NOT USAGE

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Just as in mathematics, you can eliminate parentheses from SQL expressions. The following two queries are logically equivalent:
select * from employees where NOT (ename = 'BLAKE' AND init = 'R')
select * from employees where ename <> 'BLAKE' OR init <> 'R'

In the second version, the NOT operator disappeared, the negation is applied to the two comparison operators, and last, but not least, the AND changes into an OR. You will look at this logical equivalence in more detail in one of the exercises at the end of this chapter.

SQL> select * from tt2;

     LOGID LOGTYPE
---------- --------------------
   6450749 3
   6450749 2
   6450750 3
   6450882 3
   6450882 3
   6450882 3
   6450882 1
   6461331 1
   6461399 1
   6462395 2

10 rows selected.

SQL> select * from tt2
2 where logid<>6450749 or logtype<>2;//OR的意思当一行中有一个不等于6450749或不等2那么就成立，选出结果；

     LOGID LOGTYPE
---------- --------------------
   6450749 3
   6450750 3
   6450882 3
   6450882 3
   6450882 3
   6450882 1
   6461331 1
   6461399 1
   6462395 2

9 rows selected.