比较具体的问题是:
我在学习sql语言的时候,查询语句的用法有点让人困惑
select 列 from 表 where 条件select x.s#
from sc as x, sc as y
where x.s# = y.s# and x.c# = 'c2' and y.c# = 'c'这里的意思是搜索至少选修课程号为c2和c4的学生号。
为何需要sc X sc 操作?而且 x.s# = y.s#条件是什么意思?为何不可以:
select s#
from sc
where c# = 'c2' and c# = 'c'
---------------------------------------------------检索不学习c2课程的学生姓名和年龄select sname, age
from s
where not exists (select * from sc where sc.s# = s.s# and c# = 'c2')这里为何不可以:
select sname, age
from s
where not exists (select * from sc where c# = 'c2')为何需要, sc.s# = s.s#?
from sc
where c# = 'c2' and c# = 'c'--这是同时选修课程号为c2和c4的学生号
where x.s# = y.s# and x.c# = 'c2' and y.c# = 'c'
E:\整理电子书\数据库 的目录2006-10-16 19:17 <DIR> .
2006-10-16 19:17 <DIR> ..
2006-10-16 19:17 0 a.txt
2006-10-14 20:34 <DIR> oracle
2006-07-20 20:40 1,279,478 Programming Microsoft SQL Server 2000 with XML.chm
2006-09-16 23:48 <DIR> SQL Server 2000 Developers Guide
2006-09-23 09:19 <DIR> sql server 2000实用教程
2006-07-20 20:40 36,991,853 SQL Server 2000编程员指南.pdf
2006-09-16 23:49 <DIR> SQL server 2000自学教程(PDG)
2006-07-20 20:40 4,756,398 sql server精华.chm
2006-07-29 18:07 251,904 sql2000a1-overview.ppt
2006-07-29 18:10 52,736 sql2000a10-performance.ppt
2006-07-29 18:10 107,520 sql2000a11-transferdata.ppt
2006-07-29 18:11 275,968 sql2000a12-replication.ppt
2006-07-29 18:07 218,624 sql2000a2-install.ppt
2006-07-29 18:08 112,128 sql2000a3-(t-sql).ppt
2006-07-29 18:08 61,440 sql2000a4-dml.ppt
2006-07-29 18:08 216,064 sql2000a5-database.ppt
2006-07-29 18:09 310,784 sql2000a6-security.ppt
2006-07-29 18:09 256,512 sql2000a7-backup.ppt
2006-07-29 18:10 113,664 sql2000a8-restore.ppt
2006-07-29 18:10 213,504 sql2000a9-task.ppt
2006-04-19 14:07 90,108,781 SQL编程实用大全精华版.pdf
2006-09-16 23:49 <DIR> 战胜SQL Server必做练习50题
2006-09-16 23:49 <DIR> 数据库理论PPT
2006-09-16 23:49 <DIR> 数据库系统设计(第七版)
2006-07-20 20:40 12,721,565 轻松搞定 sql server 2000 程序设计.pdf
18 个文件 148,048,923 字节
9 个目录 1,445,650,432 可用字节
from sc as x, sc as y
where x.s# = y.s# and x.c# = 'c2' and y.c# = 'c'这里的意思是搜索至少选修课程号为c2和c4的学生号。
为何需要sc X sc 操作?而且 x.s# = y.s#条件是什么意思?
re:
要将相同的两个表进行关联起进行查询为何不可以:
select s#
from sc
where c# = 'c2' and c# = 'c'
这是访问一个表,是同事学以和c的两门课程的学生信息