Transcript Creating Tables

Data Types and Table
Creation
Introduction to Database Technology
Session 2
Session learning outcomes
At the end of this session you will be able to:
• Work with your database using the MySQL
Query Browser
• Describe the relationship between fields, records
and tables
• Select appropriate data types and sizes for the
fields in a table
• Design a table
• Use SQL commands to create a table in a
MySQL database
• Use SQL Commands to enter and edit data in a
table
Introduction to Database Technology
Session 2
MySQL Query Browser
• Provides a graphical user interface (GUI)
for use with MySQL databases
• Will be used in the rest of this module
• Hands-on 3 enables you to familiarise
yourself with this interface
Introduction to Database Technology
Session 2
Table Creation Steps
1. List the attributes (fields)
2. Choose an appropriate data type for
each attribute
3. Select an appropriate primary key for the
table
4. Create the table in the database
5. Modify the table structure (if required)
6. Enter data into the table
7. Edit the data (if required)
Introduction to Database Technology
Session 2
MySQL Character Data Types
Use Character data types for fields that will contain character strings –
letters, digits and special characters.
Also use for numbers that will not be used in calculations – a customer
number, for example.
CHAR(n)
Stores character string up to n (max 255)
characters long.
If the string is less than n characters long, blank
spaces are stored for the remaining characters.
VARCHAR(n)
Stores character string up to n (max 255)
characters long.
Only stores the string with no blank characters.
Uses less storage space.
Slower processing for queries and updates
TEXT
Use to store textual data up to a maximum of
65,535 characters
Introduction to Database Technology
Session 2
MySQL Numeric Data Types
Use Numeric data types for fields whose values will be used in calculations
INT, INTEGER
Stores integers (whole numbers with no decimal
part).
Range: -2147483648 to 214748367
SMALLINT
Stores integers but uses less storage space than
INT
Range: -32768 to 32767
Use when you are sure the values will be within
this range
DECIMAL(p,q)
Stores decimal number p digits long with q decimal
places.
E.g. DECIMAL(5,2) has 3 places to the left and
two to the right of the decimal point (such as
100.00)
Use this data type for currency fields
Introduction to Database Technology
Session 2
Storing dates in MySQL
DATE
Stores dates.
Dates have format YYYY-MM-DD
Enter dates using single quote marks.
E.g. ‘2008-08-20’ is August 20, 2008
This has introduced the basic data types that will be used in the Yum
Juices database that you create during this module
You can find more information about data types in the MySQL
Reference Manual
Introduction to Database Technology
Session 2
NULL Values
• If a field does not contain data its value is
referred to as NULL
• To ensure a field cannot contain a NULL value
you need to define it as NOT NULL when you
create the table
• NULL values are appropriate when a field may
not contain data for some records
– for example, in the Customer table there are two
fields for the address but some customers only have
one address line.
Introduction to Database Technology
Session 2
Primary Keys
• A unique identifier for each record in a table
• All tables in a relational database should have a
primary key
• Often a number
– E.g. customer ID, order number
• Can be one or more fields
– Primary keys made up of two or more fields are
known as composite primary keys (covered later in
this module)
Introduction to Database Technology
Session 2
Now do…
• Hands-on Exercise 4.1
– Define data types for the fields in the Yum
Customer table.
– Decide which fields can contain NULL values.
– Select an appropriate primary key for the
table.
– Record your decisions on the sheet provided.
Introduction to Database Technology
Session 2
CREATE TABLE Syntax
CREATE TABLE tablename
( col_name datatype [NOT NULL] [PRIMARY KEY],
…, col_name datatype ) ;
Introduction to Database Technology
Session 2
Table and field name rules
• Can include alphanumeric characters and underscores _
but no other characters.
• Should not consist only of digits.- e.g. 1223
• Should not start with a digit – e.g. 1Pet
• Cannot exceed 18 characters.
• Are case sensitive - 'Pet', 'PET' and 'pet' would be
regarded as three different names, for example.
• Must not contain spaces.
• Must not be a MySQL reserved word – a word that has
special meaning in MySQL such as SELECT, for
example.
Introduction to Database Technology
Session 2
Table and field name conventions
(used in this module)
• Table names start with an initial uppercase
letter and are singular – Customer rather
than Customers, for example.
• Field names are in lower case, prefixed by
the name of the table (also lowercase)
with an underscore – customer_name, for
example.
• Will be descriptive of their contents. For
example, Customer rather than Table1.
Introduction to Database Technology
Session 2
CREATE TABLE Example
CREATE TABLE Pet
(pet_id CHAR(2) PRIMARY KEY NOT NULL,
pet_name CHAR(10),
pet_type CHAR(10),
pet_cost DECIMAL(5,2),
pet_dateofbirth DATE );
Introduction to Database Technology
Session 2
Modifying the structure of a table in
MySQL
ALTER TABLE tablename
MODIFY col_name new_datatype
- Change the data type of a specified field
ADD col_name datatype
- Add a new field to the table
CHANGE old_col_name new_col_name datatype
- Change the name of a field
DROP COLUMN col_name
- Remove a field (together with any data it contains)
Introduction to Database Technology
Session 2
ALTER TABLE Examples
ALTER TABLE Pet
MODIFY pet_name VARCHAR(25)
- Changes length of pet_name field
ADD pet_food VARCHAR(30)
- Adds pet_food field to the Pet table
CHANGE pet_dateofbirth pet_birthday DATE
- Changes name of pet_dateofbirth field to pet_birthday
DROP COLUMN pet_food
- Removes pet_food field from Pet table
Introduction to Database Technology
Session 2
MySQL Scripts
• Saving your SQL statements as a script allows you to rerun them.
• A script can contain one or more SQL statements
• Each statement must end with a semi-colon ;
• A script is simply a text file.
– You can create a script using a text editor (such as Notepad).
• In this module, you will use the Query Browser to write
and run scripts.
• Scripts saved using the Query Browser will be given a
.sql extension
– scripts saved with a .txt extension can also be opened and run
from the Query Browser.
Introduction to Database Technology
Session 2
Now do…
• The rest of Hands-on 4
– Create the Yum customer table using the
attributes and data types you listed earlier
– Save your table creation statement as a script
Introduction to Database Technology
Session 2
Entering data
INSERT INTO tablename
VALUES
(Val_1, Val_2, …, Val_n ) ;
Inserts data into all fields from left to right
You must specify a value for each field
Example:
INSERT INTO Pet
VALUES
(‘1’,’Mog’,’cat’,’20.00’,’2006-05-10’);
Introduction to Database Technology
Session 2
Entering data with NULLs
• You must explicitly enter NULL if a field
does not contain a value
• For example, to enter Mog’s details
without a date of birth:
INSERT INTO Pet
VALUES
(‘1’,’Mog’,’cat’,’20.00’,NULL);
Introduction to Database Technology
Session 2
Entering data into specific fields
• List the fields that data is to be entered into after
the table name
• You must include all fields that are defined as
NOT NULL
• Data will be entered in the order in which the
fields are specified
• Example – enter the type, name and ID for a pet
INSERT INTO Pet (pet_type, pet_name, pet_id)
VALUES
(‘cat’,’Mog’,’1’);
Introduction to Database Technology
Session 2
Updating Data
• You may make mistakes when entering
the data and need to correct them
• Data stored in the database may need to
be changed (a customer moves to a new
address, for example)
Introduction to Database Technology
Session 2
UPDATE Syntax
UPDATE tablename
SET fieldname = new_value
WHERE condition to select record
Example:
UPDATE Pet
SET pet_name = 'Moggy'
WHERE pet_id = ‘1';
Introduction to Database Technology
Session 2
Now do…
• Hands-on 5
– Enter data into the Yum Customer table that
you created in Hands-on 3
– Make sure you have downloaded the Yum
sample data before starting this exercise
Introduction to Database Technology
Session 2
SQL Command Summary
• This session has introduced the following
SQL commands:
– CREATE TABLE
– ALTER TABLE
– INSERT INTO
– UPDATE
• Use the Reference Manual to review these
commands and ensure you are familiar
with their syntax and usage.
Introduction to Database Technology
Session 2