database-querry-student-note

TABLE OF CONTENTS
1. Summary of MySQL Commands Used in this Tutorial
2. An Example for the Beginners
2.1 Creating and Deleting a Database - CREATE DATABASE and DROP DATABASE
2.2 Setting the Default Database - USE
2.3 Creating and Deleting a Table - CREATE TABLE and DROP TABLE
2.4 Inserting Rows - INSERT INTO
2.5 Querying the Database - SELECT
2.6 Producing Summary Reports
2.7 Modifying Data - UPDATE
2.8 Deleting Rows - DELETE FROM
2.9 Loading/Exporting Data from/to a Text File
2.10 Running a SQL Script
3. More Than One Tables
3.1 One-To-Many Relationship
3.2 Many-To-Many Relationship
3.3 One-to-one Relationship
3.4 Backup and Restore
4. More on Primary Key, Foreign Key and Index
4.1 Primary Key
4.2 Foreign Key
4.3 Indexes (or Keys)
5. More SQL

5.1 Sub-Query
5.2 Working with Date and Time
5.3 View
5.4 Transactions
5.5 User Variables
6. More on JOIN
6.1 INNER JOIN
6.2 OUTER JOIN - LEFT JOIN and RIGHT JOIN
7. Exercises
7.1 Rental System
7.2 Product Sales DatabaseMySQL

1. Summary of MySQL Commands Used in this Tutorial
For detailed syntax, check MySQL manual "SQL Statement Syntax" @
http://dev.mysql.com/doc/refman/5.5/en/sql-syntax.html.
-- Database-Level
DROP DATABASE databaseName -- Delete the database
(irrecoverable!)
DROP DATABASE IF EXISTS databaseName -- Delete if it exists
CREATE DATABASE databaseName -- Create a new database
CREATE DATABASE IF NOT EXISTS databaseName -- Create only if it does not
exists
SHOW DATABASES -- Show all the databases in this
server
USE databaseName -- Set the default (current)
database
SELECT DATABASE() -- Show the default database
SHOW CREATE DATABASE databaseName -- Show the CREATE DATABASE
statement
-- Table-Level
DROP TABLE [IF EXISTS] tableName, ...
CREATE TABLE [IF NOT EXISTS] tableName (
columnName columnType columnAttribute, ...
PRIMARY KEY(columnName),
FOREIGN KEY (columnNmae) REFERENCES tableName (columnNmae)
)
SHOW TABLES -- Show all the tables in the default database
DESCRIBE|DESC tableName -- Describe the details for a table
ALTER TABLE tableName ... -- Modify a table, e.g., ADD COLUMN and DROP
COLUMN
ALTER TABLE tableName ADD columnDefinition
ALTER TABLE tableName DROP columnName
ALTER TABLE tableName ADD FOREIGN KEY (columnNmae) REFERENCES tableName
(columnNmae)
ALTER TABLE tableName DROP FOREIGN KEY constraintName
SHOW CREATE TABLE tableName -- Show the CREATE TABLE statement for
this tableName
-- Row-Level
INSERT INTO tableName
VALUES (column1Value, column2Value,...) -- Insert on all
Columns
VALUES (column1Value, column2Value,...), ... -- Insert multiple
rows
INSERT INTO tableName (column1Name, ..., columnNName)
VALUES (column1Value, ..., columnNValue) -- Insert on
selected Columns
DELETE FROM tableName WHERE criteria
UPDATE tableName SET columnName = expr, ... WHERE criteria

SELECT * | column1Name AS alias1, ..., columnNName AS aliasN
FROM tableName
WHERE criteria
GROUP BY columnName
ORDER BY columnName ASC|DESC, ...
HAVING groupConstraints
LIMIT count | offset count
-- Others
SHOW WARNINGS; -- Show the warnings of the previous statement
2. An Example
A MySQL database server contains many databases (or schemas). Each database consists of one
or more tables. A table is made up of columns (or fields) and rows (records).
The SQL keywords and commands are NOT case-sensitive. For clarity, they are shown in
uppercase. The names or identifiers (database names, table names, column names, etc.) are case-
sensitive in some systems, but not in other systems. Hence, it is best to treat identifiers as case-
sensitive.
You can use SHOW DATABASES to list all the existing databases in the server.
mysql> SHOW DATABASES;
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| test |
+--------------------+
4 rows in set (0.00 sec)
The databases "mysql", "information_schema" and "performance_schema" are system
databases used internally by MySQL. A "test" database is provided during installation for your
testing.
Let us begin with a simple example - a product sales database. A product sales database
typically consists of many tables, e.g., products, customers, suppliers, orders, payments,
employees, among others. Let's call our database "southwind" (inspired from Microsoft's
Northwind Trader sample database). We shall begin with the first table called "products" with
the following columns (with the data types as indicated) and rows:
Database: southwind
Table: products
productID
INT
productCode
CHAR(3)
name
VARCHAR(30)
quantity
INT
price
DECIMAL(10,2)
1001 PEN Pen Red 5000 1.23

1002 PEN Pen Blue 8000 1.25
1003 PEN Pen Black 2000 1.25
1004 PEC Pencil 2B 10000 0.48
1005 PEC Pencil 2H 8000 0.49
2.1 Creating and Deleting a Database - CREATE DATABASE and DROP DATABASE
CREATE DATABASE and DROP DATABASE
You can create a new database using SQL command "CREATE DATABASE databaseName"; and
delete a database using "DROP DATABASE databaseName". You could optionally apply condition
"IF EXISTS" or "IF NOT EXISTS" to these commands. For example,
mysql> CREATE DATABASE southwind;
Query OK, 1 row affected (0.03 sec)
mysql> DROP DATABASE southwind;
Query OK, 0 rows affected (0.11 sec)
mysql> CREATE DATABASE IF NOT EXISTS southwind;
mysql> DROP DATABASE IF EXISTS southwind;
IMPORTANT: Use SQL DROP (and DELETE) commands with extreme care, as the deleted
entities are irrecoverable. THERE IS NO UNDO!!!
SHOW CREATE DATABASE
The CREATE DATABASE commands uses some defaults. You can issue a "SHOW CREATE
DATABASE databaseName" to display the full command and check these default values. We use
G (instead of ';') to display the results vertically. (Try comparing the outputs produced by ';'
and G.)
mysql> CREATE DATABASE IF NOT EXISTS southwind;
mysql> SHOW CREATE DATABASE southwind G
*************************** 1. row ***************************
Database: southwind
Create Database: CREATE DATABASE `southwind` /*!40100 DEFAULT CHARACTER SET
latin1 */
Back-Quoted Identifiers (`name`)
Unquoted names or identifiers (such as database name, table name and column name) cannot
contain blank and special characters or crash with MySQL keywords (such as ORDER and DESC).
You can include blanks and special characters or use MySQL keyword as identifier by enclosing

it with a pair of back-quote, i.e., `name`. For robustness, the SHOW command back-quotes all the
identifiers, as illustrated in the above example.
MySQL Version Comments
MySQL multi-line comments are enclosed within /* and */; end-of-line comments begins with -
- (followed by a space) or #.
The /*!40100 ...... */ is known as version comment, which will only be run if the server is
at or above this version number 4.01.00. To check the version of your MySQL server, issue
query "SELECT version()".
2.2 Setting the Default Database - USE
The command "USE databaseName" selects a particular database as the default (or current)
database. You can refer to a table in the default database using tableName directly; but you need
to use the fully-qualified databaseName.tableName to refer to a table NOT in the default
database.
In our example, we have a database named "southwind" with a table named "products". If we
issue "USE southwind" to set southwind as the default database, we can simply call the table as
"products". Otherwise, we need to reference the table as "southwind.products".
To display the current default database, issue command "SELECT DATABASE()".
2.3 Creating and Deleting a Table - CREATE TABLE and DROP TABLE
You can create a new table in the default database using command "CREATE TABLE tableName"
and "DROP TABLE tableName". You can also apply condition "IF EXISTS" or "IF NOT
EXISTS". To create a table, you need to define all its columns, by providing the columns' name,
type, and attributes.
Let's create the table "products" for our database "southwind".
-- Remove the database "southwind", if it exists.
-- Beware that DROP (and DELETE) actions are irreversible and not
recoverable!
mysql> DROP DATABASE IF EXISTS southwind;
-- Create the database "southwind"
mysql> CREATE DATABASE southwind;
-- Show all the databases in the server
-- to confirm that "southwind" database has been created.
mysql> SHOW DATABASES;
+--------------------+
| Database |

mysql> SHOW CREATE TABLE products G
*************************** 1. row ***************************
Table: products
Create Table:
CREATE TABLE `products` (
`productID` int(10) unsigned NOT NULL AUTO_INCREMENT,
`productCode` char(3) NOT NULL DEFAULT '',
`name` varchar(30) NOT NULL DEFAULT '',
`quantity` int(10) unsigned NOT NULL DEFAULT '0',
`price` decimal(7,2) NOT NULL DEFAULT '99999.99',
PRIMARY KEY (`productID`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
Explanations
We define 5 columns in the table products: productID, productCode, name, quantity and
price. The data types are:
 productID is INT UNSIGNED - non-negative integers.
 productCode is CHAR(3) - a fixed-length alphanumeric string of 3 characters.
 name is VARCHAR(30) - a variable-length string of up to 30 characters.
We use fixed-length string for productCode, as we assume that the productCode
contains exactly 3 characters. On the other hand, we use variable-length string for name,
as its length varies - VARCHAR is more efficient than CHAR.
 quantity is also INT.
 price is DECIMAL(10,2) - a decimal number with 2 decimal places.
DECIMAL is precise (represented as integer with a fix decimal point). On the other hand,
FLOAT and DOUBLE (real numbers) are not precise and are approximated. DECIMAL type is
recommended for currency.
The attribute "NOT NULL" specifies that the column cannot contain the NULL value. NULL is a
special value indicating "no value", "unknown value" or "missing value". We also set the default
values of all the columns. The column will take on its default value, if no value is specified
during the record creation.
We set the column productID as the so-called primary key. Values of the primary-key column
must be unique. Every table shall contain a primary key. This ensures that every row can be
distinguished from other rows. You can specify a single column or a set of columns as the
primary key. An index is build automatically on the primary-key column to facilitate fast search.
Primary key is also used as reference for other tables.
We set the column productID to AUTO_INCREMENT. with default starting value of 1. When you
insert a NULL (recommended) (or 0, or a missing value), into an AUTO_INCREMENT column, the
maximum value of that column plus 1 would be inserted. You can also insert a valid value to an
AUTO_INCREMENT column, bypassing the auto-increment.
2.4 Inserting Rows - INSERT INTO

Let's fill up our "products" table with rows. We set the productID of the first record to 1001,
and use AUTO_INCREMENT for the rest of records by inserting a NULL, or with a missing column
value. Take note that strings must be enclosed with a pair of single quotes (or double quotes).
-- Insert a row with all the column values
mysql> INSERT INTO products VALUES (1001, 'PEN', 'Pen Red', 5000, 1.23);
-- Insert multiple rows
-- Inserting NULL to an auto_increment column results in max_value + 1
mysql> INSERT INTO products VALUES
(NULL, 'PEN', 'Pen Blue', 8000, 1.25),
(NULL, 'PEN', 'Pen Black', 2000, 1.25);
Records: 2 Duplicates: 0 Warnings: 0
-- Insert value to selected columns
-- Missing value for an auto_increment column also results in max_value + 1
mysql> INSERT INTO products (productCode, name, quantity, price) VALUES
('PEC', 'Pencil 2B', 10000, 0.48),
('PEC', 'Pencil 2H', 8000, 0.49);
-- Missing columns get their default values
mysql> INSERT INTO products (productCode, name) VALUES ('PEC', 'Pencil HB');
-- 2nd column is defined to be NOT NULL
mysql> INSERT INTO products values (NULL, NULL, NULL, NULL, NULL);
ERROR 1048 (23000): Column 'productCode' cannot be null
-- Query the table
mysql> SELECT * FROM products;
+-----------+-------------+-----------+----------+------------+
| productID | productCode | name | quantity | price |
+-----------+-------------+-----------+----------+------------+
| 1001 | PEN | Pen Red | 5000 | 1.23 |
| 1002 | PEN | Pen Blue | 8000 | 1.25 |
| 1003 | PEN | Pen Black | 2000 | 1.25 |
| 1004 | PEC | Pencil 2B | 10000 | 0.48 |
| 1005 | PEC | Pencil 2H | 8000 | 0.49 |
| 1006 | PEC | Pencil HB | 0 | 9999999.99 |
+-----------+-------------+-----------+----------+------------+
-- Remove the last row
mysql> DELETE FROM products WHERE productID = 1006;
Syntax
We can use the INSERT INTO statement to insert a new row with all the column values, using the
following syntax:
INSERT INTO tableName VALUES (firstColumnValue, ..., lastColumnValue) -- All
columns

You need to list the values in the same order in which the columns are defined in the CREATE
TABLE, separated by commas. For columns of string data type (CHAR, VARCHAR), enclosed the
value with a pair of single quotes (or double quotes). For columns of numeric data type (INT,
DECIMAL, FLOAT, DOUBLE), simply place the number.
You can also insert multiple rows in one INSERT INTO statement:
INSERT INTO tableName VALUES
(row1FirstColumnValue, ..., row1lastColumnValue),
(row2FirstColumnValue, ..., row2lastColumnValue),
...
To insert a row with values on selected columns only, use:
-- Insert single record with selected columns
INSERT INTO tableName (column1Name, ..., columnNName) VALUES (column1Value,
..., columnNValue)
-- Alternately, use SET to set the values
INSERT INTO tableName SET column1=value1, column2=value2, ...
-- Insert multiple records
(column1Name, ..., columnNName)
VALUES
(row1column1Value, ..., row2ColumnNValue),
(row2column1Value, ..., row2ColumnNValue),
...
The remaining columns will receive their default value, such as AUTO_INCREMENT, default, or
NULL.
2.5 Querying the Database - SELECT
The most common, important and complex task is to query a database for a subset of data that
meets your needs - with the SELECT command. The SELECT command has the following syntax:
-- List all the rows of the specified columns
SELECT column1Name, column2Name, ... FROM tableName
-- List all the rows of ALL columns, * is a wildcard denoting all columns
SELECT * FROM tableName
-- List rows that meet the specified criteria in WHERE clause
SELECT column1Name, column2Name,... FROM tableName WHERE criteria
SELECT * FROM tableName WHERE criteria
For examples,
-- List all rows for the specified columns
mysql> SELECT name, price FROM products;
+-----------+-------+

| name | price |
+-----------+-------+
| Pen Red | 1.23 |
| Pen Blue | 1.25 |
| Pen Black | 1.25 |
| Pencil 2B | 0.48 |
| Pencil 2H | 0.49 |
+-----------+-------+
-- List all rows of ALL the columns. The wildcard * denotes ALL columns
+-----------+-------------+-----------+----------+-------+
+-----------+-------------+-----------+----------+-------+
| 1001 | PEN | Pen Red | 5000 | 1.23 |
| 1002 | PEN | Pen Blue | 8000 | 1.25 |
| 1003 | PEN | Pen Black | 2000 | 1.25 |
| 1004 | PEC | Pencil 2B | 10000 | 0.48 |
| 1005 | PEC | Pencil 2H | 8000 | 0.49 |
+-----------+-------------+-----------+----------+-------+
SELECT without Table
You can also issue SELECT without a table. For example, you can SELECT an expression or
evaluate a built-in function.
mysql> SELECT 1+1;
+-----+
| 1+1 |
+-----+
| 2 |
+-----+
1 row in set (0.00 sec)
mysql> SELECT NOW();
+---------------------+
| NOW() |
+---------------------+
| 2012-10-24 22:13:29 |
+---------------------+
// Multiple columns
mysql> SELECT 1+1, NOW();
+-----+---------------------+
| 1+1 | NOW() |
+-----+---------------------+
| 2 | 2012-10-24 22:16:34 |
+-----+---------------------+
Comparison Operators

For numbers (INT, DECIMAL, FLOAT), you could use comparison operators: '=' (equal to), '<>'
or '!=' (not equal to), '>' (greater than), '<' (less than), '>=' (greater than or equal to), '<='
(less than or equal to), to compare two numbers. For example, price > 1.0, quantity <= 500.
mysql> SELECT name, price FROM products WHERE price < 1.0;
+-----------+-------+
| name | price |
+-----------+-------+
| Pencil 2B | 0.48 |
| Pencil 2H | 0.49 |
+-----------+-------+
mysql> SELECT name, quantity FROM products WHERE quantity <= 2000;
+-----------+----------+
| name | quantity |
+-----------+----------+
| Pen Black | 2000 |
+-----------+----------+
CAUTION: Do not compare FLOATs (real numbers) for equality ('=' or '<>'), as they are not
precise. On the other hand, DECIMAL are precise.
For strings, you could also use '=', '<>', '>', '<', '>=', '<=' to compare two strings (e.g.,
productCode = 'PEC'). The ordering of string depends on the so-called collation chosen. For
example,
mysql> SELECT name, price FROM products WHERE productCode = 'PEN';
-- String values are quoted
+-----------+-------+
| name | price |
+-----------+-------+
| Pen Red | 1.23 |
| Pen Blue | 1.25 |
+-----------+-------+
String Pattern Matching - LIKE and NOT LIKE
For strings, in addition to full matching using operators like '=' and '<>', we can perform
pattern matching using operator LIKE (or NOT LIKE) with wildcard characters. The wildcard '_'
matches any single character; '%' matches any number of characters (including zero). For
example,
 'abc%' matches strings beginning with 'abc';
 '%xyz' matches strings ending with 'xyz';
 '%aaa%' matches strings containing 'aaa';
 '___' matches strings containing exactly three characters; and

 'a_b%' matches strings beginning with 'a', followed by any single character, followed
by 'b', followed by zero or more characters.
-- "name" begins with 'PENCIL'
mysql> SELECT name, price FROM products WHERE name LIKE 'PENCIL%';
+-----------+-------+
| name | price |
+-----------+-------+
| Pencil 2B | 0.48 |
| Pencil 2H | 0.49 |
+-----------+-------+
-- "name" begins with 'P', followed by any two characters,
-- followed by space, followed by zero or more characters
mysql> SELECT name, price FROM products WHERE name LIKE 'P__ %';
+-----------+-------+
| name | price |
+-----------+-------+
| Pen Red | 1.23 |
| Pen Blue | 1.25 |
+-----------+-------+
MySQL also support regular expression matching via the REGEXE operator.
Arithmetic Operators
You can perform arithmetic operations on numeric fields using arithmetic operators, as tabulated
below:
Operator Description
+ Addition
- Subtraction
* Multiplication
/ Division
DIV Integer Division
% Modulus (Remainder)
Logical Operators - AND, OR, NOT, XOR
You can combine multiple conditions with boolean operators AND, OR, XOR. You can also invert a
condition using operator NOT. For examples,
mysql> SELECT * FROM products WHERE quantity >= 5000 AND name LIKE 'Pen %';
+-----------+-------------+----------+----------+-------+
+-----------+-------------+----------+----------+-------+
| 1001 | PEN | Pen Red | 5000 | 1.23 |
| 1002 | PEN | Pen Blue | 8000 | 1.25 |
+-----------+-------------+----------+----------+-------+

mysql> SELECT * FROM products WHERE quantity >= 5000 AND price < 1.24 AND
name LIKE 'Pen %';
+-----------+-------------+---------+----------+-------+
+-----------+-------------+---------+----------+-------+
| 1001 | PEN | Pen Red | 5000 | 1.23 |
+-----------+-------------+---------+----------+-------+
mysql> SELECT * FROM products WHERE NOT (quantity >= 5000 AND name LIKE 'Pen
%');
+-----------+-------------+-----------+----------+-------+
+-----------+-------------+-----------+----------+-------+
| 1003 | PEN | Pen Black | 2000 | 1.25 |
| 1004 | PEC | Pencil 2B | 10000 | 0.48 |
| 1005 | PEC | Pencil 2H | 8000 | 0.49 |
+-----------+-------------+-----------+----------+-------+
IN, NOT IN
You can select from members of a set with IN (or NOT IN) operator. This is easier and clearer
than the equivalent AND-OR expression.
mysql> SELECT * FROM products WHERE name IN ('Pen Red', 'Pen Black');
+-----------+-------------+-----------+----------+-------+
+-----------+-------------+-----------+----------+-------+
| 1001 | PEN | Pen Red | 5000 | 1.23 |
| 1003 | PEN | Pen Black | 2000 | 1.25 |
+-----------+-------------+-----------+----------+-------+
BETWEEN, NOT BETWEEN
To check if the value is within a range, you could use BETWEEN ... AND ... operator. Again,
this is easier and clearer than the equivalent AND-OR expression.
mysql> SELECT * FROM products
WHERE (price BETWEEN 1.0 AND 2.0) AND (quantity BETWEEN 1000 AND
2000);
+-----------+-------------+-----------+----------+-------+
+-----------+-------------+-----------+----------+-------+
| 1003 | PEN | Pen Black | 2000 | 1.25 |
+-----------+-------------+-----------+----------+-------+
IS NULL, IS NOT NULL
NULL is a special value, which represent "no value", "missing value" or "unknown value". You
can checking if a column contains NULL by IS NULL or IS NOT NULL. For example,
mysql> SELECT * FROM products WHERE productCode IS NULL;
Empty set (0.00 sec)

Using comparison operator (such as = or <>) to check for NULL is a mistake - a very common
mistake. For example,
SELECT * FROM products WHERE productCode = NULL;
-- This is a common mistake. NULL cannot be compared.
ORDER BY Clause
You can order the rows selected using ORDER BY clause, with the following syntax:
SELECT ... FROM tableName
WHERE criteria
ORDER BY columnA ASC|DESC, columnB ASC|DESC, ...
The selected row will be ordered according to the values in columnA, in either ascending (ASC)
(default) or descending (DESC) order. If several rows have the same value in columnA, it will be
ordered according to columnB, and so on. For strings, the ordering could be case-sensitive or
case-insensitive, depending on the so-called character collating sequence used. For examples,
-- Order the results by price in descending order
mysql> SELECT * FROM products WHERE name LIKE 'Pen %' ORDER BY price DESC;
+-----------+-------------+-----------+----------+-------+
+-----------+-------------+-----------+----------+-------+
| 1002 | PEN | Pen Blue | 8000 | 1.25 |
| 1003 | PEN | Pen Black | 2000 | 1.25 |
| 1001 | PEN | Pen Red | 5000 | 1.23 |
+-----------+-------------+-----------+----------+-------+
-- Order by price in descending order, followed by quantity in ascending
(default) order
mysql> SELECT * FROM products WHERE name LIKE 'Pen %' ORDER BY price DESC,
quantity;
+-----------+-------------+-----------+----------+-------+
+-----------+-------------+-----------+----------+-------+
| 1003 | PEN | Pen Black | 2000 | 1.25 |
| 1002 | PEN | Pen Blue | 8000 | 1.25 |
| 1001 | PEN | Pen Red | 5000 | 1.23 |
+-----------+-------------+-----------+----------+-------+
You can randomize the returned records via function RAND(), e.g.,
mysql> SELECT * FROM products ORDER BY RAND();
LIMIT Clause
A SELECT query on a large database may produce many rows. You could use the LIMIT clause to
limit the number of rows displayed, e.g.,
-- Display the first two rows
mysql> SELECT * FROM products ORDER BY price LIMIT 2;

+-----------+-------------+-----------+----------+-------+
+-----------+-------------+-----------+----------+-------+
| 1004 | PEC | Pencil 2B | 10000 | 0.48 |
| 1005 | PEC | Pencil 2H | 8000 | 0.49 |
+-----------+-------------+-----------+----------+-------+
To continue to the following records , you could specify the number of rows to be skipped,
followed by the number of rows to be displayed in the LIMIT clause, as follows:
-- Skip the first two rows and display the next 1 row
mysql> SELECT * FROM products ORDER BY price LIMIT 2, 1;
+-----------+-------------+---------+----------+-------+
+-----------+-------------+---------+----------+-------+
| 1001 | PEN | Pen Red | 5000 | 1.23 |
+-----------+-------------+---------+----------+-------+
AS - Alias
You could use the keyword AS to define an alias for an identifier (such as column name, table
name). The alias will be used in displaying the name. It can also be used as reference. For
example,
mysql> SELECT productID AS ID, productCode AS Code, name AS Description,
price AS `Unit Price`
-- Define aliases to be used as display names
FROM products
ORDER BY ID;
-- Use alias ID as reference
+------+------+-------------+------------+
| ID | Code | Description | Unit Price |
+------+------+-------------+------------+
| 1001 | PEN | Pen Red | 1.23 |
| 1002 | PEN | Pen Blue | 1.25 |
| 1003 | PEN | Pen Black | 1.25 |
| 1004 | PEC | Pencil 2B | 0.48 |
| 1005 | PEC | Pencil 2H | 0.49 |
+------+------+-------------+------------+
Take note that the identifier "Unit Price" contains a blank and must be back-quoted.
Function CONCAT()
You can also concatenate a few columns as one (e.g., joining the last name and first name) using
function CONCAT(). For example,
mysql> SELECT CONCAT(productCode, ' - ', name) AS `Product Description`,
price FROM products;
+---------------------+-------+
| Product Description | price |
+---------------------+-------+
| PEN - Pen Red | 1.23 |

| PEN - Pen Blue | 1.25 |
| PEN - Pen Black | 1.25 |
| PEC - Pencil 2B | 0.48 |
| PEC - Pencil 2H | 0.49 |
+---------------------+-------+
2.6 Producing Summary Reports
To produce a summary report, we often need to aggregate related rows.
DISTINCT
A column may have duplicate values, we could use keyword DISTINCT to select only distinct
values. We can also apply DISTINCT to several columns to select distinct combinations of these
columns. For examples,
-- Without DISTINCT
mysql> SELECT price FROM products;
+-------+
| price |
+-------+
| 1.23 |
| 1.25 |
| 1.25 |
| 0.48 |
| 0.49 |
+-------+
-- With DISTINCT on price
mysql> SELECT DISTINCT price AS `Distinct Price` FROM products;
+----------------+
| Distinct Price |
+----------------+
| 1.23 |
| 1.25 |
| 0.48 |
| 0.49 |
+----------------+
-- DISTINCT combination of price and name
mysql> SELECT DISTINCT price, name FROM products;
+-------+-----------+
| price | name |
+-------+-----------+
| 1.23 | Pen Red |
| 1.25 | Pen Blue |
| 1.25 | Pen Black |
| 0.48 | Pencil 2B |
| 0.49 | Pencil 2H |
+-------+-----------+
GROUP BY Clause

The GROUP BY clause allows you to collapse multiple records with a common value into groups.
For example,
mysql> SELECT * FROM products ORDER BY productCode, productID;
+-----------+-------------+-----------+----------+-------+
+-----------+-------------+-----------+----------+-------+
| 1004 | PEC | Pencil 2B | 10000 | 0.48 |
| 1005 | PEC | Pencil 2H | 8000 | 0.49 |
| 1001 | PEN | Pen Red | 5000 | 1.23 |
| 1002 | PEN | Pen Blue | 8000 | 1.25 |
| 1003 | PEN | Pen Black | 2000 | 1.25 |
+-----------+-------------+-----------+----------+-------+
mysql> SELECT * FROM products GROUP BY productCode;
-- Only first record in each group is shown
+-----------+-------------+-----------+----------+-------+
+-----------+-------------+-----------+----------+-------+
| 1004 | PEC | Pencil 2B | 10000 | 0.48 |
| 1001 | PEN | Pen Red | 5000 | 1.23 |
+-----------+-------------+-----------+----------+-------+
GROUP BY by itself is not meaningful. It is used together with GROUP BY aggregate functions
(such as COUNT(), AVG(), SUM()) to produce group summary.
GROUP BY Aggregate Functions: COUNT, MAX, MIN, AVG, SUM, STD, GROUP_CONCAT
We can apply GROUP BY Aggregate functions to each group to produce group summary report.
The function COUNT(*) returns the rows selected; COUNT(columnName) counts only the non-
NULL values of the given column. For example,
-- Function COUNT(*) returns the number of rows selected
mysql> SELECT COUNT(*) AS `Count` FROM products;
-- All rows without GROUP BY clause
+-------+
| Count |
+-------+
| 5 |
+-------+
mysql> SELECT productCode, COUNT(*) FROM products GROUP BY productCode;
+-------------+----------+
| productCode | COUNT(*) |
+-------------+----------+
| PEC | 2 |
| PEN | 3 |
+-------------+----------+
-- Order by COUNT - need to define an alias to be used as reference
mysql> SELECT productCode, COUNT(*) AS count
FROM products
GROUP BY productCode

ORDER BY count DESC;
+-------------+-------+
| productCode | count |
+-------------+-------+
| PEN | 3 |
| PEC | 2 |
+-------------+-------+
Besides COUNT(), there are many other GROUP BY aggregate functions such as AVG(), MAX(),
MIN() and SUM(). For example,
mysql> SELECT MAX(price), MIN(price), AVG(price), STD(price), SUM(quantity)
FROM products;
-- Without GROUP BY - All rows
+------------+------------+------------+------------+---------------+
| MAX(price) | MIN(price) | AVG(price) | STD(price) | SUM(quantity) |
+------------+------------+------------+------------+---------------+
| 1.25 | 0.48 | 0.940000 | 0.371591 | 33000 |
+------------+------------+------------+------------+---------------+
mysql> SELECT productCode, MAX(price) AS `Highest Price`, MIN(price) AS
`Lowest Price`
FROM products
GROUP BY productCode;
+-------------+---------------+--------------+
| productCode | Highest Price | Lowest Price |
+-------------+---------------+--------------+
| PEC | 0.49 | 0.48 |
| PEN | 1.25 | 1.23 |
+-------------+---------------+--------------+
mysql> SELECT productCode, MAX(price), MIN(price),
CAST(AVG(price) AS DECIMAL(7,2)) AS `Average`,
CAST(STD(price) AS DECIMAL(7,2)) AS `Std Dev`,
SUM(quantity)
FROM products
GROUP BY productCode;
-- Use CAST(... AS ...) function to format floating-point numbers
+-------------+------------+------------+---------+---------+---------------+
| productCode | MAX(price) | MIN(price) | Average | Std Dev | SUM(quantity) |
+-------------+------------+------------+---------+---------+---------------+
| PEC | 0.49 | 0.48 | 0.49 | 0.01 | 18000 |
| PEN | 1.25 | 1.23 | 1.24 | 0.01 | 15000 |
+-------------+------------+------------+---------+---------+---------------+
HAVING clause
HAVING is similar to WHERE, but it can operate on the GROUP BY aggregate functions; whereas
WHERE operates only on columns.
mysql> SELECT
productCode AS `Product Code`,
COUNT(*) AS `Count`,
CAST(AVG(price) AS DECIMAL(7,2)) AS `Average`

FROM products
HAVING Count >=3;
-- CANNOT use WHERE count >= 3
+--------------+-------+---------+
| Product Code | Count | Average |
+--------------+-------+---------+
| PEN | 3 | 1.24 |
+--------------+-------+---------+
WITH ROLLUP
The WITH ROLLUP clause shows the summary of group summary, e.g.,
mysql> SELECT
productCode,
MAX(price),
MIN(price),
CAST(AVG(price) AS DECIMAL(7,2)) AS `Average`,
SUM(quantity)
FROM products
WITH ROLLUP; -- Apply aggregate functions to all groups
+-------------+------------+------------+---------+---------------+
| productCode | MAX(price) | MIN(price) | Average | SUM(quantity) |
+-------------+------------+------------+---------+---------------+
| PEC | 0.49 | 0.48 | 0.49 | 18000 |
| PEN | 1.25 | 1.23 | 1.24 | 15000 |
| NULL | 1.25 | 0.48 | 0.94 | 33000 |
+-------------+------------+------------+---------+---------------+
2.7 Modifying Data - UPDATE
To modify existing data, use UPDATE ... SET command, with the following syntax:
UPDATE tableName SET columnName = {value|NULL|DEFAULT}, ... WHERE criteria
For example,
-- Increase the price by 10% for all products
mysql> UPDATE products SET price = price * 1.1;
+-----------+-------------+-----------+----------+------------+
+-----------+-------------+-----------+----------+------------+
| 1001 | PEN | Pen Red | 5000 | 1.35 |
| 1002 | PEN | Pen Blue | 8000 | 1.38 |
| 1003 | PEN | Pen Black | 2000 | 1.38 |
| 1004 | PEC | Pencil 2B | 10000 | 0.53 |
| 1005 | PEC | Pencil 2H | 0 | 9999999.99 |
+-----------+-------------+-----------+----------+------------+
-- Modify selected rows

mysql> UPDATE products SET quantity = quantity - 100 WHERE name = 'Pen Red';
mysql> SELECT * FROM products WHERE name = 'Pen Red';
+-----------+-------------+---------+----------+-------+
+-----------+-------------+---------+----------+-------+
| 1001 | PEN | Pen Red | 4900 | 1.35 |
+-----------+-------------+---------+----------+-------+
-- You can modify more than one values
mysql> UPDATE products SET quantity = quantity + 50, price = 1.23 WHERE name
= 'Pen Red';
mysql> SELECT * FROM products WHERE name = 'Pen Red';
+-----------+-------------+---------+----------+-------+
+-----------+-------------+---------+----------+-------+
| 1001 | PEN | Pen Red | 4950 | 1.23 |
+-----------+-------------+---------+----------+-------+
CAUTION: If the WHERE clause is omitted in the UPDATE command, ALL ROWS will be
updated. Hence, it is a good practice to issue a SELECT query, using the same criteria, to check
the result set before issuing the UPDATE. This also applies to the DELETE statement in the
following section.

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