Creating Tables Unit 9 - Databases

Planning Your Table Before you create the table you must plan: What fields you will include What the data-types of those fields will be What names to give them Which field to use as the primary key The range of acceptable values for each field This is important as changes may be difficult later You can plan a table design using a data dictionary Creating Tables Presentation Unit 9

Data-type Each field must have a data-type Choosing the correct data-type is important Helps ensure only valid data is entered Allows you to use features related to the data-type Commonly used data-types are: Text Number Date Yes/No Creating Tables Presentation Unit 9

Other Field Settings As well as setting the name and data-type of a field there are other settings you can make. Most of the settings are dependant on the data-type Typical settings you might change include: Field length Format (e.g. date format, 20/7/05 or 20th July 2005) Default value Validation rule Creating Tables Presentation Unit 9

Text Data-type Use where text or text combined with numbers will be stored in the field Also used to store numbers that don’t require calculations to be done (e.g. phone numbers) Default length is 50 characters, maximum length is 255 characters. For longer text use the Memo data-type No preset formats Creating Tables Presentation Unit 9

Numeric Data-type Field size Not set as a value (e.g. 10 digits) but as a method of storage This sets the range of values that can be stored Integer – range of minus 32,768 to plus 32,768 Long Integer - range of minus/plus 2 million Double stores very large real numbers. Format Allows formats such as Fixed number of decimals Scientific Percent Currency Creating Tables Presentation Unit 9

Auto-number Special numeric data-type Automatically set to long integer – don’t change it. Automatically incremented by 1 when a new record is created Ensures every record in a table has a unique value for this field Ideal for primary key fields BUT you cannot modify values in this field Creating Tables Presentation Unit 9

Date/time data-type Used to store dates and or times Will not allow input of invalid values (e.g. 32/7/05) Various preset formats Order of the day, month and year is set by Regional Settings in Control Panel. Default value can be a fixed date or the =Now() function will insert the current date when inserting new records. Creating Tables Presentation Unit 9

Example You are creating a database of secondary school student details and you want a field to hold the student’s age on the 1st September in the current year What data-type should you choose? What field length? Would a default value be a good idea? Are there any validation rules you could add? Creating Tables Presentation Unit 9

Example The data-type should be Numeric Field length should be Integer Default value is probably not a good idea here Validation rule could be added to restrict values to greater than 12 and less than 21 Creating Tables Presentation Unit 9

Data Dictionary Field name Data-type Field length Key? Format Default Validation rule Student number Auto-number Long integer Yes - Surname Text 50 No First Name Date of Birth Date Short date Gender 1 M or F Creating Tables Presentation Unit 9