Creating Tables and Relationships
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Access 2007 Creating Databases - Fundamentals Contents Database Design Objectives of database design 1 Process of database design 1 Creating a New Database.............................................................................................................. 3 Tables ............................................................................................................................................ 4 Creating a table in design view 4 Defining fields 4 Creating new fields 5 Modifying table design 6 The primary key 7 Indexes 8 Saving your table 9 Field properties 9 Calculated Field Properties (Access 2010 only) 13 Importing Data ............................................................................................................................. 14 Importing data from Excel 14 Lookup fields ................................................................................................................................ 16 Modifying the Data Design of a Table ........................................................................................20 Relationships ................................................................................................................................22 Creating relationships 23 Viewing or editing existing relationships 24 Referential integrity 24 Viewing Sub Datasheets 26 . Page 2 of 29 Database Design Time spent in designing a database is time very well spent. A well-designed database is the key to efficient management of data. You need to think about what information is needed and how that information is to be used. Objectives of database design The strategy of database design is to accomplish the following objectives: To organise stored information in a timely, consistent, and economical manner. To eliminate, or minimise, the duplication of database content across the organisation. To provide rapid access to the specific elements of information in the database required by each user. To accommodate the possible expansion of the database to adapt to the needs of a growing organisation, such as the addition of new products and processes. To maintain the integrity of the database so that it contains only validated, auditable information. To prevent access to the database by unauthorised persons. Process of database design Planning your database This should be done on paper. Determine the purpose of your database. This will include deciding what information needs to be stored and what will need to be retrieved. What ‘questions’ will you need to ask your data? Tables Decide what tables you need in your database: A table should not contain duplicate information and information should not be duplicated between Tables. If information needs to be updated, it should only need updating in one place. This is more efficient and also eliminates the possibility of duplicate entries that contain different information. Do not include any calculated data (data that is the result of an 'expression') as this is effectively duplicated information. Each table should contain information about one ‘entity’ or subject. This enables you to keep information about each subject independently from other subjects. Fields Determine the fields you need in the tables: Each field should relate to the subject of the table. Each field should contain a particular type of information about the table's subject. Create fields so that you can store information in its smallest logical parts (e.g., First Name, Initial, last name, rather than just name). How small this part is will depend on how you will want to sort, filter or query your records. Primary keys Identify which fields have unique values and decide which field(s) will be your primary key(s) Page 1 of 29 Data types Determine the appropriate data types for each of your fields (e.g. Text, Currency, Date, etc). Unless you want to use the default data type (Text), you will need to assign a data type to each of your fields. All data in a single field must consist of the same data type. Relationships Identify associations between the tables (when you have more than one Table). You will need to define relationships between your tables so that Access can bring related information from different Tables back together in meaningful ways. Implementing your design The list below is a suggested step-by-step plan: Create a small database based on your design. Specify relationships between the tables Enter a few records in each table. Create rough drafts of your forms and reports to see if they contain the data you need. Try running a few queries to see if you get the answers you expect. Check your database for any unnecessary duplication of data. Check your design with anyone else who will be using your database. Decide who will be able to access the database, the tables, and the fields within the tables. Finally, enter your data into your tables. Create any queries, forms, reports, etc. that you need. Ensure all the data in your database is relevant and kept up-to-date. Remember the Data Protection Act. Page 2 of 29 Creating a New Database Before you can create objects such as tables and forms, you must first create the database file in which they will be stored. 1. On the File tab select New. 2. Click on Blank database. 3. In the File Name box, type a name for your database. 4. Click on the browse button to the right of the File name box to browse for a location for your database. 5. Click on Create. A new database will be created with a new default Table. 6. Click on Design View to start working with this Table. Page 3 of 29 Tables Tables are the fundamental objects in a database. Without any tables, no data can be stored. To create a table you need to follow these steps: Create the table object Define the fields in the table including their name, data type and description Set the properties for each field Create appropriate indexes Set the primary key Save the table Creating a table in design view 1. On the Create tab, click on Table Design. A new table will appear in design view: The table design view There are three main parts to the design view window. Upper pane Upper pane For defining fields by name, data type and description (optional). Field properties (bottom left) For defining specific properties for a field. Field properties Help (bottom right) As you move around the design view window, context- specific help is provided here. Help Defining fields Before creating the fields in a table, the following information about naming, data types and descriptions should be considered: Field naming rules and conventions The following conventions should be observed when naming fields: Field names are mandatory. They may contain up to 64 characters. Names may include embedded (but not leading) spaces and punctuation except full stops, exclamation marks and square brackets. You cannot assign the same field name to more than one field in the same table and it is good practice to use a unique field name for each field in the entire database. It is not good practice to include spaces in field names. Instead, use an underscore ( _ ) for spaces to improve the readability of field names. Page 4 of 29 Minimizing the length of field names conserves resources and saves typing when you refer to the field name in macros, etc. Avoid specifying a name for a field that could cause a conflict with a built-in Access function or property names (e.g. ‘name’, ‘date’, etc.). Data types You must assign a field data type to each field in a table unless you want to use the text data type that Access assigns by default. All data in a single field must consist of the same data type. Data Type Description Text Allows text and numbers to be stored. The default length of this field is 50 and is limited to a maximum of 255 characters. Memo Memo fields are usually used for notes, descriptions, etc and can store up to 2 gigabytes of which you can display 65,535 characters in Forms and Reports. Font formatting can also be applied to text in Memo fields. Number Allows only numbers to be stored. Used for fields which contain numbers you may wish to use for calculations. Date/Time Stores date and time formats. Currency Inserts the currency sign and decimal point. AutoNumber Automatically inserts a sequential or random number. You cannot enter data into this field – it is automatically filled in as you add records to your table. This data type can be used to create unique numbers to identify records. Yes/No Is a logical field used when you only have either a Yes or No or a True or False value. OLE Object Stores pictures, charts, etc. Can be up to 1 gigabyte. Hyperlink Stores a hyperlink address which is a path to an object, document, Web page, or other destination. Attachment Allows you to attach specific files to an individual record in a table. Calculated Allows you to add a calculation based on values from other fields and/or other values. It is also possible to use functions. Lookup This is not really a data type as such but starts up a wizard which allows you to wizard create a field that looks up data either from a list of values you type in or from another table. See page 16 for details on using the Lookup Wizard. Description This is optional but can be useful for other users of the database. If you enter a description it will be displayed in the status bar at the lower left of the Access window when you select the field for data entry or editing in the datasheet view of a table or the form view of a form. Creating new fields 1. In the first row of the Field Name column, type the name of your first field, following the naming conventions listed above. 2. Press the Tab key to take you to the Data Type field. 3. Click on the drop-down arrow that appears in the box to display a list of data types (see below) and select the appropriate one. 4. Press Tab to take you to the Description field.