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A data table is made up of columns and rows. Columns are also referred to as fields, and rows are sometimes called records. Still, in a relational database, a good deal of hidden logic exists behind a simple list of data rows. Because this chapter covers only the departments list, you ll only need to create one data table: the Department table. This table will store your departments data and is one of the simplest tables you ll work with. With the help of tools such as Visual Studio or Visual Web Developer, it s easy to create a data table in the database if you know what kind of data it will store. When designing a table, you must consider which fields it should contain and which data types should be used for those fields. Besides a field s data type, there are a few more properties to consider; we ll learn about them in the following pages. To determine which fields you need for the Department table, write down a few examples of records that would be stored in that table. Remember from the previous figures that there isn t much information to store about a department just the name and description for each department. The table containing the departments data might look like Figure 4-6.

All too often, developers get exceptions such as NullReferenceException because they didn t make sure that the state in a piece of code was valid. And if the state is not valid, an exception will occur. In fact, in this chapter, one of the previous examples has just such a situation, which you may have noticed. Here is the code that has a dumb little exception possibility:

Figure 4-6. Data from the Department table From a table like this, the names would be extracted to populate the list in the upper-left part of the web page, and the descriptions would be used as headers for the featured products list.

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The way you work with data tables in a relational database is a bit different from the way you usually work on paper. A fundamental requirement in relational databases is that each data row in a table must be uniquely identifiable. This makes sense because you usually save records into a database so that you can retrieve them later; however, you can t do that if each row isn t uniquely identifiable. For example, suppose you add another record to the Department table shown previously in Figure 4-6, making it look like the table shown in Figure 4-7.

void TestCallingExample() { CallingExample cls = null; try { cls = new CallingExample(); clsMethod(); } catch (Exception) { ;} ConsoleWriteLine("Depth is (" + clsGetDepth() + ")"); } The problem is in the bolded line, which assumes that cls will always reference a valid instance of CallingExample This is an assumption that you cannot afford to make If an exception occurred while instantiating CallingExample, cls will still be null, and the catch block will catch the instantiation exception, saving a program crash However, the use of clsGetDepth() right after that throws your protection out the window, as cls is null and will generate a NullReferenceException Here s a better way to write this code: void TestCallingExample() { CallingExample cls = null; try { cls = new CallingExample(); clsMethod(); } catch (Exception) { ;} if (cls != null) { ConsoleWriteLine("Depth is (" + cls.

The Weather worksheet consists of the following: The city and state names in which precipitation totals were collected (columns A and B) The monthly precipitation totals for each city (columns C through N) The yearly precipitation totals for each city (column O) The average monthly precipitation for each city (column P) The average monthly precipitation for each month (row 27)

Figure 4-7. Two departments with the same name Now look at this table, and tell me the description of the Balloons for Children department. Yep, we have a problem! The problem arises because there are two departments with this name. If you queried the table using the Name column and wanted to add new products to the Balloons for Children department, to change the department s name, or to do literally anything, you would get two results! This problem is addressed, in the world of relational database design, using the concept of the primary key, which allows you to uniquely identify a specific row out of many rows. Technically, the primary key is not a column itself. Instead, PRIMARY KEY is a constraint that when applied on a column guarantees that the column will have unique values across the table.

GetDepth() + ")"); } } The bolded code illustrates defensive coding that tests if cls is not null, and if so, then allows the referencing of clsGetDepth() Now the code is exception-safe It does not mean exceptions cannot occur, because calling GetDepth() could still generate an exception internally, but with respect to the TestCallingExample() method, you have made everything as safe as possible, and have assumed GetDepth() is a low exception-risk method Missing from the TestCallingExample() method is an indication of whether the processing worked It is assumed from the perspective of the caller of TestCallingExample() that calling TestCallingExample() will always result in something being done The caller of TestCallingExample() has no way of knowing that something failed, other than if an exception was thrown Code that tells you if something went wrong using an exception is both a blessing and a curse.

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