Table Relationships. Microsoft Access Learning and Development Centre

Transcription

1 Table Relationships Microsoft Access 2016 Learning and Development Centre

2 ABOUT THIS TRAINING MANUAL This manual is yours to keep and is intended as a guide to be used during the training course and as a reference once the course is completed. Each section begins with a list of topics to be explored. The courseware is designed so that each topic is fully explained and step by-step instructions are given. There are a number of conventions used in this training manual: Format BOLD ITALICS Description This is indicates a command to follow e.g. an option or button to press [ ] Keys to press are shown in square brackets e.g. [space] HEADING This marks the start of a method for performing a specific task This marks additional information This marks additional information and a warning [CTRL] + [Page Up] This means the first key is used in conjunction with the second 1

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4 Table Relationships - Course Contents TABLE RELATIONSHIPS... 5 DATABASE TYPES... 6 PLANNING YOUR DATABASE... 7 IDENTIFYING TABLE RELATIONSHIP... 8 PRIMARY KEYS CREATING TABLE RELATIONSHIPS RELATIONSHIP OPTIONS EXERCISES REVISION (CREATING A DATABASE & TABLE) CUSTOMERS & ORDERS EXERCISE MUSIC COLLECTION DATABASE BUILDING A DVD COLLECTION BUILDING A STAFF PAY DATABASE MORE INFORMATION MORE INFORMATION ACCESS KEYBOARD SHORTCUTS

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6 Topics The following topics are covered in this chapter: Database Type Planning Your Database Identifying Table Relationship Primary Keys Creating Table Relationships Relationship Options 5

7 DATABASE TYPES When building a database there are 2 basic types, flat file and relational: Database Type Flat File Description Think of an address book or telephone directory. A flat file database stores all its information names, addresses, etc - in the same place, just like the addresses are stored in an address book. These types of databases are incredibly easy to create and use but are not very powerful or suited to many business tasks Relational A flat file database would contain a table like the one show above. It holds information about customers and has no link to any other table. This kind of database contains multiple tables that are related through matching fields. Tables in a relational database are linked by a common field. Relational databases are very powerful but developing one takes a lot of skill, a lot of practice and a strong understanding of how table and fields work together. In the above example, the customer table is linked to other tables e.g. orders they have made, items they have bought etc 6

8 PLANNING YOUR DATABASE Relational databases require lots of planning ahead. Before you attempt to create your database, you should sit down with a pencil and a pad of paper and walk through the following steps: Step Descriptions Determine the purpose of the database Write down the fields you need Organise and group fields into tables Identify and add the fields common to each Table Sketch a diagram of your database Write down a list of the reports and lists that you want to come out of the database. This may seem backwards at first, but these reports are the main reason you are creating your database. Make a list of reports and lists you want to see and sketch some samples be as detailed as possible. This will help determine what tables and fields to include in your database. This should be an easy step once you have determined the purpose of your database and have sketched some reports and lists. Each table in the database should be based on only one subject. By breaking each subject into its own table, you avoid redundant information and make the database more organised. When you brain storm, try to break down your information as much as possible. Each table will be linked by a common field, but what will that be? Look at your fields and establish which field related to each table. At least one of the linking fields will need to be a primary key. Create a diagram of your database similar to the one shown on the previous page. Draw a box for each table and write the field names inside that box. Draw lines between the related fields. There are different types of relationships which you will also need to determine. All this writing and planning may seem like a lot of work, but they are both critical steps in creating a sound database. 7

9 IDENTIFYING TABLE RELATIONSHIP One of the huge advantages of a relational database is that, once you have your data held in clearly defined, compact tables, you can connect or relate the data held in different tables. There are three types of relationships between the data you are likely to encounter at this stage in the design: One to One, One to Many and Many to Many. To be able to identify these relationships, you need to examine the data and gave an understanding of what business rules apply to the data and tables. If you are not sure it can be helpful to meet with someone who does have a thorough knowledge of the data. When analysing table relationships, you need to look at the relationship from both sides. When dealing with 2 tables one will be called the Primary or Parent Table and the other is the Related or Child Table. Relationship Type One to One Descriptions This means that each record in table A relates to one, and only one, record in table B, and each record in table B relates to one, and only one record in table A. E.g. each record in the staff table is about one employee. That record relates to one and only one record in the payroll table One to Many This means a record in table A can relate to zero, one or many records in table B. Many records in table B relate to one record in table A. E.g. each record in the customers records potentially relates to many in the orders table 8

10 Many to Many This means many records in one table can relate too many in another. It is the hardest relationship to understand. The way this relationship is set up is by creating 2, one to many relationships by using what is known as a junction table. E.g. A staff member can go many courses and a course may have many staff members on them. The way you establish who has been on what course is by entering the details into the bookings table 9

11 PRIMARY KEYS When you design an Access table, you are expected to designate a primary key. If you do not, then when you save the table, you are asked if you want to. Why would you want to designate a primary key? Here is a quote from the Access help file. The power of a relational database system such as Microsoft Access comes from its ability to quickly find and bring together information stored in separate tables using queries, forms, and reports. In order to do this, each table should include a field or set of fields that uniquely identifies each record stored in the table. This information is called the primary key of the table. Every record in a table must have a primary key that differentiates it from every other record in the table. In some cases, it is only necessary to designate an existing field as the primary key if you are certain that every record in the table will have a different value for that particular field. A National Insurance number is an example of a record whose values will only appear once in a database table. If none of the existing fields in the table will produce unique values for every record, a separate field must be added. Access will often create this type of field at the beginning of the table the first time you create one. The field is named "ID" and the data type is "autonumber". Since this extra field serves no purpose to you as the user, the autonumber type automatically updates whenever a record is added so there is no extra work on your part. TO ASSIGN PRIMARY KEY FIELD 1. In design view, click into the field that is to be your unique identifying field e.g. customer ID 2. Click the Design tab (you may already be on it) 3. Click the Primary Key button to assign A key symbol will appear next to the field 10

12 CREATING TABLE RELATIONSHIPS After the tables have been designed and primary keys assigned, a user can then define a relationship. Relationships are created in the relationship window. TO DEFINE A RELATIONSHIP 1. Close any open tables 2. Click the Database tools tab 3. In the Show/Hide group, click Relationships The relationship window appears 4. Choose the tables that the relationship will be based on, then choose Add 5. Click and drag the required field from one table to the related field in another table 11

13 The Edit Relationship dialogue box appears 6. Ensure the field names in the two columns are correct 7. Choose Create The relationship will now be displayed 8. Choose File, Close 9. Choose Yes to save the relationship 12

14 TO DELETE A RELATIONSHIP 1. Enter the relationships window 2. Select the join line 3. Press [Delete] A prompt appears 4. Click Yes Using the Mouse: 1. Right click the join line 2. Press [Delete] A prompt appears 3. Click Yes 13

15 RELATIONSHIP OPTIONS As you can see, establishing a relationship is quite easy. Establishing the right kind of relationship is a little more difficult. When you attempt to establish a relationship between two tables, Access makes some decisions based on a few predefined factors: Access establishes a one-to-many relationship if one of the related fields is a primary key or has a unique index. Access establishes a one-to-one relationship if both the related fields are primary keys or have unique indexes. Access creates an indeterminate relationship if neither of the related fields is a primary key, and neither has a unique index. You cannot establish referential integrity in this case. Enforce Referential Integrity When creating the relationship between two tables you have the option to Enforce Referential Integrity. Referential integrity consists of a series of rules that the Jet Engine applies to ensure that it properly maintains the relationships between tables. At the most basic level, referential integrity rules prevent the creation of orphan records in the table on the many side of the one-to-many relationship. After establishing a relationship between a Customers Table and an Orders Table, for example, all orders in the Orders Table must be related to a particular customer in the Customers Table. Before you can establish referential integrity between two Tables, the following conditions must be met: The Matching Field on the one side of the relationship must be a Primary Key field or must have a unique index. The Matching Fields must have the same data types (for linking purposes, AutoNumber fields match Long Integer fields). With the exception of Text fields, they also must have the same size. Number fields on both sides of the relationship must have the same size (Long Integer, for example). Both Tables must be part of the same Access database. Both Tables must be stored in the proprietary Access file (.MDB) format (they cannot be external tables from other sources). Existing data within the two tables cannot violate any referential integrity rules. All orders in the Orders Table must relate to existing customers in the Customers Table, for example. 14

16 After you establish referential integrity between two tables, Access applies the following rules: You cannot enter a value in the foreign key of the related table that does not exist in the primary key of the primary table. For example, you cannot enter a value in the CustomerID Field of the Orders Table that does not exist in the CustomerID Field of the Customers Table. You cannot delete a record from the Primary Table if corresponding records exist in the related table. For example, you cannot delete a customer from the Customers Table if related records exist in the Orders Table (records with the same value in the CustomerID Field). You cannot change the value of a Primary Key on the one side of a relationship if corresponding records exist in the related table. For example, you cannot change the value in the CustomerID field of the Customers Table if corresponding orders exist in the Orders Table. If you attempt to violate any of the previous three rules and you have enforced referential integrity between the tables, Access displays an appropriate error message: Access s default behaviour is to prohibit the deletion of parent records that have associated child records and to prohibit the change of a primary key value of a parent record when that parent has associated child records. You can override these restrictions by using the two check boxes available in the Relationships dialogue when you establish or modify a relationship. Cascade Update Related Fields The Cascade Update Related Fields option is available only if you have established referential integrity between the tables. With this option selected, the user can change the primary key value of the record on the one side of the relationship. When the user tries to modify the field joining the two tables on the one side of the relationship, the Jet Engine cascades the change down to the Foreign Key Field on the many side of the relationship. This is useful if the Primary Key Field is modifiable. For example, a purchase number on a purchase order master record might be updatable. If the user modifies the purchase order number of the parent record, you would want to cascade the change to the associated detail records in the purchase order detail table. There is no need to select the Cascade Update Related Fields option when the related field on the one side of the relationship is an AutoNumber field. You can never modify an AutoNumber field. The Cascade Update Related Fields option has no effect on AutoNumber fields. 15

17 Cascade Delete Related Records The Cascade Delete Related Records option is available only if you have established referential integrity between the tables. With this option selected, the user can delete a record on the one side of a one-to-many relationship, even if related records exist in the table on the many side of the relationship. A user can delete a customer even if the customer has existing orders, for example. The Jet Engine maintains referential integrity between the tables because it automatically deletes all related records in the child table. If you attempt to delete a record from the table on the one side of a one-to-many relationship and no related records exist in the table on the many side of the relationship, you get the usual warning message, as shown on the previous page. On the other hand, if you attempt to delete a record from the table on the one side of a oneto-many relationship and related records exist in the child table, Access warns you that you are about to delete the record from the parent table, as well as any related records in the child table. TIP The Cascade Delete Related Records option is not always appropriate. It is an excellent feature, but you should use it prudently. Although it is usually appropriate to cascade delete from an Orders table to an Order Details Table, for example, it generally is not appropriate to cascade delete from a Customers Table to an Orders Table. This is because you generally do not want to delete all your order history from the Orders Table if for some reason you want to delete a customer. Deleting the order history causes important information, such as your profit and loss history, to change. It therefore is appropriate to prohibit this type of deletion and handle the customer in some other way, such as marking him as inactive, or archiving his data. On the other hand, if you delete an order because the customer cancelled it, you probably want to remove the corresponding order detail information as well. In this case, the Cascade Delete Related Records option is appropriate. You need to make the most prudent decision in each situation, based on business needs. The important thing is to carefully consider the implications of each option before making your decision. With the Cascade Update feature enabled, you are able to update the primary key value of a record that has associated child records. With the Cascade Delete feature enabled, you can delete a parent record that has associated child records. 16

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20 Topics The following topics are covered in this chapter: Revision (Creating a Database & Table) Customers & Orders Exercise CD Collection Database Building A Video Collection Building A Staff Pay Database 19

21 REVISION (CREATING A DATABASE & TABLE) 1. Create a new database using Staff Database as the file name. 2. Once in the database create a new table with the following design structure: 3. Save the table using tblstaff as the table name. 4. Switch to datasheet view and add the following records Surname Initials Gender Salary Grade Dept Code Smith A F A Smith G F A Jones L F P Webber K F D 20

22 CUSTOMERS & ORDERS EXERCISE 1. Create a new database using Customer Database as the file name. 2. Once in the database create a new table with the following design structure: 3. Save the table using tblcustomers as the table name. NOTE: Assign Customer ID as the primary key 4. Switch to Datasheet and add the following records 5. Create a new table with the following design structure: 6. Save the table using tblorders as the table name. NOTE: Assign OrderID as the primary key 21

23 MUSIC COLLECTION DATABASE 1. Build the following 2 Tables using the table names provided. 2. The fields with keys will be assigned as the Primary Key 3. Save the table as tblartists 4. Save the table as tblalbums 22

24 BUILDING A DVD COLLECTION 1. Create a new database called DVD Collection Database 2. Create the following tables The fields with keys will be assigned as the Primary Key 3. Save the table as tblfilms 4. Save the table as tblactors 23

25 BUILDING A STAFF PAY DATABASE 1. Create a new database call Staff Pay Database 2. Create the following tables in design view The fields with keys will be assigned as the Primary Key 3. Save the table as tblemployees 4. Save the table as tblpay 24

26 Topics The following topics are covered in this chapter: More Information Keyboard Shortcuts 25

27 MORE INFORMATION We hope that you have found the course helpful and this training documentation easy to use. If you require more information about other courses we run, please visit our IT Training Website at: Visit our web pages to find: A comprehensive list of all IT courses currently available with information about course contents, duration and course tutor. An up-to-date timetable of when our courses are running and availability. Online Resources including links to all our training documentation, FAQ s and training videos. Contact Information If you want to ask the IT Training Team a question about any aspect of training, please contact us at ldc@uwe.ac.uk or phone us on ext:

28 ACCESS KEYBOARD SHORTCUTS Database Actions Editing Description Shortcut Keys Description Shortcut keys Open existing database [CTRL] + [O] Select all [CTRL] + [A] Open a new database [CTRL] + [N] Copy [CTRL] + [C] Save [CTRL] + [S] Cut [CTRL] + [X] Save record [SHIFT] + [ENTER] Paste [CTRL] + [V] Print [CTRL] + [P] Undo [CTRL] + [Z] Display database window [F11] Redo [CTRL] + [Y] Find and Replace [CTRL] + [F] Find [CTRL] + [F] Copy [CTRL] + [C] Replace [CTRL]+ [H] Cut [CTRL] + [X] Spell checker [F7] Paste [CTRL] + [V] Toggle between Edit mode and Navigation mode [F2] Undo [CTRL] + [Z] Open window for editing large content fields [SHIFT] + [F2] Help [F1] Switch from current field to current record [ESC] Toggle between Form and Design view [F5] Other Navigating Through a datasheet Description Shortcut keys Description Shortcut Key Insert line break in a memo field [CTRL] + [ENTER] Next field [TAB] Insert current date [CTRL] + [;] Previous field [SHIFT] + [TAB] Insert current time [CTRL] + [:] First field of record [HOME] Copy data from previous record [CTRL] + ['] Last field of record [END] Add a record [CTRL]+ [+] Next record [DOWN ARROW] Delete a record [CTRL] + [-] Previous record [UP ARROW] First field of first record [CTRL] + [HOME] Last field of last record [CTRL] + [END]

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