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CHAPTER 2 WRITING SIMPLE SELECT QUERIES
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Figure 2-15. The results of queries using the IN operator As the WHERE clause becomes more complicated, it becomes very easy to make a mistake. Complete Exercise 2-4 to practice writing with multiple predicates andWHERE clauses with multiple predicates and the IN operator.
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Exercise 2-4
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Use the AdventureWorks2008 database to complete this exercise. Be sure to check your results to assure that they make sense. You can find the solutions in the appendix. 1. Write a query displaying the order ID, order date, and total due from the Sales.SalesOrderHeader table. Retrieve only those rows where the order was placed during the month of September 2001 and the total due exceeded $1,000. Change the query in question 1 so that only the dates September 1 3, 2001, are retrieved. See whether you can figure out three different ways to write this query. Write a query displaying the sales orders where the total due exceeds $1,000. Retrieve only those rows where the salesperson ID is 279 or the territory ID is 6. Change the query in question 3 so that territory 4 is included. Explain when it makes sense to use the IN operator.
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2. 3. 4. 5.
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Probably nothing causes more aggravation to T-SQL developers than NULL values. NULL means that a value has not been entered for a particular column in a row. Suppose you have an e-commerce application that requires the customer to fill in information such as name and address. In this example, the phone number is optional. What does it mean if the customer does not enter a phone number and
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CHAPTER 2 WRITING SIMPLE SELECT QUERIES
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the table ends up with NULL in the PhoneNumber column of the Customer table Does it mean that the customer does not have a phone That s one possibility. Another is that the customer has at least one phone number but chose not to supply it since it was not required. Either way, the end result is that you have no phone number value to work with. Think now about what would happen if you had a list of 1,000,000 phone numbers and tried to figure out whether any of the phone numbers belonged to the customer. Even if you compared each phone number to the customer s row, one by one, you would never know whether any of the phone numbers were the right one. You would never know, because you would be comparing 1,000,000 values to nothing. Conversely, can you guarantee that every one of your 1,000,000 phone numbers is not the missing phone number No, you cannot do that either, since the customer s phone number is unknown. This example should give you an idea about the challenges of working with NULL values. Type in and execute the code in Listing 2-18 to work on some examples using real data. Listing 2-18. An Example Illustrating NULL USE AdventureWorks2008; GO --1) Returns 19,972 rows SELECT MiddleName FROM Person.Person; --2) Returns 291 rows SELECT MiddleName FROM Person.Person WHERE MiddleName = 'B'; --3) Returns 11,182 but 19,681 were expected SELECT MiddleName FROM Person.Person WHERE MiddleName != 'B'; --4) Returns 19,681 SELECT MiddleName FROM Person.Person WHERE MiddleName IS NULL OR MiddleName !='B'; Query 1 with no WHERE clause returns 19,972 rows, the total number of rows in the table. Query 2 returns 291 rows with the middle name B. Logic follows that query 3 will return the difference of the two numbers: 19,681 rows. When you check the results of query 3, you will find that more than 8,000 rows are not accounted for. That is because the rows with NULL values cannot be found by the expression containing not equal. Comparing NULL to B returns UNKNOWN, so the rows are not returned. You must specifically check for NULL values by using the IS NULL operator, as shown in query 4, which returns the correct number of rows.
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