MySQL多表查詢
目錄mysql
1、準備
建表與數據準備：sql
# 建表
create table department(
id int,
name varchar(20)
);
create table employee(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);
# 插入數據
insert into department values
(200,'技術'),
(201,'人力資源'),
(202,'銷售'),
(203,'運營');
insert into employee(name,sex,age,dep_id) values
('nick','male',18,200),
('jason','female',48,201),
('sean','male',38,201),
('tank','female',28,202),
('oscar','male',18,200),
('mac','female',18,204)
;
# 查看錶結構和數據
mysql> desc department;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
mysql> desc employee;
+--------+-----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+-----------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
| sex | enum('male','female') | NO | | male | |
| age | int(11) | YES | | NULL | |
| dep_id | int(11) | YES | | NULL | |
+--------+-----------------------+------+-----+---------+----------------+
mysql> select * from department;
+------+--------------+
| id | name |
+------+--------------+
| 200 | 技術 |
| 201 | 人力資源 |
| 202 | 銷售 |
| 203 | 運營 |
+------+--------------+
mysql> select * from employee;
+----+------------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+------------+--------+------+--------+
| 1 | nick | male | 18 | 200 |
| 2 | jason | female | 48 | 201 |
| 3 | sean | male | 38 | 201 |
| 4 | tank | female | 28 | 202 |
| 5 | oscar | male | 18 | 200 |
| 6 | mac | female | 18 | 204 |
+----+------------+--------+------+--------+
表department與employee
2、多表鏈接查詢
重點：外連接語法windows
SELECT 字段列表
FROM 表1 INNER|LEFT|RIGHT JOIN 表2
ON 表1.字段 = 表2.字段;
2.1 交叉鏈接
不適用任何匹配條件。生成笛卡爾積post
mysql> select * from employee,department; +----+------------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +----+------------+--------+------+--------+------+--------------+ | 1 | nick | male | 18 | 200 | 200 | 技術 | | 1 | nick | male | 18 | 200 | 201 | 人力資源 | | 1 | nick | male | 18 | 200 | 202 | 銷售 | | 1 | nick | male | 18 | 200 | 203 | 運營 | | 2 | jason | female | 48 | 201 | 200 | 技術 | | 2 | jason | female | 48 | 201 | 201 | 人力資源 | | 2 | jason | female | 48 | 201 | 202 | 銷售 | | 2 | jason | female | 48 | 201 | 203 | 運營 | | 3 | sean | male | 38 | 201 | 200 | 技術 | | 3 | sean | male | 38 | 201 | 201 | 人力資源 | | 3 | sean | male | 38 | 201 | 202 | 銷售 | | 3 | sean | male | 38 | 201 | 203 | 運營 | | 4 | tank | female | 28 | 202 | 200 | 技術 | | 4 | tank | female | 28 | 202 | 201 | 人力資源 | | 4 | tank | female | 28 | 202 | 202 | 銷售 | | 4 | tank | female | 28 | 202 | 203 | 運營 | | 5 | oscar | male | 18 | 200 | 200 | 技術 | | 5 | oscar | male | 18 | 200 | 201 | 人力資源 | | 5 | oscar | male | 18 | 200 | 202 | 銷售 | | 5 | oscar | male | 18 | 200 | 203 | 運營 | | 6 | mac | female | 18 | 204 | 200 | 技術 | | 6 | mac | female | 18 | 204 | 201 | 人力資源 | | 6 | mac | female | 18 | 204 | 202 | 銷售 | | 6 | mac | female | 18 | 204 | 203 | 運營 | +----+------------+--------+------+--------+------+--------------+
2.2 內鏈接
只鏈接匹配的行學習
# 找兩張表共有的部分，至關於利用條件從笛卡爾積結果中篩選出了正確的結果 # department沒有204這個部門，於是employee表中關於204這條員工信息沒有匹配出來 mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id; +----+-----------+------+--------+--------------+ | id | name | age | sex | name | +----+-----------+------+--------+--------------+ | 1 | nick | 18 | male | 技術 | | 2 | jason | 48 | female | 人力資源 | | 3 | sean | 38 | male | 人力資源 | | 4 | tank | 28 | female | 銷售 | | 5 | oscar | 18 | male | 技術 | +----+-----------+------+--------+--------------+ # 上述sql等同於 mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;
2.3 外連接之左鏈接
優先顯示左表所有記錄網站
# 以左表爲準，即找出全部員工信息，固然包括沒有部門的員工 # 本質就是：在內鏈接的基礎上增長左邊有右邊沒有的結果 mysql> select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id; +----+------------+--------------+ | id | name | depart_name | +----+------------+--------------+ | 1 | nick | 技術 | | 5 | oscar | 技術 | | 2 | jason | 人力資源 | | 3 | sean | 人力資源 | | 4 | tank | 銷售 | | 6 | mac | NULL | +----+------------+--------------+
2.4 外連接之右鏈接
優先顯示右表所有記錄編碼
# 以右表爲準，即找出全部部門信息，包括沒有員工的部門 # 本質就是：在內鏈接的基礎上增長右邊有左邊沒有的結果 mysql> select employee.id,employee.name,department.name as depart_name from employee right join department on employee.dep_id=department.id; +------+-----------+--------------+ | id | name | depart_name | +------+-----------+--------------+ | 1 | nick | 技術 | | 2 | jason | 人力資源 | | 3 | sean | 人力資源 | | 4 | tank | 銷售 | | 5 | oscar | 技術 | | NULL | NULL | 運營 | +------+-----------+--------------+
2.5 全外鏈接
顯示左右兩個表所有記錄code
全外鏈接：在內鏈接的基礎上增長左邊有右邊沒有的和右邊有左邊沒有的結果 # 注意：mysql不支持全外鏈接 full JOIN # 強調：mysql可使用此種方式間接實現全外鏈接 select * from employee left join department on employee.dep_id = department.id union select * from employee right join department on employee.dep_id = department.id ; # 查看結果 +------+------------+--------+------+--------+------+--------------+ | id | name | sex | age | dep_id | id | name | +------+------------+--------+------+--------+------+--------------+ | 1 | nick | male | 18 | 200 | 200 | 技術 | | 5 | oscar | male | 18 | 200 | 200 | 技術 | | 2 | jason | female | 48 | 201 | 201 | 人力資源 | | 3 | sean | male | 38 | 201 | 201 | 人力資源 | | 4 | tank | female | 28 | 202 | 202 | 銷售 | | 6 | mac | female | 18 | 204 | NULL | NULL | | NULL | NULL | NULL | NULL | NULL | 203 | 運營 | +------+------------+--------+------+--------+------+--------------+ # 注意 union與union all的區別：union會去掉相同的紀錄
2.6 符合條件鏈接查詢
# 示例1：之內鏈接的方式查詢employee和department表，而且employee表中的age字段值必須大於25,即找出年齡大於25歲的員工以及員工所在的部門
select employee.name,department.name from employee inner join department
on employee.dep_id = department.id
where age > 25;
# 示例2：之內鏈接的方式查詢employee和department表，而且以age字段的升序方式顯示
select employee.id,employee.name,employee.age,department.name from employee,department
where employee.dep_id = department.id
and age > 25
order by age asc;
3、子查詢
- 子查詢是將一個查詢語句嵌套在另外一個查詢語句中。
- 內層查詢語句的查詢結果，能夠爲外層查詢語句提供查詢條件。
- 子查詢中能夠包含. IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等關鍵字
- 還能夠包含比較運算符. = 、 !=、> 、<等
3.1 帶IN關鍵字的子查詢
# 查詢平均年齡在25歲以上的部門名
select id,name from department
where id in
(select dep_id from employee group by dep_id having avg(age) > 25);
# 查看技術部員工姓名
select name from employee
where dep_id in
(select id from department where name='技術');
# 查看不足1人的部門名(子查詢獲得的是有人的部門id)
select name from department where id not in (select distinct dep_id from employee);
3.2 帶比較運算符的子查詢
# 比較運算符：=、!=、>、>=、<、<=、<> # 查詢大於全部人平均年齡的員工名與年齡 mysql> select name,age from emp where age > (select avg(age) from emp); +---------+------+ | name | age | +---------+------+ | jason | 48 | | sean | 38 | +---------+------+ rows in set (0.00 sec) # 查詢大於部門內平均年齡的員工名、年齡 select t1.name,t1.age from emp t1 inner join (select dep_id,avg(age) avg_age from emp group by dep_id) t2 on t1.dep_id = t2.dep_id where t1.age > t2.avg_age;
3.3 帶EXISTS關鍵字的子查詢
EXISTS關字鍵字表示存在。在使用EXISTS關鍵字時，內層查詢語句不返回查詢的記錄。blog
而是返回一個真假值。True或Falseutf-8
當返回True時，外層查詢語句將進行查詢；當返回值爲False時，外層查詢語句不進行查詢。
# department表中存在dept_id=203，Ture
mysql> select * from employee
-> where exists
-> (select id from department where id=200);
+----+------------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+------------+--------+------+--------+
| 1 | nick | male | 18 | 200 |
| 2 | jason | female | 48 | 201 |
| 3 | sean | male | 38 | 201 |
| 4 | tank | female | 28 | 202 |
| 5 | oscar | male | 18 | 200 |
| 6 | mac | female | 18 | 204 |
+----+------------+--------+------+--------+
# department表中存在dept_id=205，False
mysql> select * from employee
-> where exists
-> (select id from department where id=204);
Empty set (0.00 sec)
4、練習：查詢每一個部門最新入職的那位員工
4.1 表與數據準備
company.employee
員工id id int
姓名 emp_name varchar
性別 sex enum
年齡 age int
入職日期 hire_date date
崗位 post varchar
職位描述 post_comment varchar
薪水 salary double
辦公室 office int
部門編號 depart_id int
# 建立表
create table employee(
id int not null unique auto_increment,
name varchar(20) not null,
sex enum('male','female') not null default 'male', # 大部分是男的
age int(3) unsigned not null default 28,
hire_date date not null,
post varchar(50),
post_comment varchar(100),
salary double(15,2),
office int, # 一個部門一個屋子
depart_id int
);
# 查看錶結構
mysql> desc employee;
+--------------+-----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+-----------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | NO | | NULL | |
| sex | enum('male','female') | NO | | male | |
| age | int(3) unsigned | NO | | 28 | |
| hire_date | date | NO | | NULL | |
| post | varchar(50) | YES | | NULL | |
| post_comment | varchar(100) | YES | | NULL | |
| salary | double(15,2) | YES | | NULL | |
| office | int(11) | YES | | NULL | |
| depart_id | int(11) | YES | | NULL | |
+--------------+-----------------------+------+-----+---------+----------------+
# 插入記錄
# 三個部門：教學，銷售，運營
insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values
('nick','male',18,'20170301','老男孩上海虹橋最帥',7300.33,401,1), # 如下是教學部
('jason','male',78,'20150302','teacher',1000000.31,401,1),
('sean','male',81,'20130305','teacher',8300,401,1),
('tank','male',73,'20140701','teacher',3500,401,1),
('oscar','male',28,'20121101','teacher',2100,401,1),
('mac','female',18,'20110211','teacher',9000,401,1),
('rocky','male',18,'19000301','teacher',30000,401,1),
('成龍','male',48,'20101111','teacher',10000,401,1),
('歪歪','female',48,'20150311','sale',3000.13,402,2),# 如下是銷售部門
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),
('張野','male',28,'20160311','operation',10000.13,403,3), # 如下是運營部門
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬銀','female',18,'20130311','operation',19000,403,3),
('程咬銅','male',18,'20150411','operation',18000,403,3),
('程咬鐵','female',18,'20140512','operation',17000,403,3)
;
# ps：若是在windows系統中，插入中文字符，select的結果爲空白，能夠將全部字符編碼統一設置成gbk
4.2 答案一（連表查詢）
SELECT
*
FROM
emp AS t1
INNER JOIN (
SELECT
post,
max(hire_date) max_date
FROM
emp
GROUP BY
post
) AS t2 ON t1.post = t2.post
WHERE
t1.hire_date = t2.max_date;
4.3 答案二（子查詢）
mysql> select (select t2.name from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post; +---------------------------------------------------------------------------------------+ | (select t2.name from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) | +---------------------------------------------------------------------------------------+ | 張野 | | 格格 | | jason | | nick | +---------------------------------------------------------------------------------------+ rows in set (0.00 sec) mysql> select (select t2.id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post; +-------------------------------------------------------------------------------------+ | (select t2.id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) | +-------------------------------------------------------------------------------------+ | 14 | | 13 | | 2 | | 1 | +-------------------------------------------------------------------------------------+ rows in set (0.00 sec) # 正確答案 mysql> select t3.name,t3.post,t3.hire_date from emp as t3 where id in (select (select id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post); +--------+-----------------------------------------+------------+ | name | post | hire_date | +--------+-----------------------------------------+------------+ | nick | 老男孩上海虹橋最帥 | 2017-03-01 | | jason | teacher | 2015-03-02 | | 格格 | sale | 2017-01-27 | | 張野 | operation | 2016-03-11 | +--------+-----------------------------------------+------------+ rows in set (0.00 sec)
答案一爲正確答案，答案二中的limit 1有問題（每一個部門可能有>1個爲同一時間入職的新員工），我只是想用該例子來講明能夠在select後使用子查詢。
能夠基於上述方法解決：好比某網站在全國各個市都有站點，每一個站點一條數據，想取每一個省下最新的那一條市的網站質量信息。
5、綜合練習
5.1 init.sql文件內容
/*
數據導入：
Navicat Premium Data Transfer
Source Server : localhost
Source Server Type : MySQL
Source Server Version : 50624
Source Host : localhost
Source Database : sqlexam
Target Server Type : MySQL
Target Server Version : 50624
File Encoding : utf-8
Date: 10/21/2016 06:46:46 AM
*/
SET NAMES utf8;
SET FOREIGN_KEY_CHECKS = 0;
-- ----------------------------
-- Table structure for `class`
-- ----------------------------
DROP TABLE IF EXISTS `class`;
CREATE TABLE `class` (
`cid` int(11) NOT NULL AUTO_INCREMENT,
`caption` varchar(32) NOT NULL,
PRIMARY KEY (`cid`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of `class`
-- ----------------------------
BEGIN;
INSERT INTO `class` VALUES ('1', '三年二班'), ('2', '三年三班'), ('3', '一年二班'), ('4', '二年九班');
COMMIT;
-- ----------------------------
-- Table structure for `course`
-- ----------------------------
DROP TABLE IF EXISTS `course`;
CREATE TABLE `course` (
`cid` int(11) NOT NULL AUTO_INCREMENT,
`cname` varchar(32) NOT NULL,
`teacher_id` int(11) NOT NULL,
PRIMARY KEY (`cid`),
KEY `fk_course_teacher` (`teacher_id`),
CONSTRAINT `fk_course_teacher` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`tid`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of `course`
-- ----------------------------
BEGIN;
INSERT INTO `course` VALUES ('1', '生物', '1'), ('2', '物理', '2'), ('3', '體育', '3'), ('4', '美術', '2');
COMMIT;
-- ----------------------------
-- Table structure for `score`
-- ----------------------------
DROP TABLE IF EXISTS `score`;
CREATE TABLE `score` (
`sid` int(11) NOT NULL AUTO_INCREMENT,
`student_id` int(11) NOT NULL,
`course_id` int(11) NOT NULL,
`num` int(11) NOT NULL,
PRIMARY KEY (`sid`),
KEY `fk_score_student` (`student_id`),
KEY `fk_score_course` (`course_id`),
CONSTRAINT `fk_score_course` FOREIGN KEY (`course_id`) REFERENCES `course` (`cid`),
CONSTRAINT `fk_score_student` FOREIGN KEY (`student_id`) REFERENCES `student` (`sid`)
) ENGINE=InnoDB AUTO_INCREMENT=53 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of `score`
-- ----------------------------
BEGIN;
INSERT INTO `score` VALUES ('1', '1', '1', '10'), ('2', '1', '2', '9'), ('5', '1', '4', '66'), ('6', '2', '1', '8'), ('8', '2', '3', '68'), ('9', '2', '4', '99'), ('10', '3', '1', '77'), ('11', '3', '2', '66'), ('12', '3', '3', '87'), ('13', '3', '4', '99'), ('14', '4', '1', '79'), ('15', '4', '2', '11'), ('16', '4', '3', '67'), ('17', '4', '4', '100'), ('18', '5', '1', '79'), ('19', '5', '2', '11'), ('20', '5', '3', '67'), ('21', '5', '4', '100'), ('22', '6', '1', '9'), ('23', '6', '2', '100'), ('24', '6', '3', '67'), ('25', '6', '4', '100'), ('26', '7', '1', '9'), ('27', '7', '2', '100'), ('28', '7', '3', '67'), ('29', '7', '4', '88'), ('30', '8', '1', '9'), ('31', '8', '2', '100'), ('32', '8', '3', '67'), ('33', '8', '4', '88'), ('34', '9', '1', '91'), ('35', '9', '2', '88'), ('36', '9', '3', '67'), ('37', '9', '4', '22'), ('38', '10', '1', '90'), ('39', '10', '2', '77'), ('40', '10', '3', '43'), ('41', '10', '4', '87'), ('42', '11', '1', '90'), ('43', '11', '2', '77'), ('44', '11', '3', '43'), ('45', '11', '4', '87'), ('46', '12', '1', '90'), ('47', '12', '2', '77'), ('48', '12', '3', '43'), ('49', '12', '4', '87'), ('52', '13', '3', '87');
COMMIT;
-- ----------------------------
-- Table structure for `student`
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
`sid` int(11) NOT NULL AUTO_INCREMENT,
`gender` char(1) NOT NULL,
`class_id` int(11) NOT NULL,
`sname` varchar(32) NOT NULL,
PRIMARY KEY (`sid`),
KEY `fk_class` (`class_id`),
CONSTRAINT `fk_class` FOREIGN KEY (`class_id`) REFERENCES `class` (`cid`)
) ENGINE=InnoDB AUTO_INCREMENT=17 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of `student`
-- ----------------------------
BEGIN;
INSERT INTO `student` VALUES ('1', '男', '1', '理解'), ('2', '女', '1', '鋼蛋'), ('3', '男', '1', '張三'), ('4', '男', '1', '張一'), ('5', '女', '1', '張二'), ('6', '男', '1', '張四'), ('7', '女', '2', '鐵錘'), ('8', '男', '2', '李三'), ('9', '男', '2', '李一'), ('10', '女', '2', '李二'), ('11', '男', '2', '李四'), ('12', '女', '3', '如花'), ('13', '男', '3', '劉三'), ('14', '男', '3', '劉一'), ('15', '女', '3', '劉二'), ('16', '男', '3', '劉四');
COMMIT;
-- ----------------------------
-- Table structure for `teacher`
-- ----------------------------
DROP TABLE IF EXISTS `teacher`;
CREATE TABLE `teacher` (
`tid` int(11) NOT NULL AUTO_INCREMENT,
`tname` varchar(32) NOT NULL,
PRIMARY KEY (`tid`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of `teacher`
-- ----------------------------
BEGIN;
INSERT INTO `teacher` VALUES ('1', '張磊老師'), ('2', '李平老師'), ('3', '劉海燕老師'), ('4', '朱雲海老師'), ('5', '李傑老師');
COMMIT;
SET FOREIGN_KEY_CHECKS = 1;
5.2 從init.sql文件中導入數據
# 準備表、記錄 mysql> create database db1; mysql> use db1; mysql> source /root/init.sql
5.3 基礎練習
查詢男生、女生的人數；
查詢姓「張」的學生名單；
課程平均分從高到低顯示
查詢有課程成績小於60分的同窗的學號、姓名；
查詢至少有一門課與學號爲1的同窗所學課程相同的同窗的學號和姓名；
查詢出只選修了一門課程的所有學生的學號和姓名；
查詢各科成績最高和最低的分：以以下形式顯示：課程ID，最高分，最低分；
查詢課程編號「2」的成績比課程編號「1」課程低的全部同窗的學號、姓名；
查詢「生物」課程比「物理」課程成績高的全部學生的學號；
查詢平均成績大於60分的同窗的學號和平均成績;
查詢全部同窗的學號、姓名、選課數、總成績；
查詢姓「李」的老師的個數；
查詢沒學過「張磊老師」課的同窗的學號、姓名；
查詢學過「1」而且也學過編號「2」課程的同窗的學號、姓名；
查詢學過「李平老師」所教的全部課的同窗的學號、姓名；
5.4 進階練習
- 查詢沒有學全全部課的同窗的學號、姓名；
- 查詢和「002」號的同窗學習的課程徹底相同的其餘同窗學號和姓名；
- 刪除學習「葉平」老師課的SC表記錄；
- 向SC表中插入一些記錄，這些記錄要求符合如下條件：①沒有上過編號「002」課程的同窗學號；②插入「002」號課程的平均成績；
- 按平均成績從低到高顯示全部學生的「語文」、「數學」、「英語」三門的課程成績，按以下形式顯示： 學生ID,語文,數學,英語,有效課程數,有效平均分；
- 查詢各科成績最高和最低的分：以以下形式顯示：課程ID，最高分，最低分；
- 按各科平均成績從低到高和及格率的百分數從高到低順序；
- 查詢各科成績前三名的記錄:(不考慮成績並列狀況)
- 查詢每門課程被選修的學生數；
- 查詢同名同姓學生名單，並統計同名人數；
- 查詢每門課程的平均成績，結果按平均成績升序排列，平均成績相同時，按課程號降序排列；
- 查詢平均成績大於85的全部學生的學號. 姓名和平均成績；
- 查詢課程名稱爲「數學」，且分數低於60的學生姓名和分數；
- 查詢課程編號爲003且課程成績在80分以上的學生的學號和姓名；
- 求選了課程的學生人數
- 查詢選修「楊豔」老師所授課程的學生中，成績最高的學生姓名及其成績；
- 查詢各個課程及相應的選修人數；
- 查詢不一樣課程但成績相同的學生的學號、課程號、學生成績；
- 查詢每門課程成績最好的前兩名；
- 檢索至少選修兩門課程的學生學號；
- 查詢所有學生都選修的課程的課程號和課程名；
- 查詢沒學過「葉平」老師講授的任一門課程的學生姓名；
- 查詢兩門以上不及格課程的同窗的學號及其平均成績；
- 檢索「004」課程分數小於60，按分數降序排列的同窗學號；
- 刪除「002」同窗的「001」課程的成績；
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