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12115 A Relational Example
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Going back to the customer/orders database originally depicted as a at le in Table 123, we could structure this database as a set of four tables as shown in Figure 122 Note that each Invoice belongs to one Customer and is made up of a number of Orders Each Order is for a quantity of a particular Item In general, cross-references direct from the detail end to the master end Therefore, Orders contain a reference to Items (since one type of item can appear on many orders) and to Invoices, since one invoice can have a number of orders on it Customers contain a reference to Invoices, since one customer can have any number of invoices Note also that each table has one or more particular columns that have been highlighted This column or columns contains values that are unique to each record in the table For example, we can arrange it so that each Customer, Invoice and Item in their tables are given a unique ID number; one that we can guarantee not to use in any other record in the same table In the Orders table, the two elds Invoice-No and
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The Customer-Orders system from Table 123 as a relational database
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Item-Code will be a unique combination for each order (since an invoice will contain a list of orders for different items) We call this eld or combination of elds the primary key of a table, since it can be used to unambiguously identify each record in the table If we refer to Item-Code 0006, it can only ever be a reference to a 1/4" bolt Similarly, Customer no 0035 is always Brown of 5 Main St and Order 0123-0082 will always refer to an order of 80 6ba Wing-Nuts on an Invoice for Customer no 0011 generated on 12/02/03 Primary keys make cross-referencing between tables possible and ef cient The value of a primary key should never change, since that would break the integrity of the database by breaking the link between the record whose primary key it is and other records in other tables When a cross-reference in made by inserting the primary key eld from one table into another table, it is referred to as a foreign key or secondary key in the other table
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A relationship is a link between one item in a table and another item in the same or a different table We describe the relationships between items in terms of how many items there can be at either end For example, a single customer can have a number of invoices (as shown for Customer no 0035 in Figure 122), which we would call a one-to-many relationship Similarly, one invoice would normally contain more than one employee (eg Invoice-No 0126) However, if we included a table of credit limits into our orders database, one customer would be related to one credit limit record, and vice-versa a one-to-one relationship There is a third form of relationship, in which many items in one table can be related to many items in another This form of relationship does not come up too often, but needs to be catered for in some circumstances For example, a publisher s database could contain a table of Authors and a table of Books One author could be responsible for writing any number of books, but also a single book could have been written by a number of authors (computer books are often written in this way) Unfortunately, this form of relationship cannot be dealt with by having the relationships directly between the two tables, since a relationship is always a one-way link We can add an AuthorID column to the Books table and a particular author s id could appear in any number of rows to show what books that author had written, but if we try to reverse the relationship so that a BookID could appear in a number of author entries, the picture gets confused: has the author written the one book whose ID is in his table entry, or the many that have his ID in the Books table The way around this to create true many-to-many relationship is to add a third table, which holds only links between authors and books For example, the table excerpts shown in Figure 123 are from Microsoft s Biblio database, supplied as an example database with copies of Access, SQL Server and MSDN Three tables itemize authors ( Authors ), books ( Titles ) and the many-to-many link between authors and books ( Title Author ) The link table in the middle shows that the author with AuthorID 2340 has contributed to two books (in fact many more than this) since that author s ID appears in two rows The book with ISBN 0-2011173-4-7 was written by two authors (2340 and 7707) The result is that the link table can be used to relate many items in one table with many items in another
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