IST 601: Database Management
SQL I – Data Manipulation
(Chapter 6)
Denis L. Nkweteyim
16/11/22
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
1
Outline
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Introduction
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Simple Queries
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Sorting Results (ORDER BY Clause)
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Using the SQL Aggregate Functions
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Grouping Results (GROUP BY Clause)
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Subqueries
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ANY and ALL
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Multi-table Queries
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EXISTS and NOT EXISTS
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Combining Result Tables (UNION, INTERSECT, EXCEPT)
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Database Updates
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
2
What is SQL
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Language used for accessing and manipulating databases and
the data contained in them
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SQL is a declarative language
Unlike procedural and oo languages (C, C++, Java, Visual Basic,
etc)
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SQL keywords are not case-sensitive
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You do not specify how the DBMS should go about retrieving data
You simply specify (declare) what you want to do in SQL
DBMS uses its query optimizer to decide the best way to do it
You can use lower-, upper-, or mixed case
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
3
What is SQL
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The ISO SQL standard does not use the formal terms of relations,
attributes, and tuples, instead using the terms tables, columns, and
rows
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SQL does not adhere strictly to the definition of the relational model
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Examples
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Table produced as the result of the SELECT statement can contain
duplicate rows
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SQL imposes an ordering on the columns
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User can order the rows of a result table
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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Literals in SQL
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Literals are constants that are used in SQL statements
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Different forms of literals for every data type supported
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2 common types: Literals that are enclosed in single (or double) quotes and those that are not
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All nonnumeric data values must be enclosed in single quotes
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All numeric data values must not be enclosed in single quotes
Example: Inserting data into a table
INSERT INTO PropertyForRent(propertyNo, street, city, postcode, type, rooms, rent, ­ownerNo, staffNo,
branchNo)
VALUES (‘PA14', ‘16 Holhead', ‘Aberdeen', ‘AB7 5SU', ‘House', 6, 650.00, ‘CO46', ‘SA9', ‘B007');
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The value in column rooms is an integer literal and the value in column rent is a decimal number literal;
they are not enclosed in single quotes
All other columns are character strings and are enclosed in single quotes
Note
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MySQL is forgiving if you enclose numeric literals in quotes
16/11/22
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
5
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We shall use the Sakila database for our queries
16/11/22
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
6
Install the Sakila Database
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Download the Sakila database
archive from the following page
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https://dev.mysql.com/doc/index-o
ther.html
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Follow the instructions below to
set up the database
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Extract the installation archive to
a temporary location
Connect to the MySQL server
using the mysql command-line
client
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mysql -u root -p
Type the following commands at
the MySql prompt
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mysql> SOURCE <dir>/sakilaschema.sql;
mysql> SOURCE <dir>/sakiladata.sql;
Confirm that the sample
database is installed correctly
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mysql> USE sakila;
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Database changed
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mysql> SHOW FULL TABLES;
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
7
Select
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Most frequently used SQL command
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Purpose
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To retrieve and display data from one or more database
tables
Capable of performing the equivalent of the relational
algebra's Selection, Projection, and Join operations in a
single statement
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
8
Select Syntax
SELECT [DISTINCT | ALL] {* | [columnExpression [AS newName]] [, . . .]}
FROM TableName [alias] [, . . .]
[WHERE condition]
[GROUP BY columnList] [HAVING condition]
[ORDER BY columnList]
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columnExpression
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TableName
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Represents a column name or an
expression
Two mandatory clauses
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The name of an existing database table
or view that you have access to
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SELECT and FROM
The others are optional
alias
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An optional abbreviation for TableName
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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Select Syntax
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Sequence of processing in a
SELECT statement
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Specifies the table or tables to be used
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Filters the rows subject to some
condition
Equivalent to select condition in
relational algebra
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Forms groups of rows with the same
column value
Filters the groups subject to
some condition
SELECT
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GROUP BY
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WHERE
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HAVING
FROM
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Specifies which columns are
to appear in the output
Equivalent to projection in
relational algebra
ORDER BY
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specifies the order of the
output
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10
Simple Select Queries
SELECT actor_id, first_name, last_name, last_update from actor;
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List full details from the actor table
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Note
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No restrictions specified and so the WHERE
clause is unnecessary and all columns are
required
Like above, many SQL retrievals require all
columns of a table
Quick way to express “all columns” is to
use an asterisk (*)
SELECT * from actor;
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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Simple Select Queries
SELECT first_name, last_name from actor; from actor;
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Display just the first and last
names from the actor table
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
12
Simple Select Queries
SELECT district, city_id FROM address;
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Notice that there are several duplicates,
Unlike the relational algebra Projection operation,
SELECT does not eliminate duplicates when it
projects over one or more columns
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SQL is a bag language
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Based on bags instead of set theory
Unlike sets, bags allow for duplicates and it is easier to
implement a system based on bags than on sets
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13
Simple Select Queries
SELECT distinct district, city_id FROM address;
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To eliminate the duplicates, we use the
DISTINCT keyword
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
14
Simple Select Queries
SELECT rental_id, amount FROM payment;
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List the rental id and amount paid in the
payment table
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
15
Simple Select Queries
SELECT rental_id, amount*2 FROM payment;
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List the rental id and expected amount for 2
months
Notice that the column name for expected
amount is not very user friendly – we can
rename it
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
16
Simple Select Queries
SELECT rental_id, amount*2 AS 'Amount for 2 Months' FROM
payment;
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List the rental id and expected amount for 2
months
Notice that the column name for expected
amount is not very user friendly – we can
rename it
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
17
Row Selection (WHERE clause)
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WHERE Clause
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Used to restrict the rows that are retrieved
Consists of the keyword WHERE followed by a search condition that
specifies the rows to be retrieved
Five basic search conditions (or predicates)
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Comparison: Compare the value of one expression to the value of another
expression
Range: Test whether the value of an expression falls within a specified range of
values
Set membership: Test whether the value of an expression equals one of a set of
values
Pattern match: Test whether a string matches a specified pattern
Null: Test whether a column has a null (unknown) value
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
18
Comparison Operators
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= equals
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<> is not equal to (ISO
standard)
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! = is not equal to (allowed in
some dialects)
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< is less than
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> is greater than
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< = is less than or equal to
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> = is greater than or equal to
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More complex predicates can be
generated using the logical
operators AND, OR, and NOT
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Use parentheses (if needed or
desired) to show the order of
evaluation
an expression is evaluated left to
right;
subexpressions in brackets are
evaluated first;
NOTs are evaluated before ANDs and
ORs;
ANDs are evaluated before ORs
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
19
Comparison Search Condition
select actor_id, first_name, last_name
from actor
where first_name = 'Joe';
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ID number, first name, and last name of an actor
whose first name “Joe”
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
20
Comparison Search Condition
SELECT country_id, country FROM country where country_id = 19;
SELECT country_id, country FROM country where country_id <= '10';
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List country id and country name for country
whose country id is 19
List country id and country name for countries
whose country id is less than or equal to 10
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
21
Comparison Search Condition
SELECT address, district, city_id FROM sakila.address where
district = 'alberta' or district = 'qld';
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List address, district, city id for the districts of
Alberta and QLD
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
22
Comparison Search Condition
SELECT title, length FROM film where rating = 'pg' and length > 175;
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Titles and lengths of fils rated pg that are longer
than 175 minutes
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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Range Search Condition (BETWEEN/NOT BETWEEN)
SELECT first_name, last_name FROM actor where first_name
between 's' and 'u';
SELECT first_name, last_name FROM actor where first_name not
between 'a' and 'w';
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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Set Membership Search Condition (IN/NOT IN)
SELECT address, district, postal_code FROM address where
district in ('Texas', 'California');
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Set Membership Search Condition (IN/NOT IN)
SELECT * FROM language;
SELECT language_id, name FROM language where name not in
('English', 'French');
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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Pattern Matching
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WHERE clauses can have conditions in which a string is compared with a
pattern, to see if it matches
General form
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<Attribute> LIKE <pattern> or
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<Attribute> NOT LIKE <pattern>
Pattern is a quoted string with
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% wildcard
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_ wildcard
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Represents any sequence of zero or more characters, i.e., any string
Represents any single character
All other characters in the pattern represent themselves
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
27
Pattern Matching Examples
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LIKE 'H%'
–
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LIKE 'H_ _ _'
–
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A sequence of characters of any length containing Buea
NOT LIKE 'H%'
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Any sequence of characters, of length at least 1, with the last character an e
LIKE '%Buea%'
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There must be exactly four characters in the string, the first of which must be an H
LIKE '%e'
–
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The first character must be H, but the rest of the string can be anything
The first character cannot be an H
If the search string can include the pattern-matching character itself, we can use an
escape character to represent the pattern-matching character
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To check for the string ‘15%’, we can use the predicate: LIKE ‘15#%’ ESCAPE ‘#’
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
28
Pattern Matching Examples
select first_name, last_name from actor where last_name like '%LI%';
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Actors whose last names contain the letters LI
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Pattern Matching Examples
select first_name, last_name from actor where last_name like '%LI%';
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Address, district, and phone
numbers for all phone
numbers that begin with 99
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Pattern Matching Examples
SELECT address, district, phone FROM address where phone like '_90%';
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Address, district, and phone
numbers for all phone
numbers with 90 as the
second and third digits
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
31
Null Values
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Tuples in SQL relations can have NULL as a value for
one or more components
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Meaning of NULL varies and depends on context
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Two common cases
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Use Null for missing data: e.g., we know that an email
address exists, but we do not know what it is
Inapplicable: e.g., the value of attribute spouse for an
unmarried person
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32
Comparing Null Values
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The logic of conditions in SQL is NOT the well known 2valued logic: TRUE or FALSE
But rather a 3-valued logic: TRUE, FALSE, UNKNOWN
When any value is compared with NULL, the truth value
is UNKNOWN
But a query only produces a tuple in the answer if its
truth value for the WHERE clause is TRUE (not FALSE
or UNKNOWN)
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
33
Comparing Null Values
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To understand how AND, OR, and NOT work in 3-valued
logic, think of
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TRUE = 1, FALSE = 0, and UNKNOWN = ½.
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AND = MIN; OR = MAX, NOT(x) = 1-x.
Example
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TRUE AND (FALSE OR NOT(UNKNOWN))
= MIN(1, MAX(0, (1 - ½ )))
= MIN(1, MAX(0, ½ ))
= MIN(1, ½ ) = ½.
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
34
Three-valued Logic May Lead to Surprises
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Why?
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Some common laws, like the commutativity of AND, hold in
3-valued logic.
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But others do not
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Example: the “law of excluded middle,”
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p OR NOT p = TRUE (in two-valued logic)
In three-valued logic, when p = UNKNOWN, the left side is
MAX( ½, (1 – ½ )) = ½ (i.e., UNKNOWN)
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
35
NULL Search Condition (IS NULL/IS NOT
NULL)
SELECT first_name, last_name, username, password FROM staff;
SELECT first_name, last_name, username, password FROM staff where password is null;
SELECT first_name, last_name, username, password FROM staff where password is not null;
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
36
Sorting Results (ORDER BY Clause)
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The rows of an SQL query result table are not sorted
ORDER BY clause in the SELECT statement can be used to sort
ORDER BY clause consists of a list of column identifiers that the result is to be sorted on,
separated by commas
Column identifier may be either
A column name or
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A column number that identifies an element of the SELECT list by its position within the list, 1 being
the first (leftmost) element in the list, 2 the second element in the list, and so on
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Sorting order
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Column numbers could be used if the column to be sorted on is an expression and no AS (i.e., Rename) clause
is specified to assign the column a name
Ascending (ASC) or descending (DESC) order on any column or combination of columns
The ORDER BY clause must always be the last clause of the SELECT statement.
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Sorting Examples
SELECT * FROM category order by name asc;
SELECT * FROM category order by name desc;
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Sorting Examples
SELECT address, district FROM address where district = 'california'
order by district asc, address asc;
select first_name, last_name from actor where last_name like '%LI%'
order by first_name, last_name;
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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Using the SQL Aggregate Functions
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We often want to perform some form of summation or
aggregation of data, similar to the totals at the bottom of a
report
5 aggregation functions
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COUNT – returns the number of values in a specified column
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SUM – returns the sum of the values in a specified column
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AVG – returns the average of the values in a specified column
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MIN – returns the smallest value in a specified column
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MAX – returns the largest value in a specified column
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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Using the SQL Aggregate Functions
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Aggregate functions
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Operate on a single column of a table and return a single value
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COUNT, MIN, and MAX apply to both numeric and nonnumeric fields
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SUM and AVG may be used on numeric fields only
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Apart from COUNT(*), each function eliminates nulls first and operates only
on the remaining nonnull values
COUNT(*) is a special use of COUNT that counts all the rows of a table,
regardless of whether nulls or duplicate values occur
To eliminate duplicates before the function is applied, use the keyword
DISTINCT before the column name in the function
Can explicitly specigy the keyword ALL if we do not want to eliminate
duplicates, although ALL is assumed if nothing is specified
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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Using the SQL Aggregate Functions
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DISTINCT
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Has no effect with the MIN and MAX functions. However, it
may have an effect on the result of SUM or AVG, so
consideration must be given to whether duplicates should be
included or excluded in the computation
Can be specified only once in a query
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
42
Using the SQL Aggregate Functions
●
Some notes
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An aggregate function can be used only in the SELECT list and in the
HAVING clause
If the SELECT list includes an aggregate function and no GROUP BY
clause is being used to group data together, then no item in the
SELECT list can include any reference to a column unless that column
is the argument to an aggregate function
Example, the following query is illegal:
SELECT staffNo, COUNT(salary) FROM Staff;
The query does not have a GROUP BY clause and the column staffNo in the
SELECT list is used outside an aggregate function
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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Aggregate Functions Examples
SELECT count(*) FROM actor;
select first_name, last_name from actor where last_name like '%LI%'
SELECT count(*) FROM actor where last_name like '%LI%';
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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Aggregate Functions Examples
select count(*) category_id FROM sakila.film_category;
select count(distinct category_id) FROM film_category;
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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Aggregate Functions Examples
SELECT count(payment_id) as 'No of Payments', sum(amount) as Total FROM
payment;
SELECT min(amount) as 'Minimum Payment', max(amount) as 'Maximum
Payment', avg(amount) as 'Average Payment' from payment;
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Number of payments and
sum of payment amounts
Minimum, maximum and
average payment
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
46
Grouping Results (GROUP BY Clause)
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The aggregate functions as we have seen generaterate a single row of summary
data
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However, it is often useful to have subtotals in reports
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The GROUP BY clause of the SELECT statement can be used for this
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A grouped query groups the data from the SELECT table(s) and produces a
single summary row for each group
The columns named in the GROUP BY clause are called the grouping columns
When GROUP BY is used, each item in the SELECT list must be single-valued
per group
The SELECT clause may contain only
●
column names, aggregate functions, constants, an expression involving combinations of
these elements
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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Grouping Results (GROUP BY Clause)
●
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All column names in the SELECT list must appear in the GROUP BY
clause unless the name is used only in an aggregate function
The contrary is not true
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There may be column names in the GROUP BY clause that do not appear in the
SELECT list.
When the WHERE clause is used with GROUP BY
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The WHERE clause is applied first,
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Then groups are formed from the remaining rows that satisfy the search condition
Two nulls are considered to be equal for purposes of the GROUP BY
clause
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If two rows have nulls in the same grouping columns and identical values in all
the nonnull grouping columns, they are combined into the same group
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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GROUP BY Examples
SELECT category_id, count(film_id) FROM film_category group by
category_id;
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Film categories and the
number of films in each
category
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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GROUP BY Examples
SELECT customer_id, count(customer_id), sum(amount) FROM payment where
customer_id < 6 group by customer_id;
SELECT customer_id, staff_id, count(customer_id), sum(amount) FROM
payment where customer_id < 6 group by customer_id,staff_id;
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Restricting Groupings (HAVING clause)
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Having Clause
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Designed for use with the GROUP BY clause to restrict the groups that appear in the final
result table
Similar in syntax to WHERE, but serves a different purpose
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Column names used in the HAVING clause must also appear in the GROUP BY list or be
contained within an aggregate function
In practice, the search condition in the HAVING clause always includes at least one
aggregate function; otherwise the search condition could be moved to the WHERE clause
and applied to individual rows
●
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WHERE clause filters individual rows going into the final result table
HAVING filters groups going into the final result table
Remember that aggregate functions cannot be used in the WHERE clause
Note
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HAVING clause not a requirement for SQL
Any query expressed using a HAVING clause can always be rewritten without the HAVING clause
Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
51
HAVING Examples
SELECT customer_id, count(customer_id), sum(amount) FROM payment where
customer_id < 6 group by customer_id having count(customer_id)>22;
SELECT customer_id, count(customer_id), sum(amount) FROM payment where
customer_id < 6 group by customer_id having sum(amount) < 130;
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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HAVING Examples
SELECT customer_id, staff_id, count(customer_id), sum(amount) FROM
payment where customer_id < 6 group by customer_id,staff_id;
SELECT customer_id, staff_id, count(customer_id), sum(amount) FROM payment where
customer_id < 6 group by customer_id,staff_id HAVING sum(amount) > 55;
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Denis L. Nkweteyim@UB - IST601 (Database Management - SQL)
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Subqueries (Nested Queries)
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A subquery is
–
–
–
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A parenthesized SELECT statement embedded within another
SELECT statement
Can be used as a value in a number of places, including FROM,
WHERE, HAVING clauses
Results of this inner SELECT statement (or subselect) are used in
the outer statement to help determine the contents of the final
result
Subselects may also appear in INSERT, UPDATE, and
DELETE statements
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Subqueries (Nested Queries)
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3 types of subquery
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Scalar subquery
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●
–
Row subquery
●
●
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Returns a single column and a single row, that is, a single value
Can be used whenever a single value is needed
Returns multiple columns, but only a single row
Can be used whenever a row value constructor is needed, typically in predicates
Table subquery
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Returns one or more columns and multiple rows
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Can be used whenever a table is needed, for example, as an operand for the IN predicate
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55