DatabaseDatabase

cs453

Lab8

1Ins.Ebtesam AL-Etowi

Objectives

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Subqueries

• Describe the types of problems that subqueries can solve

• Define subqueries

• List the types of subqueries

• Write single-row and multiple-row subqueries.

Using a Subquery to Solve a Problem

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“Which employees have a salary greater than John salary”?

“what is John salary”?

To solve this problem, you need two query:First, one query to find what Jones earns andSecond query, to find who earns more than that amount

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SubqueriesSELECT select_listFROM tableWHERE expr operator

( SELECT select_list FROM table)

The subquery )inner query( executes once before the main query.

The result of the subquery is used by the main query )outer query(.

Using a Subquery

The inner query determines the salary of employee 123456789. the outer query takes the result of the inner query and uses this result to display all the employees who earn more than this amount

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SELECT Fnamefrom Employee

WHERE salary> ( SELECT salary

FROM employee WHERE Ssn = '123456789)

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Guidelines for Using SubqueriesEnclose subqueries in parentheses.

Place subqueries on the right side of the comparison operator.

Don not add an ORDER BY clause to a subquery.

Using single-row operators with single-row subqueries.

Use multiple-row operators with multiple-row subqueries

Types of Subqueries

Single-row subquery Queries that return only one row from the inner SELECT statement

Multiple-row subquery Queries that return more than one row from the inner SELECT statement

Multiple-column subquery Queries that return more than one column from the inner SELECT statement

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Single-Row Subqueries

Return only one rowUse single-row comparison operators.

Operator)>< ,=< ,< ,=> , > , =(

SELECT Fname, Dnofrom Employee

WHERE Dno=( SELECT Dno

FROM employee WHERE Ssn= '888665555)‘

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Executing Single-Row SubqueriesDisplay employee whose depatement number is the same as that employee 888665555 and whose salary is greater than that of employee 987654321.

The example consists of three query blocks: the outer query and two inner queries. The inner query blocks are executed first.

SELECT Fname, Dnofrom Employee

WHERE Dno=( SELECT Dno

FROM employee WHERE Ssn= '888665555) '

AND salary> ( SELECT salary

FROM employee WHERE Ssn = '987654321)'

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Using Group Functions in a Subquery

You can display data from a main query by using a group function in a subquery to return a single row.

Displays the employee name, department number and salary of all employee whose salary is equal to the minimum salary

SELECT Fname, Dno, SalaryFROM employee

WHERE Salary= ( SELECT MIN(Salary)

FROM employee)

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Having Clause with Subqueries

The SQL Server executes subqueries first.

The SQL Server returns results into the HAVING clause of the main query

SELECT Dno, MIN(Salary)FROM employeeGROUP BY Dno

HAVING MIN(salary)> ( SELECT MIN(Salary)

FROM employee WHERE Dno = 5)

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What is Wrong with this Statement

SELECT Dno, avg(Salary)FROM employeeGROUP BY Dno

HAVING avg(salary)> ( SELECT avg(Salary)

FROM employee GROUP BY Dno)

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

Multiple-Row Subqueries

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Return more than one rowUse Multiple-row comparison operators

OperatorMeaning

IN Equal to any member in the list

ANYCompare value to each value returned by the subquery

ALLCompare value to every value returned by the subquery

SELECT Fname, Dno, SalaryFROM employeeWHERE Salary IN (SELECT MIN(Salary)

FROM employee GROUP BY Dno)

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Find the employee salary in )30000, 5000, 2500(, first name, and departement number

SELECT Fname, Dno, SalaryFROM employeeWHERE Salary IN (30000, 25000,2500)

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Using ANY Operator in Multiple-Row Subqueries

SELECT Ssn, Fname, Dno, SalaryFROM employeeWHERE Salary < ANY

( SELECT Salary FROM employee

WHERE Dno=4)AND Dno <>4

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Using ALL Operator in Multiple-Row Subqueries

SELECT Ssn, Fname, Dno, SalaryFROM employeeWHERE Salary < ALL

( SELECT AVG(Salary) FROM employee

GROUP BY Dno)

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Exercises

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Write a query to display the first name employee and birthdates for all employee in the same department as 4.

2 -create a query to display the Ssn and Fname for all employees who salary

more than the average salary. Sort the results in descending order of salary.

3-Write a query that will display the employee number and name for all employees who work in a department with any employee whose name contains a o .

4-Display the Fname and salary of all employee who supervisor ‘333445555’

5 -Display the Dno, Fname, and salary for all emplyee in the 5 Departement number.