sql数据库总结和提升

##mysql数据库

###sql源码

mysql> select * from teachers;
+-----+--------+------+---------------------+-----------+-----------------+
| tno | tname | tsex | tbirthday | prof | depart |
+-----+--------+------+---------------------+-----------+-----------------+
| 804 | 李诚 | 男 | 1958-12-02 00:00:00 | 副教授 | 计算机系 |
| 856 | 张旭 | 男 | 1969-03-12 00:00:00 | 讲师 | 电子工程系 |
| 825 | 王萍 | 女 | 1972-05-05 00:00:00 | 助教 | 计算机系 |
| 831 | 刘冰 | 女 | 1977-08-14 00:00:00 | 助教 | 电子工程系 |
+-----+--------+------+---------------------+-----------+-----------------+
4 rows in set (0.00 sec)

mysql> select * from students;
+-----+--------+------+---------------------+-------+
| sno | sname | ssex | sbirthday | class |
+-----+--------+------+---------------------+-------+
| 108 | 曾华 | 男 | 1977-09-01 00:00:00 | 95033 |
| 105 | 匡明 | 男 | 1975-10-02 00:00:00 | 95031 |
| 107 | 王丽 | 女 | 1976-01-23 00:00:00 | 95033 |
| 101 | 李军 | 男 | 1976-02-20 00:00:00 | 95033 |
| 109 | 王芳 | 女 | 1975-02-10 00:00:00 | 95031 |
| 103 | 陆君 | 男 | 1974-06-03 00:00:00 | 95031 |
+-----+--------+------+---------------------+-------+
6 rows in set (0.00 sec)

mysql> select * from scores;
+-----+-------+--------+
| sno | cno | degree |
+-----+-------+--------+
| 103 | 3-245 | 86.0 |
| 105 | 3-245 | 75.0 |
| 109 | 3-245 | 68.0 |
| 103 | 3-105 | 92.0 |
| 105 | 3-105 | 88.0 |
| 109 | 3-105 | 76.0 |
| 101 | 3-105 | 64.0 |
| 107 | 3-105 | 91.0 |
| 108 | 3-105 | 78.0 |
| 101 | 6-166 | 85.0 |
| 107 | 6-106 | 79.0 |
| 108 | 6-166 | 81.0 |
+-----+-------+--------+
12 rows in set (0.00 sec)

mysql> select * from courses;
+-------+-----------------+-----+
| cno | cname | tno |
+-------+-----------------+-----+
| 3-105 | 计算机导论 | 825 |
| 3-245 | 操作系统 | 804 |
| 6-166 | 数据电路 | 856 |
| 9-888 | 高等数学 | 100 |
+-------+-----------------+-----+
4 rows in set (0.00 sec)

--
-- Table structure for table `courses`
--

DROP TABLE IF EXISTS `courses`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `courses` (
  `cno` varchar(5) NOT NULL,
  `cname` varchar(10) NOT NULL,
  `tno` varchar(10) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
/*!40101 SET character_set_client = @saved_cs_client */;

--
-- Dumping data for table `courses`
--

LOCK TABLES `courses` WRITE;
/*!40000 ALTER TABLE `courses` DISABLE KEYS */;
INSERT INTO `courses` VALUES ('3-105','计算机导论','825'),('3-245','操作系统','804'),('6-166','数据电路','856'),('9-888','高等数学','100');
/*!40000 ALTER TABLE `courses` ENABLE KEYS */;
UNLOCK TABLES;

--
-- Table structure for table `scores`
--

DROP TABLE IF EXISTS `scores`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `scores` (
  `sno` varchar(3) NOT NULL,
  `cno` varchar(5) NOT NULL,
  `degree` decimal(10,1) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
/*!40101 SET character_set_client = @saved_cs_client */;

--
-- Dumping data for table `scores`
--

LOCK TABLES `scores` WRITE;
/*!40000 ALTER TABLE `scores` DISABLE KEYS */;
INSERT INTO `scores` VALUES ('103','3-245',86.0),('105','3-245',75.0),('109','3-245',68.0),('103','3-105',92.0),('105','3-105',88.0),('109','3-105',76.0),('101','3-105',64.0),('107','3-105',91.0),('108','3-105',78.0),('101','6-166',85.0),('107','6-106',79.0),('108','6-166',81.0);
/*!40000 ALTER TABLE `scores` ENABLE KEYS */;
UNLOCK TABLES;

--
-- Table structure for table `students`
--

DROP TABLE IF EXISTS `students`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `students` (
  `sno` varchar(3) NOT NULL,
  `sname` varchar(4) NOT NULL,
  `ssex` varchar(2) NOT NULL,
  `sbirthday` datetime DEFAULT NULL,
  `class` varchar(5) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
/*!40101 SET character_set_client = @saved_cs_client */;

--
-- Dumping data for table `students`
--

LOCK TABLES `students` WRITE;
/*!40000 ALTER TABLE `students` DISABLE KEYS */;
INSERT INTO `students` VALUES ('108','曾华','男','1977-09-01 00:00:00','95033'),('105','匡明','男','1975-10-02 00:00:00','95031'),('107','王丽','女','1976-01-23 00:00:00','95033'),('101','李军','男','1976-02-20 00:00:00','95033'),('109','王芳','女','1975-02-10 00:00:00','95031'),('103','陆君','男','1974-06-03 00:00:00','95031');
/*!40000 ALTER TABLE `students` ENABLE KEYS */;
UNLOCK TABLES;

--
-- Table structure for table `teachers`
--

DROP TABLE IF EXISTS `teachers`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `teachers` (
  `tno` varchar(3) NOT NULL,
  `tname` varchar(4) NOT NULL,
  `tsex` varchar(2) NOT NULL,
  `tbirthday` datetime NOT NULL,
  `prof` varchar(6) DEFAULT NULL,
  `depart` varchar(10) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
/*!40101 SET character_set_client = @saved_cs_client */;

--
-- Dumping data for table `teachers`
--

LOCK TABLES `teachers` WRITE;
/*!40000 ALTER TABLE `teachers` DISABLE KEYS */;
INSERT INTO `teachers` VALUES ('804','李诚','男','1958-12-02 00:00:00','副教授','计算机系'),('856','张旭','男','1969-03-12 00:00:00','讲师','电子工程系'),('825','王萍','女','1972-05-05 00:00:00','助教','计算机系'),('831','刘冰','女','1977-08-14 00:00:00','助教','电子工程系');
/*!40000 ALTER TABLE `teachers` ENABLE KEYS */;
UNLOCK TABLES;
/*!40103 SET TIME_ZONE=@OLD_TIME_ZONE */;

基本语法

####1.数据类型

整形：

Field	Type	Null	Key
x	tinyint(4)	YES
y	smallint(6)	YES
z	mediumint(9)	YES
m	int(11)	YES
n	bigint(20)	YES

浮点形与定点形：

Field	Type	Null	Key	Default	Extra
x	float(5,1)	YES		NULL
y	double(5,1)	YES		NULL
z	decimal(5,1)	YES		NULL

日期与时间类型:

1998-08-08 08:08:08	1998-08-08	时间	10:05:05	2018
datetime	date	timestamp	time	year

字符串类型：

Field	Type	Null	Key	Default	Extra
x	char(4)	YES		NULL
y	varchar(4)	YES		NULL
z	binary(4)	YES		NULL
m	varbinary(4)	YES		NULL
n	blob	YES		NULL
n1	text	YES		NULL
n2	enum('first','second')	YES		NULL
n3	set('a','b','c')	YES		NULL

####2.数据库database的操作开始

新建用户用作远程登录
grant all on . to username@'%' identified by 123;
flush privileges;//刷新权限
mysql -h 172.16.72.195 -u username -p passwd

create database mydata; //创建数据库
drop database mydata; //删除数据库
mysqldump -u root -p phpmyadmin>php.sql//备份数据库
或者如下：
show database;//查看数据库
use databasename;//选择数据库

####3.table的操作

create table tabname(col1 type1 [not null] [primary key],col2 type2 [not null],..)//新建表
create table tab_new like tab_old//旧表新用
show tables;//查看数据库的表
desc tablename;//查看具体的表
drop table tabname;//删除表
alter table courses add n6 varchar(10);//添加列
alter table courses drop n6;//删除列
alter table courses modify n6 int(10);//修改列
alter table students add primary key(sno);//添加主键
alter table students drop primary key;// 删除主键
alter table scores add foreign key(sno) references students(sno);//添加外键（子父的字段类型相同，父字段为主键）
show create table scores;//查看构建的代码
alter table scores drop foreign key scores_ibfk_1;//删除外键
alter table scores drop KEY sno ;//也不知为啥遗留了KEY，删了

####4.index

create index myindex on students(sname,sno);//创建索引
drop index myindex on students;//删除索引
ALTER TABLE用来创建普通索引、UNIQUE索引或PRIMARY KEY索引。//创建
ALTER TABLE table_name ADD INDEX index_name (column_list)
ALTER TABLE table_name ADD UNIQUE (column_list)
ALTER TABLE table_name ADD PRIMARY KEY (column_list)
CREATE INDEX index_name ON table_name (column_list)
CREATE UNIQUE INDEX index_name ON table_name (column_list)
DROP INDEX index_name ON talbe_name//删除
ALTER TABLE table_name DROP INDEX index_name
ALTER TABLE table_name DROP PRIMARY KEY
show index from tblname;//查看
show keys from tblname;

5.视图

create view myview as select * from students;
select * from myview;
alter view myview sa select * from courses;
drop view myview;

6.数据操作

Insert into 表名（x1,x2,x3） values（x1,x2,x3）
Delete from 表名 Where
Update 表名 Set x=xx Where

Where 里面的用词
（比较运算）>、=、<、<=、>=、<>(!=) 
（逻辑+is null）and/or/not/is null
(设范围+in)between and/in
(匹配)like


聚合查询
函数有sum count avg只有在having里面使用
Where是对元组进行过滤，having对分组进行过滤


集合运算
Union except intersect（SQLserver不支持）

存在量词
Exists

基本的sql语句

选择：select * from table1 where 范围
插入：insert into table1(field1,field2) values(value1,value2)
删除：delete from table1 where 范围
更新：update table1 set field1=value1 where 范围
查找：select * from table1 where field1 like ’%value1%’ ---like的语法很精妙，查资料!
排序：select * from table1 order by field1,field2 [desc]
总数：select count as totalcount from table1
求和：select sum(field1) as sumvalue from table1
平均：select avg(field1) as avgvalue from table1
最大：select max(field1) as maxvalue from table1
最小：select min(field1) as minvalue from table1
A： UNION 运算符(合并select ，select的结果和顺序必须相同)

UNION 运算符通过组合其他两个结果表（例如 TABLE1 和 TABLE2），不加all会消去重复。

例子：

select * from myview where sno in (105,103) 
union
select * from myview where sno in (109,101,103);

+-----+--------+------+---------------------+-------+
| sno | sname | ssex | sbirthday | class |
+-----+--------+------+---------------------+-------+
| 103 | 陆君 | 男 | 1974-06-03 00:00:00 | 95031 |
| 105 | 匡明 | 男 | 1975-10-02 00:00:00 | 95031 |
| 109 | 王芳 | 女 | 1975-02-10 00:00:00 | 95031 |
| 101 | 李军 | 男 | 1976-02-20 00:00:00 | 95033 |
+-----+--------+------+---------------------+-------+

select * from myview where sno in (105,103)  
union all  
select * from myview where sno in (109,101,103);

+-----+--------+------+---------------------+-------+
| sno | sname | ssex | sbirthday | class |
+-----+--------+------+---------------------+-------+
| 103 | 陆君 | 男 | 1974-06-03 00:00:00 | 95031 |
| 105 | 匡明 | 男 | 1975-10-02 00:00:00 | 95031 |
| 103 | 陆君 | 男 | 1974-06-03 00:00:00 | 95031 |
| 109 | 王芳 | 女 | 1975-02-10 00:00:00 | 95031 |
| 101 | 李军 | 男 | 1976-02-20 00:00:00 | 95033 |
+-----+--------+------+---------------------+-------+

EXCEPT （相减，select11中不出现select2中有的东西）
INTERSECT 运算符(相交，都有)
注：使用运算词的几个查询结果行必须是一致的。

left （outer） join：
左外连接（左连接）：结果集几包括连接表的匹配行，也包括左连接表的所有行。
mysql> select * from view1;
+--------+-----------------+--------+
| sname | cname | degree |
+--------+-----------------+--------+
| 匡明 | 操作系统 | 75.0 |
| 匡明 | 计算机导论 | 88.0 |
| 曾华 | 计算机导论 | 78.0 |
| 曾华 | 数据电路 | 81.0 |
| 李军 | 计算机导论 | 64.0 |
| 李军 | 数据电路 | 85.0 |
| 王丽 | 计算机导论 | 91.0 |
| 王芳 | 操作系统 | 68.0 |
| 王芳 | 计算机导论 | 76.0 |
| 陆君 | 操作系统 | 86.0 |
| 陆君 | 计算机导论 | 92.0 |
+--------+-----------------+--------+
11 rows in set (0.00 sec)

mysql> select * from view2;
+--------+-------+
| sname | class |
+--------+-------+
| 陆君 | 95031 |
| 陆君 | 95031 |
| 匡明 | 95031 |
| 匡明 | 95031 |
| 王芳 | 95031 |
| 王芳 | 95031 |
| 李军 | 95033 |
| 李军 | 95033 |
| 王丽 | 95033 |
| 曾华 | 95033 |
| 曾华 | 95033 |
+--------+-------+
11 rows in set (0.00 sec)

select view1.* ,view2.class from view1 left join view2 on view1.sname=view2.sname;

+--------+-----------------+--------+-------+
| sname | cname | degree | class |
+--------+-----------------+--------+-------+
| 匡明 | 操作系统 | 75.0 | 95031 |
| 匡明 | 操作系统 | 75.0 | 95031 |
| 匡明 | 计算机导论 | 88.0 | 95031 |
| 匡明 | 计算机导论 | 88.0 | 95031 |
| 曾华 | 计算机导论 | 78.0 | 95033 |
| 曾华 | 计算机导论 | 78.0 | 95033 |
| 曾华 | 数据电路 | 81.0 | 95033 |
| 曾华 | 数据电路 | 81.0 | 95033 |
| 李军 | 计算机导论 | 64.0 | 95033 |
| 李军 | 计算机导论 | 64.0 | 95033 |
| 李军 | 数据电路 | 85.0 | 95033 |
| 李军 | 数据电路 | 85.0 | 95033 |
| 王丽 | 计算机导论 | 91.0 | 95033 |
| 王芳 | 操作系统 | 68.0 | 95031 |
| 王芳 | 操作系统 | 68.0 | 95031 |
| 王芳 | 计算机导论 | 76.0 | 95031 |
| 王芳 | 计算机导论 | 76.0 | 95031 |
| 陆君 | 操作系统 | 86.0 | 95031 |
| 陆君 | 操作系统 | 86.0 | 95031 |
| 陆君 | 计算机导论 | 92.0 | 95031 |
| 陆君 | 计算机导论 | 92.0 | 95031 |
+--------+-----------------+--------+-------+

右外连接(右连接)：结果集既包括连接表的匹配连接行，也包括右连接表的所有行。

select count(*) from view1 right join view2 on view1.sname=view2.sname;

full/cross （outer） join：
全外连接：不仅包括符号连接表的匹配行，还包括两个连接表中的所有记录。

select count(*) from view1 full join view2;

group by having:(分组，然后进行计算，但是分组后返回的值只相当于一个字段组,如果select a,b ...group by xx将会报错。a,b必须为聚合函数avg,sum,count等,或者为xx，也就是能操作一个组的东西。
组相关的信息：（统计信息） count,sum,max,min,avg 分组的标准
例子：平均成绩大于八十的学生名字，学科，分数

select sname ,cname,degree
 from view1 
 where sname in 
    (select sname from view1 group by sname having avg(degree)

sname	cname	degree
匡明	操作系统	75.0
匡明	计算机导论	88.0
王丽	计算机导论	91.0
陆君	操作系统	86.0
陆君	计算机导论	92.0

###自我提升

1、 查询Student表中的所有记录的Sname、Ssex和Class列。

select sname, ssex,class
  from students;

2、 查询教师所有的单位即不重复的Depart列。

select distinct depart 
 from teachers;

3、 查询Student表的所有记录。

select * 
from students;

4、 查询Score表中成绩在60到80之间的所有记录。

select * 
from scores 
where degree between 60 and 80;

5、 查询Score表中成绩为85，86或88的记录。

select * 
from scores 
where degree in (85,86,88);

6、 查询Student表中“95031”班或性别为“女”的同学记录。

select * 
from students 
where class='95031' or ssex='女';

7、 以Class降序查询Student表的所有记录。

select * 
from students 
where class='95031' or ssex='女';

8、 以Cno升序、Degree降序查询Score表的所有记录。

select * 
from scores 
order by cno ,degree desc;

9、 查询“95031”班的学生人数。

select count(*) as studentnum 
from students 
where class='95031';

10、查询Score表中的最高分的学生学号和课程号。

 select sno,cno 
 from scores 
 order by degree desc limit 1;
 or
 select sno,cno 
 from scores 
 where degree = (select max(s.degree) from scores s);

11、查询‘3-105’号课程的平均分。

select avg(degree) 
from scores 
where cno='3-105';

12、查询Score表中至少有5名学生选修的并以3开头的课程的平均分数。

select cno, avg(degree)
 from scores 
 where cno like '3%' 
 group by cno 
 having count(sno)>=5;

13、查询最低分大于70，最高分小于90的Sno列。

 select sno 
 from scores 
 group by sno 
 having min(degree)>70 and max(degree)<90;

14、查询所有学生的Sname、Cno和Degree列。

 select a.sname,c.cno,b.degree 
 from students a,scores b,courses c 
 where a.sno=b.sno and b.cno=c.cno;

17、查询“95033”班所选课程的平均分。

select avg(degree) 
from students a,scores b,courses c 
where a.sno=b.sno and b.cno=c.cno and class='95033' 
group by a.class;

18、假设使用如下命令建立了一个grade表：
create table grade(low int(3),upp int(3),rank char(1));
insert into grade values(90,100,'A');
insert into grade values(80,89,'B');
insert into grade values(70,79,'C');
insert into grade values(60,69,'D');
insert into grade values(0,59,'E');
commit;
现查询所有同学的Sno、Cno和rank列。

select sno,cno,rank 
from scores left join grade on (scores.degree>=grade.low  and scores.degree<=grade.upp) 
order by sno desc ,cno desc ;
or
select sno,cno,rank 
from scores inner join grade on (scores.degree>=grade.low  and scores.degree<=grade.upp) 
order by sno;

19、查询选修“3-105”课程的成绩高于“109”号同学成绩的所有同学的记录。

select a.* 
from scores a left join scores b on(a.cno=b.cno and a.degree>b.degree) 
where a.cno='3-105' and b.sno='109';
or 
 select a.*
  from scores a 
  where a.cno='3-105' and a.degree > (select degree from scores where sno='109' and cno='3-105');

20、查询score中选学一门以上课程的同学中分数为非最高分成绩的记录。

select b.* 
from scores b ,(select cno ,max(degree) de from scores group by cno) a 
where b.sno in (select sno from scores group by sno having count(degree)>1) and b.cno=a.cno and a.de!=b.degree;
or
select b.* 
from scores b left join (select cno ,max(degree) de from scores group by cno) a  on (a.cno=b.cno and a.de!=b.degree) 
where b.sno in (select sno from scores group by sno having count(degree)>1) and  a.cno=b.cno;

+-----+-------+--------+
| sno | cno | degree |
+-----+-------+--------+
| 105 | 3-245 | 75.0 |
| 109 | 3-245 | 68.0 |
| 105 | 3-105 | 88.0 |
| 109 | 3-105 | 76.0 |
| 101 | 3-105 | 64.0 |
| 107 | 3-105 | 91.0 |
| 108 | 3-105 | 78.0 |
| 108 | 6-166 | 81.0 |
+-----+-------+--------+

21、查询成绩高于学号为“109”、课程号为“3-105”的成绩的所有记录。

select a.* 
from scores a left join scores b on (a.cno=b.cno and a.degree>b.degree) 
where a.cno='3-105' and b.sno='109' order by a.sno

22、查询和学号为108的同学同年出生的所有学生的Sno、Sname和Sbirthday列。

select a.sno,a.sname,a.sbirthday 
from students a left join students b on (year(a.sbirthday)=year(b.sbirthday)) 
where a.sno='108';

23、查询“张旭“教师任课的学生成绩。

select sno,degree 
from scores left join courses on(scores.cno=courses.cno) left join teachers on(teachers.tno=courses.tno) 
where teachers.tname='张旭';
or
 select a.sno,a.degree 
 from scores a,teachers b,courses c 
 where a.cno=c.cno and c.tno=b.tno and b.tname='张旭';

24、查询选修某课程的同学人数多于5人的教师姓名。

select teachers.* 
from teachers ,courses 
where courses.cno=(select courses.cno from scores left join courses on(scores.cno=courses.cno) left join teachers on(teachers.tno=courses.tno) 
group by courses.cno 
having count(courses.cno)>5) and courses.tno=teachers.tno;

25、查询95033班和95031班全体学生的记录。

select * 
from students 
where class in ('95033','95031')

26、查询存在有85分以上成绩的课程Cno.

select distinct cno 
from scores 
where degree>85         

27、查询出“计算机系“教师所教课程的成绩表。

 select d.tname,c.cname,a.sname,b.degree 
 from students a,scores b,courses c,teachers d 
 where a.sno=b.sno and b.cno=c.cno and c.tno=d.tno and d.depart='计算机系';
or
select d.tname,c.cname,a.sname,b.degree  
from students a  left join scores b on(a.sno=b.sno) left join courses c on (b.cno=c.cno) left join teachers d  on ( c.tno=d.tno) 
where  d.depart='计算机系';

28、查询“计算机系”与“电子工程系“不同职称的教师的Tname和Prof。

select distinct a.tname,a.prof 
from teachers a 
where depart='计算机系' and depart not in (select distinct b.depart from teachers b where b.depart='电子工程系');

29、查询选修编号为“3-105“课程且成绩至少高于选修编号为“3-245”的同学的Cno、Sno和Degree,并按Degree从高到低次序排序。

select a.cno,a.sno,a.degree 
from scores a left join scores b on (a.sno=b.sno) 
where a.cno='3-105'  and b.cno='3-245' and a.degree>b.degree 
order by a.degree desc;

30、查询选修编号为“3-105”且成绩高于选修编号为“3-245”课程的同学的Cno、Sno和Degree.

 select a.cno,a.sno,a.degree  
 from scores a left join scores b on (a.sno=b.sno) 
 where a.cno='3-105'  and b.cno='3-245' and a.degree>b.degree 
 order by a.degree desc;

31、查询所有教师和同学的name、sex和birthday

 select sname ,ssex,sbirthday 
 from students  
 union
  select tname,tsex,tbirthday 
  from teachers;

32、查询所有“女”教师和“女”同学的name、sex和birthday.

select sname ,ssex,sbirthday 
from students where ssex='女'  
union 
select tname,tsex,tbirthday 
from teachers where tsex='女' ;

33、查询成绩比该课程平均成绩低的同学的成绩表。

select a.* 
from scores a left join (select cno ,avg(degree) varage from scores group by cno) b on(a.cno=b.cno and a.degree > b.varage) ;

34、查询所有任课教师的Tname和Depart.

 select tname,depart 
 from teachers 
 where tno in (select tno from courses );

35 查询所有未讲课的教师的Tname和Depart.

 select tname,depart
  from teachers 
  where tno not in (select tno from courses );

36、查询至少有2名男生的班号。

select class ,count(1) boycount 
from students 
where ssex='男' group by class ;

37、查询Student表中不姓“王”的同学记录。

select * 
from students 
where sname not like '王%'

38、查询Student表中每个学生的姓名和年龄。

 select sname,year(now())-year(sbirthday) age 
 from students;

39、查询Student表中最大和最小的Sbirthday日期值。

select max(sbirthday),min(sbirthday) 
from students

40、以班号和年龄从大到小的顺序查询Student表中的全部记录。

select * from students order by class desc, sbirthday ;

41、查询“男”教师及其所上的课程。

 select tname,cname 
 from teachers,courses 
 where tsex='男' and teachers.tno=courses.tno;

42、查询最高分同学的Sno、Cno和Degree列。(单科成绩最高分)

 select d.* 
 from scores d left  join (select a.cno,max(a.degree) ma from scores a group by a.cno) b on (b.cno=d.cno )
 where b.ma=d.degree;

43、查询和“李军”同性别的所有同学的Sname.

select * 
from students 
where ssex=(select ssex from students where sname='李军');
or 
 select a.* 
 from students a left join (select ssex from students where sname='李军') b on(b.ssex=a.ssex) 
 where a.ssex=b.ssex;

44、查询和“李军”同性别并同班的同学Sname.

select a.* 
from students a left join (select class, ssex from students where sname='李军') b on(b.ssex=a.ssex) 
where a.ssex=b.ssex and b.class=a.class;
or
select * 
from students 
where ssex=(select ssex from students where sname='李军' ) and class=(select class from students where sname='李军' )  ;

45、查询所有选修“计算机导论”课程的“男”同学的成绩表

select a.sname,c.cname,b.degree 
from  students a left  join scores b on (a.sno=b.sno) left  join   courses c on(c.cno=b.cno) 
where c.cname='计算机导论';