print barcode labels vb.net Figure 7-3 The Customer table in Visual C#

Generation Code 3 of 9 in Visual C# Figure 7-3 The Customer table

Figure 7-3 The Customer table
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Microsoft Visual Studio 2010: A Beginner s Guide
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The CustomerID has a primary key symbol, which is created by right-clicking the column and selecting Set Primary Key If you needed a composite key (multiple columns that define a key), which you don t in this simple example, you would press CTRL and click each column that belongs to the key and then right-click and select Set Primary Key
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When working with LINQ, which we discuss later in this chapter, it is absolutely essential that you give every table a primary key
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In addition to setting the primary key, it s helpful to make the key number autoincrement so that it will have a unique value for every record you insert In Figure 7-3, you can see that CustomerID is selected and Column Properties has scrolled down to the Identity Specification property By default, Identity Specification is collapsed and set to No You ll need to expand the Identity Specification property by selecting the arrow on the left, and change the value to Yes by selecting the drop-down arrow on the right of the (Is Identity) setting, which by default is No This will also enable Identity Increment, which specifies the number to add for each new record, and the Identity Seed, which specifies what the first number will be The effect of setting Identity Increment is that the first record added to the table will have a CustomerID with the value 1 (Identity Seed) and subsequent records will have a CustomerID with the values 2, 3, 4, and so on (Identity Increment) The value for CustomerID in each record creates a unique value that identifies the record and makes it easy to program data applications When you re done creating the table, click the Save button on the toolbar and name the table Customer when prompted You can add data to the Customer table by opening the database in Server Explorer, navigate to the Tables folder in the MyShop database, right-click Customer, and select Show Table Data You ll see a grid similar to Figure 7-4 where you can enter some
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Figure 7-4 Adding data to a table
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7: Working with Data
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Figure 7-5 The Order table
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customer data Notice that you need only type a name in the Name column (replacing the word NULL) and do not need to enter a value for the CustomerID, since we ve made the CustomerID column auto-increment In a database of any sophistication, you have multiple tables In the MyShop database, a Customer has an Order So, create a new table named Order, shown in Figure 7-5, that has Primary Key OrderID, a datetime field called OrderDate, and an int field called CustomerID The Description in the Column Properties for the CustomerID field says FK to Customer FK is an abbreviation for foreign key, which is used to create a relationship between a parent table and a child table The next section explains more about what a foreign key is and how to create one
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Relating Tables with Foreign Keys
Foreign keys allow you to establish a relationship between two tables You can think of this relationship as being parent/child, master/detail, or one-to-many; each analogy being equal, I ll use parent/child The preceding section shows how to create Customer and Order tables The relationship between these two tables is that Customer is the parent and Order is the child One record in Customer can have many records in Order; customers
Microsoft Visual Studio 2010: A Beginner s Guide
can have zero or more orders A foreign key can help manage the relationship between Customer and Order The mechanics of creating a foreign key relationship is that you put the foreign key column in the child table, Order, and have the foreign key column refer to the primary key of the parent table, Customer In this case, the reference is made simply by matching the integer value; if the ID number is the same in both tables, then the records are related As shown in Figure 7-5, Order does have a CustomerID column, of type int, and we will make this the foreign key that will refer to CustomerID primary key in Customer, shown in Figure 7-3 To create this foreign key relationship in VS, right-click the CustomerID column in the Order table and select Relationships We re going to create the foreign key relationship that you see in Figure 7-6 Next, click Add, select the Tables And Columns Specific property, and click the ellipses button that appears on the far right of your selection (the ellipses does not appear until you first click Tables And Columns Specific below (General) in the Foreign Key Relationships dialog window This will open the Tables And Columns window shown in Figure 7-7
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