Access Primer

UoN workshopNaivasha, 30 July – 4 August

2006

MS Access

Relational Database Management System Data/information resides in series of

related tables Principle of normalisation

Double system (Microsoft!) .mdb: stand-alone files .adp: front-end to MS SQL Server database

Access elements

Tables: to store data Queries: to ask questions from

database Forms: to create a user interface Macros: to automate repetitive tasks Modules: create non-standard

functionality, using Visual Basic

Tables

Must be defined before they can be used Vaguely resemble Excel sheets but:

Unit of information is a ‘record’ (line) Saved automatically as soon as one moves to

another record Consists of ‘fields’

Type must be chosen All fields of a record remain together

No restriction on the number of lines Excel: 64,000 lines

Order of records is not fixed!

Types of fields

Text (length; max = 255) Memo Integer (normal or long)

Autonumber Real (single or double) Date Boolean (yes/no)

Indices

Facilitate sorting and selective extraction of records

Special index: primary key unique identifier for a record often ‘autonumber’

Other indices: Unique or not Slow data entry down

Normalisation

All data should be in the database once and only once Repeated data go to separate table Relationships between the tables are

part of the ‘model’ of the databaseLink between tables defined on pair of fields,

one in each table, that are of same typeRecords of the two tables are ‘related’ if

value in the link fields is the same

Non-normalised Table

Species # legs # eyes place Country dateAsterias rubens 5 0 Oostende Belgium 12/3/2004Asterias rubens 5 0 Zeebrugge Belgium 13/3/2005Asterias rubens 5 0 Zeebrugge Belgium 14/3/2005Cancer pagurus 10 2 De Panne Belgium 12/3/2004Cancer pagurus 10 2 Oostende Belgium 12/3/2004Cancer pagurus 10 2 Zeebrugge Belgium 14/3/2004Asterias rubens 5 0 Wimereux France 13/3/2005Asterias rubens 5 0 Wimereux France 14/3/2005Cancer pagurus 10 2 Wimereux France 12/3/2004

Normalisation

Table with morphological characters fields: species, number of legs, number of eyes

Table with distribution records fields: species, place, date

The field ‘Species’ is common to the two tables, and makes it possible to link records between two tables

Creating relations

‘one to many’ relations Defined on fields of the same type

In case of text fields: same length

On the many side: primary key Helps in maintaining the integrity of the

data Enforce presence of a ‘related record’ in the

‘related table’ Avoid spelling variations (Oostende, Ostend, Ostende…)

Relations

Best on fields that do not carry any real-world information Autonumber

In the example: add field autonumber in the species table, and use this as a code in the distributions table; make autonumber field the primary key

Also best to add Autonumber as primary key in distributions table

And more Relations…

Also the locality of the species distribution record should be in a separate table With fields for position, country…

Queries

Select and sort records of a table Pull together information from related tables Simple statistics

In principle: SQL But access interface hides SQL from the user

In pratice: query grid Define which tables will be used, fields that have

to be shown, criteria for selection and sorting…

SQL statement

SELECT Places.PlaceName, Species.SpeciesName

FROM Species INNER JOIN (Places INNER JOIN distributions ON Places.PlaceID = distributions.placeID) ON Species.speciesID = distributions.speciesID

ORDER BY Places.PlaceName;

Forms

Create an interface for the users Hide codes/autonumbers (combobox) Elements of the form are linked to

fields in the tables Can show single records at a time, or

‘repeated forms’ Entries/changes to the form are written

automatically to the table

ComboBox

Limit possible entries into a field Hide codes/autonumbers from users Two different representations of the

same information are combined Code, to be entered into the table Full text equivalent, for the benefit of

the user

Subforms

To display the content of a many-to-many relationship We don’t know how many related records

there will be, so impossible to solve with static, predefined structure

A form can be repeated, showing multiple lines Number of lines is not limited

First create the subform

Subforms

And then the main form

And now…

How to decide on the structure of a database? Which information do we want to store? Which tables do we need? Relationships

between them? Which indices (primary and others) do we

need?

Start from an analysis of user needs In principle also, decide on a platform

Task

Create ‘model’ for biodiversity/biogeography database Which standard vocabularies to use? What are important elements of

information?SourcesBiology

Classification, synonymyGeography