CSE 311L(Database Management

System)
LAB-Week 02

Topics:
Basic SELECT Statement
Selecting All Columns, Specific Columns
Arithmetic Expressions, Using Arithmetic Operators, Parenthesis
Defining a Column Alias

BASIC QUERIES IN SQL

 SQL has one basic statement for retrieving information from a database; the SELECT
statement

 This is not the same as the SELECT operation of the relational algebra

 Important distinction between SQL and the formal relational model;

 SQL allows a table (relation) to have two or more tuples that are identical in all their
attribute values

 Hence, an SQL relation (table) is a multi-set (sometimes called a bag) of tuples; it is not
a set of tuples

 SQL relations can be constrained to be sets by using the CREATE UNIQUE INDEX
command, or by using the DISTINCT option
  Basic form of the SQL SELECT statement is called a mapping of a SELECT-FROM-
WHERE block

SELECT <attribute list> FROM <table list> WHERE <condition>

 <attribute list> is a list of attribute names whose values are to be retrieved by the query

 <table list > is a list of the relation names required to process the query

 <condition> is a conditional (Boolean) expression that identifies the tuples to be

retrieved by the query

SIMPLE SQL QUERIES

Basic SQL queries correspond to using the following operations of the relational algebra:

SELECT

PROJECT

JOIN

Example of a simple query on one relation (company2.sql)

Basic SELECT Statement

SELECT *|{[DISTINCT] column|expression [alias],...}
FROM table;

Arithmetic Operators
SELECT last_name, salary, 12*(salary+100)
FROM emps;

Using Column Aliases

SELECT last_name "Name", salary*12 "Annual Salary"
FROM emps;
Activity 01:
Write a query that displays the last name , weekly salary, department number of the
employees. Name the salary column as "Weekly Salary".

Run: Populate the table with data given and running company,sql in the mysql promt

All subsequent examples uses COMPANY database as shown below:

Example of a simple query on one relation

Query 0: Retrieve the birth date and address of the employee whose name is 'John B.

Smith'.

Q0: SELECT BDATE, ADDRESS FROM EMPLOYEE

WHERE FNAME='John' AND MINIT='B‘ AND LNAME='Smith‘

The SELECT-clause specifies the projection attributes and the WHERE-clause specifies the
selection condition However, the result of the query may contain duplicate tuples
Example of a simple query on two relations

Query 1: Retrieve the name and address of all employees who work for the 'Research'

department.

Q1: SELECT FNAME, LNAME, ADDRESS FROM EMPLOYEE, DEPARTMENT

WHERE DNAME='Research' AND DNUMBER=DNO

Similar to a SELECT-PROJECT-JOIN sequence of relational algebra operations

(DNAME='Research') is a selection condition (corresponds to a SELECT operation in relational
algebra) (DNUMBER=DNO) is a join condition (corresponds to a JOIN operation in relational
algebra)

Example of a simple query on three relations

Query 2: For every project located in 'Stafford', list the project number, the controlling
department number, and the department manager's last name, address, and birth date.

Q2: SELECT PNUMBER, DNUM, LNAME, BDATE, ADDRESS FROM PROJECT,

DEPARTMENT, EMPLOYEE WHERE DNUM=DNUMBER AND MGRSSN=SSN
AND PLOCATION='Stafford'
In Q2, there are two join conditions The join condition DNUM=DNUMBER relates a project to
its controlling department The join condition MGRSSN=SSN relates the controlling department
to the employee who manages that department

ALIASES, * AND DISTINCT, EMPTY WHERE-CLAUSE

 In SQL, we can use the same name for two (or more) attributes as long as the attributes
are in different relations

 A query that refers to two or more attributes with the same name must qualify the
attribute name with the relation name by prefixing the relation name to the attribute
name Example: EMPLOYEE.LNAME, DEPARTMENT.DNAME

 Some queries need to refer to the same relation twice. In this case, aliases are given to
the relation name

Example

Query 3: For each employee, retrieve the employee's name, and the name of his or her

immediate supervisor.
Q3: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAME FROM EMPLOYEE E S
WHERE E.SUPERSSN=S.SSN

In Q3, the alternate relation names E and S are called aliases or tuple variables for the
EMPLOYEE relation We can think of E and S as two different copies of EMPLOYEE; E
represents employees in role of supervisees and S represents employees in role of supervisors

Aliasing can also be used in any SQL query for convenience. Can also use the AS
keyword to specify aliases

Q3: SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAME FROM EMPLOYEE AS

E, EMPLOYEE AS S WHERE E.SUPERSSN=S.SSN

UNSPECIFIED WHERE-clause
 A missing WHERE-clause indicates no condition; hence, all tuples of the relations in the
FROM-clause are selected. This is equivalent to the condition WHERE TRUE

Example:

Query 4: Retrieve the SSN values for all employees.

Q4: SELECT SSN FROM EMPLOYEE

If more than one relation is specified in the FROM-clause and there is no join condition, then
the CARTESIAN PRODUCT of tuples is selected

Example:

Q5: SELECT SSN, DNAME FROM EMPLOYEE, DEPARTMENT

Note: It is extremely important not to overlook specifying any selection and join conditions in
the WHERE-clause; otherwise, incorrect and very large relations may result

USE OF *

To retrieve all the attribute values of the selected tuples, a * is used, which stands for all the
attributes

Examples:

Retrieve all the attribute values of EMPLOYEES who work in department 5.

Q1a: SELECT * FROM EMPLOYEE WHERE DNO=5

Retrieve all the attributes of an employee and attributes of DEPARTMENT he works in

for every employee of ‘Research’ department.
Q1b: SELECT * FROM EMPLOYEE, DEPARTMENT WHERE DNAME='Research'

AND DNO=DNUMBER

USE OF DISTINCT

SQL does not treat a relation as a set; duplicate tuples can appear. To eliminate duplicate
tuples in a query result, the keyword DISTINCT is used

Example: the result of Q1c may have duplicate SALARY values whereas Q1d does not have
any duplicate values

Q1c: SELECT SALARY FROM EMPLOYEE

Q1d: SELECT DISTINCT

SALARY FROM EMPLOYEE

Activity 02:
Find the results in SQL for these queries:
1) Find the first name and Last name of the employees who are supervised by “Franklin
Wong’?
2) Find the last and first name of the female employees who have a dependent with the same
first name as themselves?
3) For each department find out the department manager’s last name, his start date and the
name his dependents (if any)?
4) For each employee find out the employee’s last and first name, the department name in
which he works and the project name he works in and the number of hours he work in
those projects.