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By default, null values are displayed on your computer screen as nothing, as shown earlier in Listings 4-15 and 4-16. You can change this behavior in SQL Developer at the session level. You can specify how null values appear at the session level by modifying the Display NULL Value AS environment setting, available in the SQL Developer Preferences dialog box, shown in Figure 4-5. Select the Tools Preferences menu option to open this dialog box.
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The Nature of Null Values
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Null values sometimes behave counter-intuitively. Compare the results of the two queries in Listing 439.
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Figure 4-5. The SQL Developer Preferences dialog box Listing 4-39. Comparing Two Complementary Queries select empno, ename, comm from employees where comm > 400; EMPNO -------7521 7654 ENAME COMM -------- -------WARD 500 MARTIN 1400
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select empno, ename, comm from employees where comm <= 400;
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EMPNO ENAME COMM -------- -------- -------7499 ALLEN 300 7844 TURNER 0 The first query in Listing 4-39 returns 2 employees, so you might expect to see the other 12 employees in the result of the second query, because the two WHERE clauses complement each other. However, the two query results actually are not complementary. If Oracle evaluates a condition, there are three possible outcomes: the result can be TRUE, FALSE, or UNKNOWN. In other words, the SQL language is using three-valued logic. Only those rows for which the condition evaluates to TRUE will appear in the result no problem. However, the EMPLOYEES table contains several rows for which both conditions in Listing 4-39 evaluate to UNKNOWN. Therefore, these rows (ten, in this case) will not appear in either result. Just to stress the nonintuitive nature of null values in SQL, you could say the following: In SQL, NOT is not not The explanation of this (case-sensitive) statement is that in three-valued logic, the NOT operator is not the complement operator anymore: NOT TRUE is equivalent with FALSE not TRUE is equivalent with FALSE OR UNKNOWN
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The IS NULL Operator
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Suppose you are looking for all employees except the lucky ones with a commission greater than 400. In that case, the second query in Listing 4-39 does not give you the correct answer, because you would expect to see 12 employees instead of 2. To fix this query, you need the SQL IS NULL operator, as shown in Listing 4-40. Listing 4-40. Using the IS NULL Operator select from where or empno, ename, comm employees comm <= 400 comm is null; ENAME COMM -------- -------SMITH ALLEN 300 JONES BLAKE CLARK SCOTT KING TURNER 0 ADAMS JONES FORD MILLER
EMPNO -------7369 7499 7566 7698 7782 7788 7839 7844 7876 7900 7902 7934
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Note Oracle SQL provides some functions with the specific purpose of handling null values in a flexible way (such as NVL and NVL2). These functions are covered in the next chapter.
The IS NULL operator just like BETWEEN, IN, and LIKE has its own built-in negation option. See Listing 4-41 for an example. Listing 4-41. Using the IS NOT NULL Operator select ename, job, msal, comm from employees where comm is not null; ENAME -------ALLEN WARD MARTIN TURNER JOB MSAL COMM -------- -------- -------SALESREP 1600 300 SALESREP 1250 500 SALESREP 1250 1400 SALESREP 1500 0
Note The IS NULL operator always evaluates to TRUE or FALSE. UNKNOWN is an impossible outcome.
Null Values and the Equality Operator
The IS NULL operator has only one operand: the preceding column name (or column expression). Actually, it is a pity that this operator is not written as IS_NULL (with an underscore instead of a space) to stress the fact that this operator has just a single operand. In contrast, the equality operator (=) has two operands: a left operand and a right one. Watch the rather subtle syntax difference between the following two queries: select * from registrations where evaluation IS null select * from registrations where evaluation = null If you were to read both queries aloud, you might not even hear any difference. However, the seemingly innocent syntax change has definite consequences for the query results. They don t produce error messages, because both queries are syntactically correct. If one (or both) of the operands being compared by the equality comparison operator (=) evaluates to a null value, the result is UNKNOWN. In other words, you cannot say that a null value is equal to a null value. The following shows the conclusions:
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