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CHAPTER 2 WRITING SIMPLE SELECT QUERIES
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Be aware that any word listed immediately after a column within the select list is treated as an alias. If you forget to add the comma between two column names, the second column name will be used as the alias for the first. Omitting the comma like that is a common error. Look carefully at the query in Listing 2-4, and you ll see that the intent is to display the LoginID and JobTitle columns. Because the comma was left out between those two column names, the name of the LoginID column was changed to JobTitle. JobTitle was treated as an alias rather than as an additional column. Watch for and avoid that common mistake. Reading about T-SQL and typing in code examples are wonderful ways to learn. The best way to learn, however, is to figure out the code for yourself. Imagine learning how to swim by reading about it instead of jumping into the water. Practice now with what you have learned so far. Follow the instructions in Exercise 2-1, and write a few queries to test what you know.
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Exercise 2-1
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For this exercise, switch to the AdventureWorksLT2008 database. That database is a very simplified version of AdventureWorks2008, but it is perfect for learning queries that access only one table. You can find the solutions in the appendix. Remember that you can expand the tables in the Object Explorer to see the list of table names and then expand the table to see the list of column names. Now, try your hand at writing the following tasks: 1. 2. 3. 4. Write a SELECT statement that lists the customers along with their ID numbers. Include the last names, first names, and company names. Write a SELECT statement that lists the name, product number, and color of each product. Write a SELECT statement that lists the customer ID numbers and sales order ID numbers from the SalesLT.SalesOrderHeader table. Answer this question: why should you specify column names rather than an asterisk when writing the select-list Give at least two reasons.
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Filtering Data
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Usually an application requires only a fraction of the rows from a table at any given time. For example, an order-entry application that shows the order history will need to display the orders for only one customer at a time. There might be millions of orders in the database, but the operator of the software will view only a handful of rows instead of the entire table. Filtering data is a very important part of T-SQL.
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Adding a WHERE Clause
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To filter the rows returned from a query, you will add a WHERE clause to your SELECT statement. The database engine processes the WHERE clause second, right after the FROM clause. The WHERE clause will contain expressions, called predicates, that can be evaluated to TRUE, FALSE, or UNKNOWN. You will learn
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CHAPTER 2 WRITING SIMPLE SELECT QUERIES
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more about UNKNOWN in the Working with Nothing section later in the chapter. The WHERE clause syntax is as follows: SELECT <column1>,<column2> FROM <schema>.<table> WHERE <column> = <value>; Listing 2-5 shows the syntax and some examples demonstrating how to compare a column to a literal value. The following examples are from the AdventureWorks2008 database unless specified otherwise. Be sure to type each query into the query window and execute the statement to see how it works. Make sure you understand how the expression in the WHERE clause affects the results returned by each query. Notice that tick marks, or single quotes, have been used around literal strings and dates. Listing 2-5. How to Use the WHERE Clause USE AdventureWorks2008; GO --1 SELECT CustomerID, SalesOrderID FROM Sales.SalesOrderHeader WHERE CustomerID = 11000; --2 SELECT CustomerID, SalesOrderID FROM Sales.SalesOrderHeader WHERE SalesOrderID = 43793; --3 SELECT CustomerID, SalesOrderID, OrderDate FROM Sales.SalesOrderHeader WHERE OrderDate = '2001-07-01'; --4 SELECT BusinessEntityID, LoginID, JobTitle FROM HumanResources.Employee WHERE JobTitle = 'Chief Executive Officer'; Each query in Listing 2-5 returns rows that are filtered by the expression in the WHERE clause. Be sure to check the results of each query to make sure that the expected rows are returned (see Figure 2-5). Each query returns only the information specified in that query s WHERE clause.
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