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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
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 | 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:
| 1 2 3 4 5 6 7 8 9 10 11 12 | 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 2 3 4 5 6 7 8 9 10 11 12 13 14 | 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. |