Clauses and Logical Operators for Retrieving Table Data

How to code the WHERE clause

The WHERE clause in a SELECT statement filters the rows in the base table so that only the rows you need are retrieved. In the topics that follow, you’ll learn a variety of ways to code this clause.

How to use the comparison operators

Figure 3-11 shows you how to use the comparison operators in the search condition of a WHERE clause. As you can see in the syntax summary at the top of this figure, you use a comparison operator to compare two expressions. If the result of the comparison is True, the row being tested is included in the query results.

The examples in this figure show how to use some of the comparison operators. The first WHERE clause, for example, uses the equal operator (=) to retrieve only those rows whose vendor_state column have a value of IA. Since the state code is a string literal, it must be enclosed in single quotes.

In contrast, a numeric literal like the one in the second WHERE clause isn’t enclosed in quotes. This clause uses the greater than (>) operator to retrieve only those rows that have a balance due greater than zero.

The third WHERE clause illustrates another way to retrieve all the invoices with a balance due. Like the second clause, it uses the greater than operator. Instead of comparing the balance due to a value of zero, however, it compares the invoice total to the total of the payments and credits that have been applied to the invoice.

The fourth WHERE clause illustrates how you can use comparison operators other than the equal operator with string data. In this example, the less than operator (<) is used to compare the value of the vendor_name column to a literal string that has the letter M in the first position. That will cause the query to return all vendors with names that begin with the letters A through L.

You can also use the comparison operators with date literals, as illustrated by the fifth and sixth WHERE clauses. The fifth clause will retrieve rows with invoice dates on or before May 31, 2008, and the sixth clause will retrieve rows with invoice dates on or after May 1, 2008. Like string literals, date literals must be enclosed in single quotes. In addition, you can use different formats to specify dates, as shown by the two date literals shown in this figure. You’ll learn more about the acceptable date formats and date comparisons in chapter 8.

The last WHERE clause shows how you can test for a not equal condition. To do that, you code a less than sign followed by a greater than sign. In this case, only rows with a credit total that’s not equal to zero will be retrieved.

Whenever possible, you should compare expressions that have similar data types. If you attempt to compare expressions that have different data types, Oracle may implicitly convert the data types for you. Although implicit conversions are often acceptable, they will occasionally yield unexpected results. In chapter 8, you’ll learn how to explicitly convert data types so your comparisons will always yield the results that you want.

The syntax of the WHERE clause with comparison operators

Invoices with a balance due (two variations)

Vendors with names from A to L

Invoices on or before a specified date

Invoices on or after a specified date

Invoices with credits that don’t equal zero

Description

1. You can use a comparison operator to compare any two expressions that result in like data types. Although unlike data types may be converted to data types that can be compared, the comparison may produce unexpected results.
   
2. If the result of a comparison results in a True value, the row being tested is included in the result set. If it’s False or Unknown, the row isn’t included.
   
3. To use a string literal or a date literal in a comparison, enclose it in quotes. To use a numeric literal, enter the number without quotes.
   
4. Character comparisons are case-sensitive. ‘CA’ and ‘Ca’, for example, are not equivalent.

How to use the comparison operators