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that evaluate to true are returned as part of the query result. Those that evaluate to unknown or false are not included in the results. For a better understanding of how each row is evaluated, let s take a closer look at the <search condition> placeholder. The search condition is made up of one or more predicates that are used to test the contents returned by the FROM clause. A predicate is an SQL expression that defines a fact about any row returned by the SELECT statement. You have already seen examples of predicates in s 4 and 5. For instance, one example of a view definition (in 5) includes the following SELECT statement:
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SELECT CD_TITLE, COPYRIGHT, IN_STOCK FROM COMPACT_DISC_INVENTORY WHERE COPYRIGHT > 1989 AND COPYRIGHT < 2000;
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This statement is querying three columns in the COMPACT_DISC_INVENTORY table. The SELECT clause specifies the columns to be returned, and the FROM clause specifies the source table. The WHERE clause determines which rows (based on the FROM clause) are included in the results. In this case, the WHERE clause contains two predicates that are connected by the AND keyword. The first predicate (COPYRIGHT > 1989) specifies that all rows included in the query results must contain a value greater than 1989 in the COPYRIGHT column. The second predicate (COPYRIGHT < 2000) specifies that all rows included in the query results must contain a value less than 2000 in the COPYRIGHT column. As rows are evaluated, each predicate is evaluated on an individual basis to determine whether the row meets the condition defined by that predicate. Returning to the last example, the first predicate sets the condition that values must be greater than 1989. If the COPYRIGHT value for a particular row is more than 1989, the condition is met and the predicate evaluates to true. If the value is not greater than 1989, the predicate evaluates to false. If SQL cannot determine whether or not the value meets the condition (as would be the case if the value is null), the predicate evaluates to unknown. Every predicate evaluates to true, false, or unknown. If more than one predicate is included in the WHERE clause, they are joined together by the OR keyword or the AND keyword. If OR is used, then at least one of the predicates on either side of OR must evaluate to true for the row to pass the filter, and therefore appear in the query results. If AND is used, then predicates on either side must evaluate to true for the row to pass the filter. For instance, the WHERE clause in the last example includes two predicates that are connected by the AND keyword. This means that the first predicate must evaluate to true and the second predicate must evaluate to true. If OR had been used instead of AND, then only one of the predicates must evaluate to true, which is a bit nonsensical in this case because all values except null are either above 1989 or below 2000. Ultimately, the WHERE clause as a whole must evaluate to true in order for a row to be included in the query results. If the WHERE clause includes more than one predicate, SQL follows specific guidelines for how the statement as a whole is evaluated. Let s start by looking at the OR keyword. Table 7-1 lists the evaluation of a search condition if the OR keyword is used to separate two predicates. To use the table, match a condition in the left column to a condition in the top row. The result (where a row and column intersect to form a cell) shows how the search condition is evaluated based on how each predicate is evaluated.
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