database management & administration_lecture 4

8/6/2019 Database Management & Administration_Lecture 4

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IAC, CSE, BUET:

P re p a re d B y A ru p R a to n R o y

Join & Subquery

Database Management &

Administration


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IAC, CSE, BUET 2:


The number of columns and the data types ofthe columns being selected must be identical

Union Returns all the tuples that are in any ofthe table eliminating the duplicates

Intersect Returns all the tuples that are inboth the tables

Minus Returns all the tuples that are in Table

1 but not in Table 2

Union, Intersection & Minus


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IAC, CSE, BUET 3:


Employee_Name Salary

Mr. A 12000

Mr. B 12300

Mr. C 14000

Mr. D 15070

Employee


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Employee2 TableEmployee_Name SalaryMr. B 12300

Mr. F 14600

Mr. E 17250

Mr. D 15070Mr. G 18250


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Select Employee_Name, Salary from Employeeunion select Employee_Name, Salary fromEmployee 2

Insert

Employee_Name SalaryMr. C 14000

Mr. D 15070


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Select Employee_Name, Salary from Employeeintersect select Employee_Name, Salaryfrom Employee 2

Union

Employee_Name SalaryMr. A 12000

Mr. B 12300

Mr. C 14000

Mr. D 15070

Mr. F 14600

Mr. E 17250

Mr. G 18250


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IAC, CSE, BUET 7:


Select Employee_Name, Salary from Employeeminus select Employee_Name, Salary fromEmployee 2

Minus

Employee_Name SalaryMr. A 12000

Mr. B 12300


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IAC, CSE, BUET 8:


Joins are very important for relationaldatabases.

Mainly used when we need to find informationthat distributes over multiple tables.

Join


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IAC, CSE, BUET 9:P re p a re d B y A ru p R a to n R o y

Employee Branch

Employee & Branch


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create table employee( employee_id number primary key,name varchar2(50),

branc_id number references branch(branch_id));

Employee Table


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create table branch(branch_id number primary key,

branch_name varcha2(50));

Branch Table


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Employee_id Name Brach_id1 Mr. A 2

2 Mr. B 1

3 Mr. C 1

4 Mr. D 15 Mr. E 2

Employee Table With Data


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Branch_id Branch_Name

1 Polashi2 Bokshibazar

Branch Table With Data


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select * from employee, branch;

Joining

join


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All the tuples from a join are not consistent.We have to eliminate the inconsistent tuples.To select the consistent tuples, where clause

should be used with joinTo uniquely identify the column name (if same

name), the name should be preceded withtable_nameand dot(.).

If the column names do not conflict then they

can be used without the table name.select * from employee, branch where

employee.branch_id = branch.branch_id;

To Selecting The Tuples


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Eliminitating Inconsistent Data

Employee_id

Name Branch_id

Branch_id

Branch_Name1 Mr. A 2 2 Bokshibazar

2 Mr. B 1 1 Polashi3 Mr. C 1 1 Polashi

4 Mr. D 1 1 Polashi

5 Mr. E 2 2 Bokshibazar


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Any number of tables can be joined in joiningselect * from table1, table2, ....... tableN

where table1.col11=table2.col21 and ......andtablen1 .coln1=tablem1 .colm1 ;

Joining More


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Here Manager_id is the employee_id of aemployees manager

Joining Table with itself

Employee_id Name Manager_id1 Mr. A 2

2 Mr. B 4

3 Mr. C null

4 Mr. D 3

5 Mr. E 2

Employee

Manager


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List the names of all employees together withthe name of their manager.

Two same tables. S0 removing the conflict aliasshould be given to table

select e1.name, e2.name from employeeas e1, employee as e2wheree1.manager_id=e2.employee_id;

Example


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The joining done so far also called Inner Join.Oracle has another syntax.

select * from

table1 inner join table2 on table1.col1 = table2.col2select * from employee inner join branch

on employee.branch_id =

branch.branch_id;

Inner Join


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Automatically joins the two tables that have acommonly named and defined field.

The join returns the tuples which have thesame value in the columns of same name.

select * from employee natural join branch;

Here, branch_id is the same column exists inboth employee and branch table.

Natural Join


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Three typesLeft Outer JoinRight Outer JoinFull Outer Join

Outer Join


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All tuples from the left table must be in thetuples selected.

If a tuple from the left table is not matchedwith any tuples in the right table, it will have

null in all the fields for the right table column

Left Outer Join


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All tuples from the right table must be in thetuples selected.

If a tuple from the right table is not matchedwith any tuples in the left table, it will have

null in all the fields for the left table column

Right Outer Join


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All tuples from both the table must be in theselected tuple.

If a tuple from the left table is not matchedwith any tuple from the right table, then it will

have null values in the right table columnIf a tuple from the right table is not matched

with any tuple from the left table, then it willhave null values in the left table column

Full Outer Join


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Employee and Branch DataModifiedEmployee_id Name Branch_id1 Mr. A 2

2 Mr. B 1

3 Mr. C 1

4 Mr. D 15 Mr. E 2

6 Mr. F 5

Branch_id Branch_Name1 Polashi

2 Bokshibazar

3 Motijheel


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select * from employee left outer join branchon employee.branch_id = branch.branch_id;

select * from employee, branch whereemployee.branch_id = branch.branch_id(+);

Left Outer Join


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select * from employee right outer join branchon employee.branch_id = branch.branch_id;

select * from employee, branch whereemployee.branch_id(+) = branch.branch_id;

Right Outer Join


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select * from employee full outer join branch onemployee.branch_id = branch.branch_id;

Full Outer Join


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34:P re p a re d B y A ru p R a to n R o y

a query result can also be used in a conditionof a where clause.

in such a case the query is called a subqueryand the complete select statement is called a

nested query.We can refer the top query table from sub

query.

View the sub query as if it is executed for eachtuple in top query

Sub Query


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A respective condition in the where clause then canhave one of the following forms: 1. Set-valued subqueries [not] in () [any|all]

()An can either be a column or a

computed value. 2. Test for (non)existence [not] exists ()

In a where clause conditions using subqueries canbe combined arbitrarily by using the logicalconnectives and and or.

In Where Clause


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Select the employees who works in PalashiBranchselect * from employee where branch_id

in(select branch_id from branch where

name=Palashi);Select the employees who does not work in

Palashi Branchselect * from employee where branch_id not

in(select branch_id from branch wherename=Palashi);

Example


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You can use (=) for checking condition, if youare sure enough that your subquery returns asingle value.

Better, use in. Because it eliminates the

necessity of single return value. And alsoworks for the single return value.

Caution


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Often a query result depends on whethercertain rows do (not) exist in (other) tables.Such type of queries is formulated using theexists operator.

List all branch that have no employeesselect * from branch

where not exists (select * from employee where branch_id = branch.branch_id);

Exists/Not Exists


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Modified Employee Table

Employee_id Name Branch_id Salary

1 Mr. A 2 12000

2 Mr. B 1 140003 Mr. C 1 12000

4 Mr. D 1 15000

5 Mr. E 2 14000

6 Mr. F 5 13000


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For the clause any, the condition evaluatesto true if there exists at least on rowselected by the sub query for which thecomparison holds.

If the sub query yields an empty result set, thecondition is not satisfied.

Retrieve all employees who are working inbranch 1 and who earn at least as much as

any employee working in department 2.select * from employee where branch_id = 1

and salary>=any(select salary fromemployee where branch_id=2);

Any


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For the clause all, the condition evaluatesto true if for all rows selected by the subquery the comparison holds.

In this case the condition evaluates to true if

the sub query does not yield any row orvalue.

select the employee with maximum salaryselect * from employee where salary

>=all(select salary from employee);

All


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As any select query returns a temporary table,thus subquery can be joined with a table.

select ............ from table1, table2...., (select ..... from

table_s ......)subqueryName, ..... here, subqueryNamecan be used to

reference a column from subquery.select the employee, salary and average salary.

select employee_name, salary,subquery.average_salary from employee,(select avg(salary) as average_salary fromemployee)subquery;

Subquery in From Clause

Employee_name Salary Average_salary


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Oracle provides an object called a Sequencethat can generate numeric value.

ProvidesGenerating the number in ascending or

descending orderThe maximum number that can be generated

by sequenceThe increment value for generating the next

number

Sequence


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CREATE SEQUENCE [INCREMENT BY START WITH MAXVALUE / NOMAXVALUEMINVALUE / NOMINVALUECYCLE/NOCYCLECACHE /NOCACHEORDER/NOORDER]

Syntax


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create sequence employee_id_seq Incrementedby 1 start with 1 minvalue 1 maxvalue 999cycle;

Example


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Every time nextval references a sequence itsoutput automatically incremented.

select .NextVal from dual;

select employee_id_seq .nextval from dual;

Referencing a Sequence


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Alter Sequence [ newValue];

property name is any property that is used insequence parameter.

start value cannot be alteredalter sequence employee_id_seq Incremeneted

by 2;

Altering a Sequence


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drop sequence drop sequence employee_id_seq ;

Dropping a Sequence


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Greatest(expr1, expr2, ......, exprn)select greatest(4, 5, 17) from dual;

Greatest can also be used to find the maxvalue row wise. [note: Max search column

wise]select greatest(A, B) from table1;

Greatest

A B

5 1025 15

Table1


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Least(expr1, expr2, ......, exprn)select greatest(4, 5, 17) from dual;

Least can also be used to find the min valuerow wise. [note: Min search column wise]

select least(A, B) from table1;

Least

A B

5 1025 15

Table1

database management & administration_lecture 4

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