Database
cs453
Lab8
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Objectives
Subqueries
• Describe the types of problems that subqueries can solve
• Define subqueries
• List the types of subqueries
• Write single-row and multiple-row subqueries.
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Using a Subquery to Solve a Problem
“ 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 and
Second query, to find who earns more than that
amount
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Subqueries
SELECT select_list
FROM table
WHERE 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 Fname
from Employee
WHERE salary >
(SELECT salary
FROM employee
WHERE Ssn = '123456789)
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Guidelines for Using Subqueries
Enclose 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
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SELECT statement
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Single-Row Subqueries
Return only one row
Use single-row comparison operators.
Operator (= , > , >=, <, <=, <>)
SELECT Fname, Dno
from Employee
WHERE Dno=
(SELECT Dno
FROM employee
WHERE Ssn= '888665555‘)
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Executing Single-Row Subqueries
Display 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, Dno
from 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, Salary
FROM 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 employee
GROUP 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 employee
GROUP BY Dno
HAVING avg(salary) >
(SELECT avg(Salary)
FROM employee
GROUP BY Dno)
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Subquery returned more than 1 value. This is not
permitted when the subquery follows =, !=, <, <= , >,
>= or when the subquery is used as an expression.
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Multiple-Row Subqueries
Return more than one row
Use Multiple-row comparison operators
Operator
Meaning
IN
Equal to any member in the
list
ANY
Compare value to each value
returned by the subquery
ALL
Compare value to every value
returned by the subquery
SELECT Fname, Dno, Salary
FROM employee
WHERE 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, Salary
FROM employee
WHERE Salary IN (30000, 25000,2500)
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Using ANY Operator in Multiple-Row
Subqueries
SELECT Ssn, Fname, Dno, Salary
FROM employee
WHERE 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, Salary
FROM employee
WHERE Salary < ALL
(SELECT AVG(Salary)
FROM employee
GROUP BY Dno)
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Exercises
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.
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