CUST_NBR FROM
CUST_ORDER O, EMPLOYEE E WHERE
O.SALES_EMP_ID = E.EMP_ID AND E.LNAME =
'MARTIN');

9. • UNION - Combines the results of two SELECT
statements into one result set, and then
eliminates any duplicate rows from that result
set.
Eg - SELECT CUST_NBR, NAME FROM
CUSTOMER WHERE REGION_ID = 5 UNION
SELECT C.CUST_NBR, C.NAME FROM
CUSTOMER C WHERE C.CUST_NBR IN
(SELECT O.CUST_NBR FROM CUST_ORDER
O, EMPLOYEE E WHERE O.SALES_EMP_ID =
E.EMP_ID AND E.LNAME = 'MARTIN');

10.  MINUS - Takes the result set of one SELECT
statement, and removes those rows that are also
returned by a second SELECT statement.
 Eg - SELECT CUST_NBR, NAME FROM CUSTOMER
WHERE REGION_ID = 5 MINUS SELECT
C.CUST_NBR, C.NAME FROM CUSTOMER C WHERE
C.CUST_NBR IN (SELECT O.CUST_NBR FROM
CUST_ORDER O, EMPLOYEE E WHERE
O.SALES_EMP_ID = E.EMP_ID AND E.LNAME =
'MARTIN');

11.  INTERSECT - Returns only those rows that are returned
by each of two SELECT statements.
 Eg - SELECT CUST_NBR, NAME FROM CUSTOMER
WHERE REGION_ID = 5 INTERSECT SELECT
C.CUST_NBR, C.NAME FROM CUSTOMER C WHERE
C.CUST_NBR IN (SELECT O.CUST_NBR FROM
CUST_ORDER O, EMPLOYEE E WHERE
O.SALES_EMP_ID = E.EMP_ID AND E.LNAME =
'MARTIN');