The Language of SQL- P7 potx

Specifying Columns So far, we’ve done nothing more than simply display all the data in a table. But what if you wanted to select only certain columns? Working from the same table, you might want to display only the customer’s last name, for example. The SELECT statement would then look like: SELECT LastName FROM Customers and the resulting data would be: LastName Smith Lopez Harper If you want to select more than one, but not all columns, the SELECT might look like: SELECT FirstName, LastName FROM Customers and the output would appear as: FirstName LastName William Smith Natalie Lopez Brenda Harper The general format of this statement is: SELECT columnli st FROM table The important thing to remember is that if you need to specify more than one column in the columnlist, then those columns must be separated by commas. Chapter 2 ■ Basic Data Retrieval16 Also notice that we placed each column in the columnlist on separate lines. This was done to improve readability. Column Names with Embedded Spaces What if a column contains a space in its name? Let’s say, for example, that the LastName column was specified as Last Name instead (with a space inserted between the two words). Clearly, the following would not work: SELECT Last Name FROM Customers This statement would be considered invalid since Last and Name are not column names. And even if Last and Name were proper column names, they would need to be separated by a comma. The solution is to use a special character around any column name containing spaces. The character to use differs, depending on which database you’re using. For Microsoft SQL Server, the characters to use are square brackets, and the proper syntax is: SELECT [Last Name] FROM Customers One additional syntax note: Just as keywords are not case sensitive, it’s also true that table and column names are not case sensitive. For example, the previous example is identical to: select [last name] from customers For clarity’s sake, I will print all keywords in all caps and capitalize table and column names in this book, but that is not truly necessary. DATABASE DIFFERENCES: MySQL and Oracle For MySQL, the character to use around column names containing spaces is an accent grave (`). The syntax for the above example is: SELECT `Last Name` FROM Customers; Column Names with Embedded Spaces 17 For Oracle, the character to use around column names containing spaces is the double quote. The syntax for the example is: SELECT "Last Name" FROM Customers; Additionally, unlike Microsoft SQL Server and MySQL, column names surrounded by double quotes are case sensitive. That means that the previous statement is not equivalent to: SELECT "LAST NAME" FROM Customers; Looking Ahead In this chapter, we’ve begun our exploration of how to use the SELECT state- ment to retrieve data. We learned about basic syntax and have seen how to select specific columns. In reality, however, this allows us to accomplish very little of a practical nature. Most significantly, we have not yet learned how to apply any type of selection criteria to our data retrieval efforts. For example, while we know how to select all customers, we don’t yet know how to only select customers from the state of New York. As it happens, I won’t be covering selection criteria until Chapter 7. What will we be doing until then? In the next few chapters, we’re going to build on what can be done with the columnlist component of the SELECT statement. In the next chapter, I’ll be moving on to more variations on column selection, allowing us to create complex calculations in a single column. I’ll also be talking about ways to rename columns to make them more descriptive. Similarly, Chapters 4 through 6 will build on your ability to create even more complex and powerful columnlists, so when we finally get to the topic of selection criteria, you will have a full arsenal of techniques available at your disposal. Chapter 2 ■ Basic Data Retrieval18 chapter 3 Calculations and Aliases Keyword Introduced: AS The topics covered in this chapter will allow you to present information in a more convenient and interesting format for anyone viewing your data. The main technique to be discussed is known as calculated fields. This technique will allow you to perform calculations on individual data items that you retrieve from a database. Using this approach, customer names can be formatted exactly as desired. Numeric calculations specific to your business or organization can be made and presented. As a SQL developer, you often need the ability to customize the con- tent of individual columns in order to successfully turn data into more intelligent content. Calculated fields are very useful tools that can help you accomplish that goal. Calculated Fields When selecting data from a table, you are not restricted to the columns that happen to be in the table. The concept of calculated fields allows for a number of other possibilities. With calculated fields, you can do the following: ■ Select specific words or values ■ Perform calculations on single or multiple columns ■ Combine columns and literal values together 19 Let’s look at a few examples, all coming from this Orders table: OrderID FirstName LastName QuantityPurchased PricePerItem 1 William Smith 4 2.50 2 Natalie Lopez 10 1.25 3 Brenda Harper 5 4.00 Literal Values Our first example of a calculated field isn’t really a calculation at all. We’re going to select a specific value as a column, even though the literal value has nothing to do with data in the table. This type of expression is called a literal value. Here’s an example: SELECT 'First Name: ', FirstName FROM Orders This statement will return this data: (no column name) FirstName First Name: William First Name: Natalie First Name: Brenda In this statement, we are selecting two data items. The first is the literal value 'First Name:'. Note that single quote marks are used to indicate that this is a literal with character data. The second data item is the FirstNa me column. Notice two things. First, the literal ‘First Name’ is repeated on every row. Second, there is no header information for the first column. When run in Microsoft SQL Server, the column header displays (‘‘no column name’’). The reason why there is no header is simply because this is a calculated field. There isn’t a column name that can be used for the header. Chapter 3 ■ Calculations and Aliases20 . not restricted to the columns that happen to be in the table. The concept of calculated fields allows for a number of other possibilities. With calculated fields, you can do the following: ■ Select. Oracle, the character to use around column names containing spaces is the double quote. The syntax for the example is: SELECT "Last Name" FROM Customers; Additionally, unlike Microsoft. column. Notice two things. First, the literal ‘First Name’ is repeated on every row. Second, there is no header information for the first column. When run in Microsoft SQL Server, the column header displays