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RETRIEVAL: THE BASICS
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In case there is no employee with the employee number specified, you get the no rows selected message. You might expect an error message like single row subquery returns no rows (actually, this error message once existed in Oracle, many releases ago), but apparently there is no problem. See Listing 4-37 for an example. Listing 4-37. When the Subquery Returns No Rows select ename, init, bdate from employees where bdate > (select bdate from employees where empno = 99999); no rows selected The subquery (returning no rows, or producing an empty set) is treated like a subquery returning one row instead, containing a null value. In other words, SQL treats this situation as if there were an employee 99999 with an unknown date of birth. This may sound strange; however, this behavior is fully compliant with the ANSI/ISO SQL standard.
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When a Single-Row Subquery Returns More Than One Row
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In case the subquery happens to produce multiple rows, the Oracle DBMS reacts with the error message shown in Listing 4-38. Listing 4-38. Error: Single-Row Subquery Returns More Than One Row select ename, init, bdate from employees where bdate > (select bdate from employees where ename = 'JONES'); where bdate > (select bdate * ERROR at line 3: ORA-01427: single-row subquery returns more than one row In this example, the problem is that we have two employees with the same name (Jones). Note that you always risk this outcome, unless you make sure to use an equality comparison against a unique column of the table accessed in the subquery, as in the example in Listing 4-36. So far, we have investigated subqueries only in the WHERE clause of the SELECT statement. Oracle SQL also supports subqueries in other SELECT statement clauses, such as the FROM clause and the SELECT clause. 9 will revisit subqueries.
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4.9 Null Values
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If a column (in a specific row of a table) contains no value, we say that such a column contains a null value. The term null value is actually slightly misleading, because it is an indicator of missing information. Null marker would have been a better term, because a null value is not a value. There can be many different reasons for missing information. Sometimes, an attribute is inapplicable; for example, only sales representatives are eligible for commission. An attribute value can also be unknown; for example, the person entering data did not know certain values when the data was entered. And, sometimes, you don t know whether an attribute is applicable or inapplicable; for example, if you don t know the job of a specific employee, you don t know whether a commission value is applicable. The REGISTRATIONS table provides another good example. A null value in the EVALUATION column can mean several things: the course did not yet take place, the attendee had no opinion, the attendee refused to provide her opinion, the evaluation forms are not yet processed, and so on. It would be nice if you could represent the reason why information is missing, but SQL supports only one null value, and according to Ted Codd s rule 3 (see 1) null values can have only one context-independent meaning.
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Caution Don t confuse null values with the number zero (0), a series of one or more spaces, or even an empty string. Although an empty string ('') is formally different from a null value, Oracle sometimes interprets empty strings as null values (see 6 for some examples). However, you should never rely on this (debatable) interpretation of empty strings. You should always use the reserved word NULL to refer to null values in your SQL commands. Furthermore, the Oracle documentation states that empty strings may no longer be interpreted as NULL at some point in the future.
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