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CHAPTER 8 WORKING WITH THE DATA
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Figure 8-12. LastColl constraint in the Check Constraints dialog box 11. Now it s time to test the constraints to ensure that they work. First of all, we want to check the AmountToCollect constraint. Enter the following code, which will fail as the amount to collect is a negative amount. INSERT INTO CustomerDetails.CustomerProducts (CustomerId,FinancialProductId,AmountToCollect,Frequency, LastCollected,LastCollection,Renewable) VALUES (1,1,-100,0,'24 Aug 2005','24 Aug 2005',0) 12. When you execute the code in Query Editor, you will see the following result. Instantly you can see that the constraint check (CK_CustProds_AmtCheck) has cut in and the record has not been inserted. Msg 547, Level 16, State 0, Line 1 The INSERT statement conflicted with the CHECK constraint "CK_CustProds_AmtCheck". The conflict occurred in database "ApressFinancial", table "CustomerDetails.CustomerProducts", column 'AmountToCollect'. The statement has been terminated. 13. We alter this now to have a positive amount, but change the LastCollection so that we break the CK_CustProd_LastColl constraint. Enter the following code: INSERT INTO CustomerDetails.CustomerProducts (CustomerId,FinancialProductId,AmountToCollect,Frequency, LastCollected,LastCollection) VALUES (1,1,100,0,'24 Aug 2005','23 Aug 2005')
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CHAPTER 8 WORKING WITH THE DATA
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14. When the preceding code is executed, you will see the following error message: Msg 547, Level 16, State 0, Line 1 The INSERT statement conflicted with the CHECK constraint "CK_CustProd_LastColl". The conflict occurred in database "ApressFinancial", table "CustomerDetails.CustomerProducts". The statement has been terminated. Adding a constraint occurs through the ALTER TABLE statement as has just been demonstrated. However, the ADD CONSTRAINT command is quite a flexible command and can achieve a number of different goals. The preceding example uses the ADD CONSTRAINT to insert a primary key, which can be made up of one or more columns (none of which can contain a NULL value), and also to insert a validity check and a set of default values. The only option not covered in the example is the addition of a foreign key, but this is very similar to the addition of a primary key. The first constraint added is the primary key, which we saw in 5. The second constraint definition builds a column check to ensure that the data entered is valid: ADD CONSTRAINT constraint_name CHECK (constraint_check_syntax) The syntax for a CHECK constraint is a simple true or false test. When adding in a constraint for checking the data, the information to be inserted is valid (true) or invalid (false) when the test is applied. As you will see, using mathematical operators to test a column against a single value or a range of values will determine whether the data can be inserted. Notice in the example that the ADD CONSTRAINT command is preceded with a WITH NOCHECK option on the ALTER TABLE statement. This informs SQL Server that any existing data in the table will not be validated when it adds the table alteration with the constraint, and that only data modified or inserted after the addition of the constraint will be checked. If you do wish the existing rows to be checked, then you would use the WITH CHECK option. The advantage of this is that the existing data is validated against that constraint, and if the constraint was added to the table successfully, then you know your data is valid. If any error was generated, then you know that there was erroneous data, and that you need to fix that data before being able to add the constraint. This is just another method of ensuring that your data is valid. Finally, for adding a default value, the ADD CONSTRAINT syntax is very simple. ADD CONSTRAINT constraint_name DEFAULT default_value FOR column_to_receive_the_value The only part of the preceding syntax that requires further explanation is the default_value area. default_value can be a string, a numeric, NULL, or a system function (for example, GETDATE(), which would insert the current date and time). So the default value does not have to be fixed; it can be dynamic.
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