09 Nested Queries

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CS 338: Computer Applications in Business: Databases (Fall 2014)

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.)

CS 338: Computer Applications in Business: Databases

Nested Queries

1992-2014 by Addison Wesley & Pearson Education, Inc., McGraw Hill, Cengage Learning Slides adapted and modified from Fundamentals of Database Systems (5/6) (Elmasri et al.), Database System Concepts (5/6) (Silberschatz et al.), Database Systems (Coronel et al.), Database Systems (4/5) (Connolly et al. ), Database Systems: Complete Book (Garcia-Molina et al.) Rice University Data Center

Fall 2014

Chapter 5

Some queries require that existing values in the database be fetched and then used in a comparison condition

Nested queries: complete select-from-where blocks within WHERE clause of another query

Nested queries are also called subqueries, or inner queries

Outer query is the remaining part of the query

There are a number of ways a nested query can be used:

1. Nested queries can return a single constant, and this constant can be compared with another value in a WHERE clause

2. Nested queries can return relations that can be used in various ways in WHERE clauses

3. Nested queries can appear in FROM clauses, followed by a tuple variable representing the tuples in the result of the nested query

Nested Queries

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It is often necessary to process data based on other processed data

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Nested Queries Produce Scalar Values

An atomic value that can appear as one component of a tuple is referred to as a scalar

A select-from-where expression can produce a relation with any

number of attributes in its schema, and there can be any number of

tuples in the relation

However, we are often interested in values of a single attribute.

If so, we can use this select-from-where expression surrounded by

parentheses, as if it were a constant

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Nested Queries Produce Scalar Values: Example 1

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Method 1: Step 1: SELECT Position FROM Employee WHERE Name = Victor;

Output: Manager

Step 2: SELECT * FROM Employee WHERE Position = Manager; Output:

111 John Sales Manager 444 Victor Marketing Manager 555 Mark Purchase Manager

Example 1

Employee

EmployeeID Name Dept Position Salary

111 Sue Sales Manager 65000

222 John Purchase Accountant 42000

333 Mary Sales Clerk 28000

444 Victor Marketing Manager 52000

555 Mark Purchase Manager 52000

666 Tony Marketing Assistant Manager 38000

Find all employees who have the same position as Victor

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Nested Queries Produce Scalar Values: Example 2

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Example 2

Employee








Find all employees who have the same position as Victor

Method 2: Use Nested Query or Subquery

SELECT *

FROM Employee

WHERE Position = ( SELECT Position

FROM Employee

WHERE Name = Victor);

outer query inner query

Nested Queries Conditions Involving Relations

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There are a number of SQL operators that we can apply to a relation R and produce a Boolean result Relation R must be expressed as a nested query or subquery

Operators: IN, ALL, and ANY

1. EXISTS R is a condition that is true if and only if R is not empty 2. Scalar value IN R is true if and only if scalar value is equal to one of the

values in R (compares value v with a set of values V) Likewise, scalar value NOT IN R is true if and only if scalar value is equal to no value

in R

3. Scalar value > ALL R is true if and only if scalar value is greater than every value in unary relation R The > operator could be replaced by any of the other comparison operators

4. Scalar value > ANY R is true if and only if scalar value is greater than at least one value in unary relation R The > operator could be replaced by any of the other comparison operators

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Nested Queries Using IN Operator: Example

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Example

Employee








Find all employees with non-managerial positions

SELECT * FROM Employee WHERE Position IN ( SELECT Position FROM Employee WHERE Position NOT LIKE Manager%);

IN equivalent to = ANY

Nested Queries Using ALL Operator: Example

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Example

Employee








Find all employees who earn more than all (every) employees in the Purchase department

SELECT * FROM Employee WHERE Salary > ALL ( SELECT Salary FROM Employee WHERE Dept = Purchase);

>= ALL equivalent to = (SELECT MAX() )

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Nested Queries Using ANY Operator: Example

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Example

Employee








Find all employees who earn more than any (or at least one) employee in the Purchase department

SELECT * FROM Employee WHERE Salary > ANY ( SELECT Salary FROM Employee WHERE Dept = Purchase);

can use SOME instead of ANY

Multiple Nested Queries

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A WHERE clause of a query may contain one or more subqueries combined using operators AND or OR

Example

List all employees with either work in the same department as Sue or have a salary same or greater than Mary

SELECT * FROM Employee WHERE Dept = ( SELECT Dept FROM Employee WHERE Name = Sue ) OR Salary >= ( SELECT Salary FROM Employee WHERE Name = Mary );

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Correlated Nested Queries

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Correlated nested query Inner query refers to a table from the outer query Inner query evaluated once for each tuple in the outer query

Example

SELECT * FROM Employee as E1 WHERE Salary > ( SELECT AVG(Salary) FROM Employee as E2 WHERE E1.Dept = E2.Dept )

Find all employees who earn more than the average salary in their own department

EXISTS and UNIQUE Function in SQL

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EXISTS function Check whether the result of a correlated nested query is empty or not

Produces simple TRUE or FALSE result

TRUE if the nested query result contains at least one row

FALSE if nested query result contains no rows

NOT EXISTS is the opposite of EXISTS

EXISTS and NOT EXISTS Typically used in conjunction with a correlated nested query

UNIQUE(Q)function Returns TRUE if there are no duplicate tuples in the result of query Q

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EXISTS Function: Example

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Example

SELECT * FROM Employee as E WHERE EXISTS ( SELECT * FROM Department as D WHERE E.DeptID = D.DeptID AND D.DeptName = Sales );

Find all sales employees

Employee

EmployeeID Name DeptID Position Salary

111 Sue 1 Manager 65000

222 John 2 Accountant 42000

333 Mary 1 Clerk 28000

444 Victor 3 Manager 52000

555 Mark 2 Manager 52000

666 Tony 3 Assistant Manager 38000

DeptID DeptName Location

1 Sales Room 245

2 Purchase Room 430

3 Marketing Room 212

Department

NOT EXISTS Function: Example

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Example

SELECT * FROM Employee as E WHERE NOT EXISTS ( SELECT * FROM Department as D WHERE E.DeptID = D.DeptID AND D.DeptName = Sales);

Find all non-sales employees

Employee

EmployeeID Name DeptID Position Salary

111 Sue 1 Manager 65000

222 John 2 Accountant 42000

333 Mary 1 Clerk 28000

444 Victor 3 Manager 52000

555 Mark 2 Manager 52000

666 Tony 3 Assistant Manager 38000

DeptID DeptName Location

1 Sales Room 245

2 Purchase Room 430

3 Marketing Room 212

Department

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Nested Queries in FROM Clauses

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In a FROM list, instead of a stored relation, we may use a parenthesized nested query Because we do not have a name for the result of this nested query, we

must give it a tuple-variable alias

Example

SELECT * FROM ( SELECT * FROM Employee as E WHERE Salary > 45000 ) as SalariedEmployees WHERE Dept = Sales ;

Find all the employees earning more than 45000 and work for the Sales department

09 Nested Queries

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