barcode generator java source code free Databases: A Beginner s Guide in Software

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Databases: A Beginner s Guide
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Figure 2-9
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Microsoft Access 2007 Relationships panel
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symbol 1 near the parent table (the one side) and the mathematical symbol for infinity (a sideways figure 8) near the child table (the many side) These constraints are defined simply by dragging the name of the primary key in the parent table to the name of the foreign key in the child table A pop-up window is then automatically displayed to allow the definition of options for the referential constraint, as shown in Figure 2-10 At the top of the Edit Relationships panel, the two table names appear with the parent table on the left and the child table on the right If you forget which is which, the Relationship Type field at the bottom of the panel should remind you Under each table
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2: Exploring Relational Database Components
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Figure 2-10 Microsoft Access 2007 Edit Relationships panel
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name are rows for selection of the column names that make up the primary key and foreign key Figure 2-10 shows the primary key column ID in the Customers table and foreign key column Customer ID in the Orders table The check boxes provide some options:
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Enforce Referential Integrity If this box is checked, the constraint is enforced; unchecking the box turns off constraint enforcement Cascade Update Related Fields If this box is checked, any update to the primary key value in the parent table will cause automatic corresponding updates to the related foreign key values An update of primary key values is a rare situation Cascade Delete Related Records If the box is checked, a delete of a parent table row will cause the automatic cascading deletion of the related child table rows Think carefully here There are times when you should use this, such as the constraint between Orders and Order Details, and times when the option can lead to the disastrous unwanted loss of data, such as deleting an employee (perhaps accidentally) and having all the orders that employee handled automatically deleted from the database
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Intersection Tables
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The discussion of many-to-many relationships earlier in this chapter pointed out that relational databases cannot implement these relationships directly and that an intersection table is formed to establish them Figure 2-11 shows the implementation of the Order Details intersection table in Microsoft Access
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Databases: A Beginner s Guide
Figure 2-11 Order Details intersection table (Microsoft Access 2007)
The many-to-many relationship between orders and products in the conceptual design becomes an intersection table (Order Details) in the logical design The relationship is then implemented as two, one-to-many relationships with the intersection table on the many side of each The primary key of the Order Details table could be formed using the combination of Order ID and Product ID, with Order ID being a foreign key to the Orders table and Product ID being a foreign key to the Products table In this case, however, the designer chose to add a single unique key value, ID, as the primary key of the Order Details table This arrangement is known as a surrogate key, because the so-called natural key has been replaced with another one Take a moment to examine
2: Exploring Relational Database Components
the contents of the intersection table and the two referential constraints Understanding this arrangement is fundamental to understanding how relational databases work Here are some points to consider:
Each row in the Order Details intersection table belongs to the intersection of one product and one order It would not make sense to include Product Name in this table because that name is the same every time the product appears on an order Also, it would not make sense to include Customer ID in Order Details because all line items on the same order belong to the same customer Each Products table row may have many related Order Details rows (one for each order line item on which the product was ordered), but each Order Details row belongs to one and only one Products table row Each Orders table row may have many related Order Details rows (one for each line item for that particular order), but each Order Details row belongs to one and only one Orders table row
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