Creating the Employees Table
is a particular kind of constraint. When the primary key constraint is set on a column, the database will refuse to store duplicate values in that column.
Constraints in general, and primary keys in particular, represent a means by which the database can maintain the integrity and consistency of data.
Primary keys composed of a single column, such as Employee ID, are frequently used in conjunction with the IDENTITY property. The primary key constraint guarantees that duplicate values cannot be inserted into the table. The IDENTITY property helps us by always generating a new value that hasn’t already been used in the primary key.
Primary Keys and the IDENTITY Property
Using the IDENTITY property for a column doesn’t mean we can avoid specifying a primary key. It’s true that the IDENTITY property always generates unique values, but it doesn’t necessarily enforce them.
For example, say we have a table with a number of columns, one of which has the IDENTITY property set. This table contains three records that are likely to contain the automatically generated values 1, 2, and 3 in the IDENTITY column. Provided the INDENTITY_INSERT property for this table is enabled (by default it’s disabled, but it’s quite easy to enable), it’s quite simple to insert another record with the value 2. The IDENTITY column will continue to generate unique values (4, 5, 6, and so on), but it doesn’t guarantee the column remains unique.
Creating the Employees Table
In this section, I’ll show you how to use both Visual Web Developer and SQL Server Management Studio, but this time we’ll create a new data table. If you’re using Visual Web Developer, expand the database node in Database Explorer, right-click Tables, and select Add New Table, as shown in Figure 7.8.
Figure 7.8. Adding a new table in Visual Web Developer
267
Chapter 7: Database Design and Development
If you prefer SQL Server Management Studio, you need to follow a similar procedure. Expand the Dorknozzle database node, right-click Tables, and select New Table…, as illustrated in Figure 7.9.
Figure 7.9. Adding a new table with SQL Server Management Studio
The window that appears as the result of the above procedures is shown in Figure 7.10—it looks the same in both Visual Web Developer and SQL Server Management Studio. The main editing window lets you specify the column’s three main properties: Column Name, Data Type, and Allow Nulls. To set additional properties, you need to use the Column Properties pane.
To add the IDENTITY property to a column, locate the Identity Specification row in the Column Properties pane and expand it. This will reveal the (Is Identity) dropdown list, which should be set to Yes for an IDENTITY column, as Figure 7.10 indicates.
To set a column as the primary key, we can select Table Designer > Set Primary Key, or click the little golden key icon in the Table Designer toolbar while the column is selected. When a column is set as a primary key, a little golden key appears next to it, as Figure 7.11 illustrates.
268
Creating the Employees Table
Figure 7.10. Specifying column properties
Figure 7.11. The Employees table
269
Chapter 7: Database Design and Development
Now, let’s create a table called Employees by adding the columns described in Table 7.1.
Table 7.1. The structure of the Employees table
Column |
SQL Data |
Identity |
Allow Nulls |
Primary Key |
Name |
Type |
|
|
|
EmployeeID |
int |
Yes |
No |
Yes |
DepartmentID |
int |
No |
No |
No |
Name |
nvarchar(50) |
No |
No |
No |
Username |
nvarchar(50) |
No |
No |
No |
Password |
nvarchar(50) |
No |
Yes |
No |
Address |
nvarchar(50) |
No |
Yes |
No |
City |
nvarchar(50) |
No |
Yes |
No |
State |
nvarchar(50) |
No |
Yes |
No |
Zip |
nvarchar(50) |
No |
Yes |
No |
HomePhone |
nvarchar(50) |
No |
Yes |
No |
Extension |
nvarchar(50) |
No |
Yes |
No |
MobilePhone |
nvarchar(50) |
No |
Yes |
No |
After entering this information, press Ctrl-S to save the table. When you’re asked to name the table, type Employees and click OK. When you’re done, your table will resemble Figure 7.11.
After you create the table, you’ll see it appear under the Tables node in the Object Explorer (or Database Explorer in Visual Web Developer). SQL Server Management Studio prepends dbo. to the table’s name; dbo is the default “database owner” user. Don’t worry about this for now—we’ll explore the topic of database users in some detail later.
If you close the table designer window, you can open it later by right-clicking the
Employees table and selecting Open Table Definition in Visual Web Developer, or Modify in SQL Server Management Studio. You’ll be taken back to the screen that shows the structure of the table (shown in Figure 7.11).
270