joins in database

Database Systems
Types of Joins
Syed Sarmad AliDatabase Management Systems
1
Lecture 12

Today’s agenda(Joins in database)
Natural Join
Inner Join
Equi Join
Theta Join
Semi Join
Anti Join
Cross Join
Outer Join
 Left Outer Join
 Right Outer Join
 Full Outer Join
Self Join
Database Management Systems
2

Join
 Join is a special form of cross product of two tables.
 In Cartesian product we join a tuple of one table with the tuples
of the second table. But in join there is a special requirement of
relationship between tuples.
 For example if there is a relation STUDENT and a relation BOOK
then it may be required to know that how many books have
been issued to any particular student. Now in this case the
primary key of STUDENT that is stId is a foreign key in BOOK table
through which the join can be made.

Natural Join
 Natural join ( ) is a binary operator that is written as (R S)
where R and S are relations.
 In particular, natural join allows the combination of relations that
are associated by a foreign key.

Semi Join ( ) and ( )
 The left semijoin is joining similar to the natural join and written as R S
where R and S are relation.
 The result of this semijoin is only the set of all tuples in R for which there is a
tuple in S that is equal on their common attribute names.
 For an example consider the tables Employee and Dept and their semi join:

Semi Join (Example)
SELECT * FROM departments
WHERE EXISTS
(SELECT * FROM employees WHERE departments.department_id =
employees.department_id AND employees.salary > 2500)
ORDER BY department_name;

Outer Join
 An outer join does not require each record in the two
joined tables to have a matching record. The joined table
retains each record—even if no other matching record
exists. Outer joins subdivide further into left outer joins, right
outer joins, and full outer joins, depending on which
table(s) one retains the rows from (left, right, or both).

Left Outer Join
 The result of a left outer join (or simply left join) for table A and B always
contains all records of the "left" table (A), even if the join-condition does not
find any matching record in the "right" table (B).
 This means that if the ON clause matches 0 (zero) records in B, the join will still
return a row in the result—but with NULL in each column from B.
 This means that a left outer join returns all the values from the left table, plus
matched values from the right table (or NULL in case of no matching join
predicate).
 If the right table returns one row and the left table returns more than one
matching row for it, the values in the right table will be repeated for each
distinct row on the left table.

Employee
Dept
Employee =X Dept

Right Outer Join
 A right outer join (or right join) closely resembles a left outer join, except
with the treatment of the tables reversed. Every row from the "right" table
(B) will appear in the joined table at least once. If no matching row from
the "left" table (A) exists, NULL will appear in columns from A for those
records that have no match in B. A right outer join returns all the values from
the right table and matched values from the left table (NULL in case of no
matching join predicate).

Employee
Dept
Employee X= Dept

Full Outer Join
 Conceptually, a full outer join combines the effect of applying both left
and right outer joins. Where records in the FULL OUTER JOINed tables do not
match, the result set will have NULL values for every column of the table
that lacks a matching row. For those records that do match, a single row
will be produced in the result set (containing fields populated from both
tables).

Employee
Dept
Employee =X= Dept

Cartesian Product
 In mathematics, it is a set of all pairs of elements (x, y) that can be
constructed from given sets, X and Y, such that x belongs to X and y to Y.
 It defines a relation that is the concatenation of every tuple of relation R
with every tuple of relation S.

Inner join
 An inner join is the most common join operation used in
applications and can be regarded as the default join-type.
 Inner join creates a new result table by combining column values of
two tables (A and B) based upon the join-predicate.
 The query compares each row of A with each row of B to find all
pairs of rows which satisfy the join-predicate. When the join-
predicate is satisfied, column values for each matched pair of rows
of A and B are combined into a result row.

Equijoin
 An equi-join, also known as an equijoin, is a specific type of
comparator-based join, or theta join, that uses
only equality comparisons in the join-predicate. Using other
comparison operators (such as <) disqualifies a join as an equi-join.

θ-join
 Consider tables Car and Boat which list models of cars and boats and their
respective prices.
 Suppose a customer wants to buy a car and a boat, but she does not
want to spend more money for the boat than for the car.
 The θ-join on the relation CarPrice ≥ BoatPrice produces a table with all
the possible options. If you are using a condition where the attributes are
equal the like Price the condition may be specified as Price=Price or
alternatively (Price) itself.

Antijoin( )
 The antijoin, written as R S where R and S are relations, is similar to
the natural join, but the result of an antijoin is only those tuples in R for
which there is no tuple in S that is equal on their common attribute
names.
 For an example consider the tables Employee and Dept and their
antijoin:

Anti Join (Example)
 SELECT * FROM employees
WHERE department_id NOT IN
(SELECT department_id FROM departments WHERE location_id = 1700)
ORDER BY last_name;

Self Join
 A self-join is joining a table to itself. This is best illustrated by an example.
 A query to find all pairings of two employees in the same country is desired.
If there were two separate tables for employees and a query which
requested employees in the first table having the same country as
employees in the second table, a normal join operation could be used to
find the answer table. However, all the employee information is contained
within a single large table.

Query: Find the name of the sailor who
reserved boat 101.

joins in database

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