Using Defaults with NULL in .NET

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Using Defaults with NULL
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Another place that DEFAULT Definitions can be very useful is when adding columns to a table with the NOT NULL definition If you try to add a column to a table with data using the NOT NULL argument, it will fail Think about it If you add a column to a table that has existing data, the new column won t have any data, so it will be blank for each row Typically, you would define a column as NOT NULL to ensure the column is filled in, but if you specify NOT NULL
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2: SQL Server 2005 Database Basics
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(meaning that it can t be blank), you would suddenly have a lot of rows that don t comply with the table definition (the rows that already exist in the table) SQL recognizes this, so the ALTER table definition will fail and give an error The only way to succeed with the task of adding a column with the NOT NULL setting after data is in the table is by adding a DEFAULT definition along with the NOT NULL The default data could be something as simple as Not Defined Yet EXAM TIP A key point here is that DEFAULT definitions are valuable to use when you are adding new columns that won t accept NULL data It is simply not possible to add a column using NOT NULL if the table has existing data, unless you use a DEFAULT definition The following exercise shows how adding a column using NOT NULL will fail without the DEFAULT, and then how the DEFAULT definition will allow it to succeed Exercise 26 Add a Column with and without a Default Definition Watch the benefit of using DEFAULT with NULL in video 2-6
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1 Try to add a column named Country to your address table using the following script:
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USE 2; GO ALTER TABLE Address Add Country VARCHAR(50) NOT NULL;
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2 Execute the script and notice the following error: ALTER TABLE only allows columns to be added that can contain nulls, or have a DEFAULT definition specified, or the column being added is an identity or timestamp column, or alternatively if none of the previous conditions are satisfied, the table must be empty to allow addition of this column Column Country cannot be added to nonempty table Address because it does not satisfy these conditions In other words, we already have a row with data from the previous exercise (232 Turning Court) If we add a Country column, it will be null for this row However, the ALTER TABLE statement specifies that the Country column can t be null Thus, we have a catch-22 situation We ll see later in this exercise how we can use a DEFAULT to avoid this error 3 It is possible to add columns without the NULL argument Change the script to the following script (remove NOT in NOT NULL) and execute it
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USE 2; GO ALTER TABLE Address Add Country VARCHAR(50) NULL;
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MCITP SQL Server 2005 Database Developer All-in-One Exam Guide
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4 Drop the column using the following script We will add the column again using the NOT NULL argument and a DEFAULT definition in the next step:
USE 2; GO Alter TABLE Address Drop Column Country;
5 Create the new column with the DEFAULT using the following script:
USE 2; GO ALTER TABLE Address Add Country VARCHAR(50) NOT NULL CONSTRAINT Country_USA_def DEFAULT 'USA';
6 After executing the script, right-click the Address table, and select Open Table Notice that the Country column has been created and populated with USA for each of the existing rows in the table
Computed Columns
Tables in SQL Server 2005 can also have computed columns A computed column is an expression that defines the value of the column By default, data is not contained within the column, but instead it is a virtual column As an example, a parts table could have an InventoryCost column created as a computed column The expression would be QuantityOnHand * Cost (assuming we have both these columns in the table) Once created, computed columns can be used within T-SQL queries just like a regular column T-SQL will be explored in much more depth in 4, but be aware that the computed columns can be included in SELECT lists, WHERE clauses, ORDER BY clauses, and more Computed columns can t have data inserted or updated via INSERT or UPDATE statements The following script would create the InventoryCost computed column within a table The bolded line shows the computed column
USE 2; GO CREATE TABLE ComputedColumnTest ( QuantityOnHand int NULL, Cost money NULL, InventoryCost AS (QuantityOnHand * Cost) );
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