The Language of SQL- P8 pdf

DATABASE DIFFERENCES: MySQL and Oracle Both MySQL and Oracle will return a value in the header row for literal values. In MySQL, the header for the first column in the previous example will appear as: First Name: In Oracle, the header for the first column will appear as: 'FIRSTNAME:' One question you might very well ask is why the header row is important at all. If you are using the SELECT statement to bring back data in a computer program, then you probably don’t care about the header. You only need the data. However, if you are using the SELECT statement to retrieve data for a report displayed to a user, either on paper or on a computer screen, then the header might be relevant. After all, when users look at a column of data, they generally want to know the meaning of the column. In the case of a literal value, there really is no meaning to the column, so a header isn’t truly necessary. But in other types of calculated fields, there may be a meaningful label that could be applied to the column. Later in this chapter, we will discuss the concept of column aliases, which is a way of providing a header in this type of situation. In addition to providing a column header where there is none, column aliases also allow you to change the name of a column to something that may be more meaningful for the person viewing the data. For example, a database designer may have given your last name column the obscure name of LstNm222. A col- umn alias can be employed to change it to something more descriptive. One more point about literals. You might think that all literals need quotation marks, but this is not necessarily true. For example, the following statement: SELECT 5, FirstName FROM Orders will return this data: (no column name) FirstName 5 William 5 Natalie 5 Brenda Literal Values 21 Even though the literal value 5 is completely meaningless, it is still a valid value. Since it doesn’t have quote marks, the 5 is interpreted as a numeric value. Arithmetic Calculations Let’s return to a more typical example of a calculated field. Arithmetic calcula- tions allow you to perform a calculation on one or more columns in a table. For example: SELECT OrderID, QuantityPurchased, PricePerItem, QuantityPurchased * PricePerItem FROM Orders will return this data: OrderID QuantityPurchased PricePerItem (no column name) 1 4 2.50 10.00 2 10 1.25 12.50 3 5 4.00 20.00 The first three columns of the above SELECT are nothing different from what you’ve previously seen. The fourth column is a calculated column with this arithmetic expression: QuantityPurchased * PricePerItem In this case, the asterisk is a symbol that denotes multiplication. It doesn’t mean ‘‘all columns,’’ as was seen in the last chapter. In addition to the asterisk, several other arithmetic operators are allowed. The most common are the following: Arithmetic Operator Meaning þ addition À subtraction à multiplication = division Chapter 3 ■ Calculations and Aliases22 Also note that, as with the literals, the fourth column has no header, due to the fact that it isn’t derived from a single column. Concatenating Fields Concatenation is a fancy computer term that means to combine or join character data together. Just as arithmetic operations can be performed on numeric data, character data can be concatenated together. The syntax for concatenation var- ies, depending on the database you’re working with. Here’s an example from Microsoft SQL Server: SELECT OrderID, FirstName, LastName, FirstName þ ''þ LastName FROM Orders The data retrieved is: OrderID FirstName LastName (no column name) 1 William Smith William Smith 2 Natalie Lopez Natalie Lopez 3 Brenda Harper Brenda Harper Again, the first three columns are nothing new. The fourth column is this expression: FirstName þ ''þ LastName The plus sign denotes concatenation. Since the operation involves character rather than numeric data, SQL is smart enough to know that the plus sign means concatenation and not addition. In this case, the concatenation expressed is composed of three terms: the FirstName column, a literal space (' '), and the LastName column. The literal space is necessary so that William Smith doesn’t display as Willi amSmith. Concatenating Fields 23 DATABASE DIFFERENCES: MySQL and Oracle MySQL doesn’t use a symbol (such as +) to denote concatenation, but it does require you to use a function called CONCAT. We’ll be covering this function in the next chapter, but for now this is a taste of what the same statement looks like in MySQL: SELECT OrderID, FirstName, LastName, CONCAT (FirstName, ' ', LastName) FROM Orders; Oracle uses two vertical bars (||) rather than a plus sign (+) to denote concatenation. The equivalent statement in Oracle is: SELECT OrderID, FirstName, LastName, FirstName || ' ' || LastName FROM Orders; Column Aliases In all the prior examples in this chapter, you have seen calculated fields with a nondescriptive header. We’re now going to address the question as to how a header can be specified for these types of columns. The answer is to use a column alias. The term alias means an alternate name. Here’s an example of how to use a column alias with the Microsoft SQL Server version of the previous SELECT statement: SELECT OrderID, FirstName, LastName, FirstName þ ''þ LastName AS 'Name' FROM Orders Notice that the column alias of ‘Name’ is surrounded by single quotes. The output is: Chapter 3 ■ Calculations and Aliases24 OrderID FirstName LastName Name 1 William Smith William Smith 2 Natalie Lopez Natalie Lopez 3 Brenda Harper Brenda Harper The fourth column now has a header. The keyword AS is used to specify a col- umn alias, which immediately follows the keyword. DATABASE DIFFERENCES: MySQL and Oracle The equivalent of the statement in MySQL is: SELECT OrderID, FirstName, LastName, CONCAT (FirstName, ' ', LastName) AS 'Name' FROM Orders; Oracle does not require single quotes around column alias names. However, if the column alias contains embedded spaces, then double quotes should be used. The same statement in Oracle is: SELECT OrderID, FirstName, LastName, FirstName || ' ' || LastName AS Name FROM Orders; In addition to providing a header for a calculated field, column aliases are often useful if a column in a table has a cryptic name that you’d like to change. For example, if a table has a column with a name of Qty, you could issue this state- ment to display the column as Quantity Purchased: SELECT Qty AS 'Quantity Purchased' FROM table Column Aliases 25 . data, they generally want to know the meaning of the column. In the case of a literal value, there really is no meaning to the column, so a header isn’t truly necessary. But in other types of calculated fields,. types of columns. The answer is to use a column alias. The term alias means an alternate name. Here’s an example of how to use a column alias with the Microsoft SQL Server version of the previous SELECT statement: SELECT OrderID, FirstName, LastName, FirstName. calculated fields, there may be a meaningful label that could be applied to the column. Later in this chapter, we will discuss the concept of column aliases, which is a way of providing a header in this type of