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RETRIEVAL: THE BASICS
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NULL = NULL NULL IS NULL
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UNKNOWN TRUE
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This explains why the query in Listing 4-42 doesn t return all 14 rows of the EMPLOYEES table. Listing 4-42. Example of a Counterintuitive WHERE Clause select ename, init from employees where comm = comm; ENAME -------ALLEN WARD MARTIN TURNER INIT ----JAM TF P JJ
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In mathematical logic, we call expressions always evaluating to TRUE a tautology. The example in Listing 4-42 shows that certain trivial tautologies from two-valued logic (such as COMM = COMM) don t hold true in SQL.
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Null values in SQL often cause trouble. You must be aware of their existence in the database and their odds of being generated by Oracle in (intermediate) results, and you must continuously ask yourself how you want them to be treated in the processing of your SQL statements. Otherwise, the correctness of your queries will be debatable, to say the least. You have already seen that null values in expressions generally cause those expressions to produce a null value. In the next chapter, you will learn how the various SQL functions handle null values. It is obvious that there are many pitfalls in the area of missing information. It may be possible to circumvent at least some of these problems by properly designing your databases. In one of his books, Ted Codd, the inventor of the relational model, even proposed introducing two types of null values: applicable and inapplicable. This would imply the need for a four-valued logic (see Ted Codd, 1990).
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Tip If you are interested in more details about the trouble of null values (or other theoretical information about relational databases and pitfalls in SQL), the books written by Chris Date are the best starting point for further exploration. In particular, his Selected Writings series is brilliant. Chris Date s ability to write in an understandable, entertaining, and fascinating way about these topics far exceeds others in the field.
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RETRIEVAL: THE BASICS
Here s a brain-twister to finish this section about null values: why does the query in Listing 4-43 produce no rows selected There are registrations with evaluation values 4 and 5, for sure... Listing 4-43. A Brain-Twister select * from registrations where evaluation not in (1,2,3,NULL); no rows selected The following WHERE clause: where evaluation not in (1,2,3,NULL) is logically equivalent with the following iterated AND condition: where AND AND AND evaluation evaluation evaluation evaluation <> <> <> <> 1 2 3 NULL
If you consider a row with an EVALUATION value of 1, 2, or 3, it is obvious that out of the first three conditions, one of them returns FALSE, and the other two return TRUE. Therefore, the complete WHERE clause returns FALSE. If the EVALUATION value is NULL, all four conditions return UNKNOWN. Therefore, the end result is also UNKNOWN. So far, there are no surprises. If the EVALUATION value is 4 or 5 (the remaining two allowed values), the first three conditions all return TRUE, but the last condition returns UNKNOWN. So you have the following expression: (TRUE) and (TRUE) and (TRUE) and (UNKNOWN) This is logically equivalent with UNKNOWN, so the complete WHERE clause returns UNKNOWN.
4.10 Truth Tables
Section 4.5 of this chapter showed how to use the AND, OR, and NOT operators to build compound conditions. In that section, we didn t worry too much about missing information and null values, but we are now in a position to examine the combination of three-valued logic and compound conditions. This is often a challenging subject, because three-valued logic is not always intuitive. The most reliable way to investigate compound conditions is to use truth tables. Table 4-3 shows the truth table of the NOT operator. In truth tables, UNK is commonly used as an abbreviation for UNKNOWN.
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