Oracle Study之---Oracle IN和NOT IN的使用

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NOT IN 与  IN 的区别:
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not In 相当于 <> all,如果 Not In 后面跟的是子查询的话，子查询中只要包含一个 null 的返回值，则会造成
整个 Not in 字句返回空值，结果就是查询不会返回任何结果。

而 in 相当于 =any 的意思,可以有效处理子查询中返回空值的情况,返回正确的结果。
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NOT IN示例：

--该例子想要返回没有下属的职员的姓名，如果子查询中有空值返回的话，则整个查询将没有结果返回

11:20:02 SYS@ test3 >conn scott/tiger  
Connected.  
11:21:18 SCOTT@ test3 >select * from emp;  
     EMPNO ENAME      JOB              MGR HIREDATE                   SAL       COMM     DEPTNO  
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------  
      7369 SMITH      CLERK           7902 1980-12-17 00:00:00        800                    20  
      7499 ALLEN      SALESMAN        7698 1981-02-20 00:00:00       1600        300         30  
      7521 WARD       SALESMAN        7698 1981-02-22 00:00:00       1250        500         30  
      7566 JONES      MANAGER         7839 1981-04-02 00:00:00       2975                    20  
      7654 MARTIN     SALESMAN        7698 1981-09-28 00:00:00       1250       1400         30  
      7698 BLAKE      MANAGER         7839 1981-05-01 00:00:00       2850                    30  
      7782 CLARK      MANAGER         7839 1981-06-09 00:00:00       2450                    10  
      7788 SCOTT      ANALYST         7566 1987-04-19 00:00:00       3000                    20  
      7839 KING       PRESIDENT            1981-11-17 00:00:00       5000                    10  
      7844 TURNER     SALESMAN        7698 1981-09-08 00:00:00       1500          0         30  
      7876 ADAMS      CLERK           7788 1987-05-23 00:00:00       1100                    20  
      7900 JAMES      CLERK           7698 1981-12-03 00:00:00        950                    30  
      7902 FORD       ANALYST         7566 1981-12-03 00:00:00       3000                    20  
      7934 MILLER     CLERK           7782 1982-01-23 00:00:00       1300                    10  
14 rows selected.  
11:20:11 SCOTT@ test3 >select empno from emp  
11:20:21   2            where empno NOT IN (select mgr from emp);  
no rows selected

说明：
Null Values in a Subquery
The SQL statement in the slide attempts to display all the employees who do not have any 
subordinates. Logically, this SQL statement should have returned 12 rows. However, the SQL 
statement does not return any rows. One of the values returned by the inner query is a null value and, 
therefore, the entire query returns no rows

The reason is that all conditions that compare a null value result in a null. So whenever null values 
are likely to be part of the resultsset of a subquery, do not use the NOT INoperator. The NOT IN
operator is equivalent to <> ALL.
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IN的示例：

Notice that the null value as part of the results set of a subquery is not a problem if you use the IN
operator. The IN operator is equivalent to =ANY. For example, to display the employees who have 
subordinates(下属), use the following SQL statement:

11:20:42 SCOTT@ test3 >select empno from emp  
11:21:04   2      where empno in  (select mgr from emp);  
  
     EMPNO  
----------  
      7566  
      7698  
      7782  
      7788  
      7839  
      7902  
6 rows selected.

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Alternatively, a WHERE clause can be included in the subquery to display all employees who do not 
have any subordinates:
--使用 Not In 的话，要注意除掉子查询中将要返回的空值

1. 11:27:01 SCOTT@ test3 >select empno from emp  
   11:27:12   2  where empno NOT IN (select mgr from emp WHERE MGR IS NOT NULL);  
     
        EMPNO  
   ----------  
         7844  
         7521  
         7654  
         7499  
         7934  
         7369  
         7876  
         7900  
   8 rows selected.

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