Session 2Introduction to Database Technology Data Types and Table Creation

Session 2Introduction to Database Technology

Data Types and Table Creation


Session learning outcomesAt 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


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


Table Creation Steps1. 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)


MySQL Character Data Types

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

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.


MySQL Numeric Data Types

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

Use Numeric data types for fields whose values will be used in calculations


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


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.


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)


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.


CREATE TABLE Syntax

CREATE TABLE tablename

( col_name datatype [NOT NULL] [PRIMARY KEY],

…, col_name datatype ) ;


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.


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.


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 );


Modifying the structure of a table in MySQL

ALTER TABLE tablenameMODIFY 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)


ALTER TABLE Examples

ALTER TABLE PetMODIFY 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


MySQL Scripts

• Saving your SQL statements as a script allows you to re-run 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.


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


Entering data

INSERT INTO tablename VALUES (Val_1, Val_2, …, Val_n ) ;

Inserts data into all fields from left to rightYou must specify a value for each field

Example:INSERT INTO PetVALUES (‘1’,’Mog’,’cat’,’20.00’,’2006-05-10’);


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);


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 petINSERT INTO Pet (pet_type, pet_name, pet_id)

VALUES

(‘cat’,’Mog’,’1’);


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)


UPDATE Syntax

UPDATE tablename

SET fieldname = new_value

WHERE condition to select record

Example:UPDATE Pet

SET pet_name = 'Moggy'

WHERE pet_id = ‘1';


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


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.

Documents

Session 2Introduction to Database Technology Data Types and Table Creation