Basic of Join

INNER JOIN

This join returns rows when there is at least one match in both the tables.

OUTER JOIN

There are three different Outer Join methods.

LEFT OUTER 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.


RIGHT OUTER 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.


FULL OUTER JOIN
This join combines left outer join and right after join. It returns row from either table when the conditions are met and returns null value when there is no match.

CROSS JOIN

This join is a Cartesian join that does not necessitate any condition to join. The resultset contains records that are multiplication of record number from both the tables.

Additional Notes related to JOIN:

The following are three classic examples to display where Outer Join is useful. You will notice several instances where developers write query as given below.

SELECT t1.*FROM Table1 t1WHERE t1.ID NOT IN (SELECT t2.ID FROM Table2 t2)GO

The query demonstrated above can be easily replaced by Outer Join. Indeed, replacing it by Outer Join is the best practice. The query that gives same result as above is displayed here using Outer Join and WHERE clause in join.

/* LEFT JOIN - WHERE NULL */SELECT t1.*,t2.*FROM Table1 t1LEFT JOIN Table2 t2 ON t1.ID = t2.IDWHERE t2.ID IS NULL

The above example can also be created using Right Outer Join.


NOT INNER JOIN
Remember, the term Not Inner Join does not exist in database terminology. However, when full Outer Join is used along with WHERE condition, as explained in the above two examples, it will give you exclusive result to Inner Join. This join will give all the results that were not present in Inner Join.

Relational Algebra

What is Relational Algebra?

1.    The relation algebra is a procedure query language.

2.    It consists of set of operation that takes one or two relation as input and produced a new relation as result.

3.    The select, project, and rename operation are called unary operation because they operate on one relation.

4.    Other operations operate on more than one relation and therefore it’s called binary operation.

Six basic operators:

·       Unary Operators

1)   Select    -  σ  (Sigma)

2)   Project   -  ∏ (PI)

3)    Rename -  ρ  (rho)

·       Binary Operators

1)    Union    -  U

2)    Set Difference - –

3)    Cartesian Product - X

Four Additional operators:

1)    Set intersection - ∩

2)    Natural join      -  

                    I.        Left Outer Join    -  

                  II.        Right Outer Join -  

                 III.        Full Outer Join   -

3)    Division - ÷

4)    Assignment - ←

Three Database Modification operators:

1)    Insert Operator  r ← r U E

2)    Delete Operator  r ← r – E

3)    Update Operator r ← ∏ F1,F2….Fn  (r)

Projection

Example:   (Table) EMPLOYEE

nr	name	salary
1	John	100
5	Sarah	300
7	Tom	100

SQL	Result	Relational algebra
select salary from Employee	salary 100 300	∏salary(Employee)
select nr, salary from Employee	nr salary 1 100 5 300 7 100	∏nr, salary(E)

Note that there are no duplicate rows in the result.

Selection

SQL	Result	Relational algebra
select * from Employee where salary < 200	nr name salary 1 John 100 7 Tom 100	σsalary < 200(Employee)
select * from Employee where salary < 200 and nr >= 7	nr name salary 7 Tom 100	σsalary < 200 and nr >= 7(Employee)

Cartesian product

The cartesian product of two tables combines each row in one table with each row in the other table.
Example: The table  (for EMPLOYEE)

enr	ename	dept
1	Parag	A
2	Kapil	C
3	Deven	A

(DEPARTMENT)

dnr	dname
A	Marketing
B	Sales
C	Legal

SQL	Result	Relational algebra
select * from E, D	enr ename dept dnr dname 1 Parag A A Marketing 1 Parag A B Sales 1 Parag A C Legal 2 Kapil C A Marketing 2 Kapil C B Sales 2 Kapil C C Legal 3 Deven A A Marketing 3 Deven A B Sales 3 Deven A C Legal	Employee X Department

• Seldom useful in practice.
• Usually an error.
• Can give a huge result.

The Natural join Operation:

•   The Natural join is a binary operation that allows us to combine certain selections and Cartesian product into one operation.
•   It is denoted by the join symbol ( ).
•   The natural join operation forms a Cartesian product of its two arguments, performs a selection forcing equality on those attribute that appear in both relation, and removes the duplicate attributes.

Example

                                       EMP

Emp No	Emp Name	City	Salary	Deptno
1	Parag	Snagar	17000	10
2	Kapil	Sidhpur	18000	20
3	Deven	Junagadh	20000	10
4	Rahul	Rajkot	22000	30
5	Saurav	Jamnagar	35000	Null

    DEPT

Deptno	Deptname	Location
10	Computer	Ahmedabad
20	Account	Surat
30	Finance	Banglore
40	Research	Bombay
50	Marketing	Rajkot

∏ Emp.Empno,Empname,Salary,Deptname

 

(σEmp.deptno=dept.deptno ^{(EMP                                DEPT)})

Output:

Empno	Empname	Salary	Deptname
1	Parag	17000	Computer
2	Kapil	18000	Account
3	Deven	20000	Computer
4	Rahul	22000	Finance

•   The result of this expression is shown in above table. Notice that we have lost the record about Saurav because he is not working in any department.
•   Similarly, we have lost the record of deptno 40 and 50 from Dept because there is no employee working in deptno 40 and 50.
•   In short, it returns record on base of deptno.