Simple Queries
Chapter 1
Objectives
Basic SELECT statement
 Computed columns
 Aliases
 Concatenation operator
 Use of DISTINCT to eliminate
duplicates
 Sort data by using an ORDER BY
clause
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Basic SELECT statement
SELECT expression1, expression2, …
FROM table
 An expression may involve one or more
column(s), literal(s), and/or arithmetic
operator(s), … ; eg. last_name, salary * 1.1
 Example:
SELECT last_name, first_name,
commission_pct, commission_pct*1.2
FROM employees
 An * in the SELECT clause represents all
columns, eg:
SELECT *
FROM employees;
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Aliases
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An alias can be provided for any expression
in a SELECT clause and is used as an
alternate column heading
Keyword AS is optional when specifying alias
Alias follows expression in SELECT clause
If alias is made up of more than a single word
then alias must be enclosed within double
quotation marks
Example:
SELECT last_name, first_name,
commission_pct*1.2 AS “Increased Comm”
FROM employees;
Literals
Literals are actual values that can be
used in an SQL statement
 May be numeric value, eg 2, 1.2
 May be character strings, eg
‘Employee:’, ‘2’
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Concatenation operator

Concatenation operator || is used to join strings
and is often used to format output
 Example:
SELECT first_name || last_name || ‘ has
commission rate of’ || commission_pct
FROM employees;
 Normally good to use alias for expressions
involving || operation for meaningful heading:
SELECT first_name || last_name || ‘ has
commission rate of’ || commission_pct AS
“EMPLOYEE COMMISSIONS”
FROM employees;
DISTINCT

Duplicate values may be returned by a
SELECT statement.
 Use of DISTINCT allows duplicate rows of
output to be eliminated.
 Example: list the department for all employees:
SELECT department_id
FROM employees;
 Output from query contains duplicate values of
department # and is not clear. Use of
DISTINCT in query removes duplicate rows
from output:
SELECT DISTINCT department_id
FROM employees;
ORDER BY
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Sort output by using an ORDER BY clause
May sort by one or more expressions
First expression is primary sort field, second
expression is then used, …
Example of listing employee names in
alphabetical order:
SELECT first_name || ‘ ’ || last_name AS
NAME
FROM employees
ORDER BY last_name, first_name;
But is this output in alphabetical order by last
name and first name? Can’t tell because there
are not 2 employees with the same last name!
Testing of SQL Statements
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SQL is a programming language
SQL statements must be tested just as you
must test C or C++ programs
It is not okay to assume that output and/or
results produced by an SQL statement you
write is correct!
Many important business decisions may be
made based on the output produced by SQL
statements that you write – therefore every
statement you write must be thoroughly
tested.
To test SQL statements you must have a wide
range of data values in your test database
Testing of SQL Statements (ctd)
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You should determine what results you expect
based on your client’s requirements and the
data contained in the database
Then develop your SQL statement (often in
stages)
Then execute your statement
Compare the output and/or results with what
you predicted. If they are not the same then
verify if the results you predicted are correct.
If the predicted results are correct then you
must find and correct a logical error in your
SQL statement
It is very easy to write syntactically valid but
logically incorrect SQL statements
Testing of ORDER BY
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Example of listing employee names in
alphabetical order:
SELECT first_name || ‘ ’ || last_name
FROM employees
ORDER BY last_name, first_name;
 How to test if the output produced by the
query is in alphabetical order by last and first
name? Must add another employee with
same last name as an existing employee
 INSERT INTO employees VALUES(15,
‘John',‘King', ‘JKing@senecac.on.ca',
NULL,'12-NOV-04', 'IT_PROG', NULL, NULL,
NULL, NULL);
 Now rerun query and verify results
ORDER BY clause (ctd)
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Expression(s) used in ORDER BY clause
may or may not be included in SELECT
clause
 Example: list employee names in order by
department id and then for any employees
located in the same department in
alphabetical order by last and first name:
SELECT first_name || ‘ ’ || last_name AS
NAME
FROM employees
ORDER BY department_id, last_name,
first_name;
ORDER BY clause (ctd)
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Data may be sorted in ascending or
descending order by using the keywords ASC
or DESC
 Default order is ascending
 Example: list employee names from the
highest numbered employee:
SELECT first_name || ‘ ’ || last_name AS
NAME
FROM employees
ORDER BY employee_id DESC