一句SQL的理解求组 A表100条数据 字段 NSELECT count(*) from A where A.N=A.N;结果集是200条数据 怎么理解呢? 解决方案 » 免费领取超大流量手机卡,每月29元包185G流量+100分钟通话, 中国电信官方发货 -- where 条件是多余的,返回的其实就是表A的记录行总数!-- 可以看出:你做事情喜欢画蛇添足! --应该只有100条的SQL> select * from a; N---------- 1 2 3 4 5 6 7 . . . 94 95 96 97 98 99 100 100 rows selected SQL> select count(*) from a where a.n=a.n; COUNT(*)---------- 100 SQL> scott@SZTYORA> select * from emp; EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO---------- -------------------- ------------------ ---------- ------------------- ---------- ---------- ---------- 111 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 0087-07-13 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 0087-07-13 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 5201 10 8000 luoyoumou MANAGER 7782 2011-04-02 16:32:43 5000 1000 20已选择15行。scott@SZTYORA> select count(*) from emp e 2 where e.ename=e.ename; COUNT(*)---------- 15scott@SZTYORA> select count(*) from emp e 2 where e.deptno=e.deptno; COUNT(*)---------- 15scott@SZTYORA> SELECT count(*) from A where A.N=A.N;这个会把A.N is null的数据过滤掉,怎么会200条呢,搞错了吧 where A.N=A.N;-- 等同于一个永真条件(即:不管什么情况,永远是真的,我们一般习惯:where 1=1 ) n的值是null,你也永真????? 在oracle9i的表中误删除几行记录,怎么恢复? 大家帮帮忙,有点乱 有没有Oracle for OleDb的独立驱动? 为何我新建的一个数据库打不开啊??? 如何查询数据库中有哪些表?谢谢!!---------------------------------急 更改字符集数据库不能启动!请高手回答,必送分! 请问安装Oracle失败后怎样卸载,重装? 谁能解释一下在oracle中number(4),与在mysql 中int(4),中的4指的是什么? ocijdbc8.dll 那儿可以找到下载。。。?+++在线等等 存储过程中的动态SQL 一个表空间能多个用户来关联吗 请教这个sql怎么写
-- 可以看出:你做事情喜欢画蛇添足!
SQL> select * from a;
N
----------
1
2
3
4
5
6
7
.
.
.
94
95
96
97
98
99
100
100 rows selected
SQL> select count(*) from a where a.n=a.n;
COUNT(*)
----------
100
SQL>
---------- -------------------- ------------------ ---------- ------------------- ---------- ---------- ----------
111 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 0087-07-13 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 0087-07-13 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 5201 10
8000 luoyoumou MANAGER 7782 2011-04-02 16:32:43 5000 1000 20已选择15行。scott@SZTYORA> select count(*) from emp e
2 where e.ename=e.ename; COUNT(*)
----------
15scott@SZTYORA> select count(*) from emp e
2 where e.deptno=e.deptno; COUNT(*)
----------
15scott@SZTYORA>
n的值是null,你也永真?????