4 Database Queries

4 Database QueriesPeeking into Computer Science

Reading Assignment

Mandatory: Chapter 4 – Sections 4.6 &4.7

Join Queries

Objectives

By the end of this section, you will be able to:

1. Formulate queries on multiple tables2. Understand how natural joins work3. Determine the result of a multi-table

JT’s Extra: Set Multiplication

Recall: set multiplication ‘determines all possible combinations of elements from each set’.

In actual databases not all combinations may occur.◦E.g., an employee typically is just a member of

one department not all departments.The database implementation of a set

multiplication is ‘join’.

JT’s Extra: Join Example

JT’s Extra: Query For Join Example

SELECT *FROM Employees, Departments;

JT’s Extra: Result Of The Query

James Tam is only in Department 1 but the query yields all possible combinations (whether they actually occur in the data or not)

PROJECT x DEPARTMENT9

JT’s Extra: Join Of Actual Cases From The Database

In the previous example this would only include the cases where the department number of the Employees table matched the department number of the Departments table.◦(It should exclude non-existent combinations of

employees and departments.

JT’s Extra: MS-Access SQL Query

SELECT Departments.*, Employees.*FROM Departments INNER JOIN Employees

ON ◦Departments.[Department Number] =

Employees.[Department number];

Results

Employees table

Where Dnumbers Match14

Natural Join15

Natural Join Condition16

Natural Join Result17

Natural Join Result18

Default QBE Joins in ACCESS

Can be also done explicitly

QBE with Explicit Join20

JT’s Extra: Effect Of QBE With Explicit Join

Query criteria

Query result

Join Example22

Retrieve the address of each employee of the IT department

Determining the Result of a Join Query

Reduce the size of the joined tables

Multiply the resulting relations and project

Apply the join condition27

Aggregate Functions, Ordering, & Grouping

Objectives

By the end of this section, you will be able to:

1. Use aggregate functions2. Group the calculations of aggregate

functions3. Formulate SQL queries with HAVING and

ORDER BY clauses

JT’s Extra: Basic Parts Of An SQL Query

◦SELECT: Specifies the fields/columns shown in the query results e.g., SIN field.

◦FROM: Lists the tables from which the data is to be selected e.g., look in the Employees table.

◦WHERE: Provides the conditions to determine if rows/records are shown by the query.

◦ORDER BY: Specifies the order in which rows are to be returned by the query.

Original table

SELECT Employees.Name, Employees.SalaryFROM EmployeesORDER BY Employees.Salary;

JT’s Extra: Query Results

Order By Clause

Orders the result of a query

Descending Order34

Order By Clause35

Aggregate Functions1

SumAverageMinimumMaximumCount

1: JT’s extra, Aggregate functions allow you to perform calculations on multiple rows of data, but will only return a single value in the response.

Original table

QBE SQL

SELECT Sum(salary) AS Expr1FROM Employees;

Query result

COUNT39

Exercise

Retrieve the min salary of female employees who work for the Finance department

Use function Min()

Exercise

Retrieve the max salary of employees who work on projects located in Toronto

Use function Max()

Grouping Calculations

How to find the sum of salary per department

SQL has the GROUP BY clauseGroups calculations of an aggregate

function

Employees table

SELECT department_number, sum(salary) AS DEPT_TOTAL

FROM EmployeesGROUP BY department_number;

Query result

GROUP BY47

HAVING Clause

To show only some of the groups

WHERE filters tuples

HAVING filters groups

JT’s Extra: Group By (All Departments)

Employees table

SELECT Department_number, COUNT(*) as DepartmentEmpTotalsFROM EmployeesGROUP BY Department_number;

Query result

JT’s Extra: Having Clause (Bigger Dept.’s)

Employees table

Query result

SELECT Department_number, COUNT(*) as DepartmentEmpTotalsFROM EmployeesGROUP BY Department_numberHAVING COUNT(*)>1

HAVING Clause51

HAVING Clause52

HAVING Clause53

EXISTS – Optional Reading54

4 Database Queries

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