参考资料来源于Bilibili《一天学会MYSQL数据库》
- 条件查询
- 排序
- 子查询
- 其它(按所选字段查询、查询数目)
1、准备数据
创建四个表:学生表、教师表、课程表和成绩表。
学生表(学号 姓名 性别 出生日期 所在班级)
CREATE TABLE student(s_no VARCHAR(10) PRIMARY KEY,
s_name VARCHAR(20) NOT NULL,
s_sex CHAR(2) NOT NULL,
s_birth DATETIME NOT NULL,
s_class VARCHAR(10) NOT NULL);
教师表(教师编号、教师名字、教师性别、出生日期、职称、所在部门)
CREATE TABLE teacher(t_no VARCHAR(10) PRIMARY KEY,
t_name VARCHAR(20) NOT NULL,
t_sex CHAR(2) NOT NULL,
t_birth DATETIME NOT NULL,
t_prof VARCHAR(20) NOT NULL,
t_depart VARCHAR(20) NOT NULL);
课程表(课程号,课程课程名称,教师编号)
CREATE TABLE course(c_no VARCHAR(10) PRIMARY KEY,
c_name VARCHAR(20) NOT NULL,
t_no VARCHAR(10),
FOREIGN KEY(t_no) REFERENCES teacher(t_no));
成绩表(学号,课程号,成绩)
CREATE TABLE score(s_no VARCHAR(10) ,
c_no VARCHAR(10),
grade DECIMAL,
FOREIGN KEY(s_no) REFERENCES student(s_no),
FOREIGN KEY(c_no) REFERENCES course(c_no),
PRIMARY KEY(s_no,c_no));
向表中添加数据
--学生表数据
INSERT INTO student VALUES('101','曾华','男','1977-09-01','95033');
INSERT INTO student VALUES('102','匡明','男','1975-10-02','95031');
INSERT INTO student VALUES('103','王丽','女','1976-01-23','95033');
INSERT INTO student VALUES('104','李军','男','1976-02-20','95033');
INSERT INTO student VALUES('105','王芳','女','1975-02-10','95031');
INSERT INTO student VALUES('106','陆军','男','1974-06-03','95031');
INSERT INTO student VALUES('107','王尼玛','男','1976-02-20','95033');
INSERT INTO student VALUES('108','张全蛋','男','1975-02-10','95031');
INSERT INTO student VALUES('109','赵铁柱','男','1974-06-03','95031');
--教师表数据
INSERT INTO teacher VALUES('804','李诚','男','1958-12-02','副教授','计算机系');
INSERT INTO teacher VALUES('856','张旭','男','1969-03-12','讲师','电子工程系');
INSERT INTO teacher VALUES('825','王萍','女','1972-05-05','助教','计算机系');
INSERT INTO teacher VALUES('831','刘冰','女','1977-08-14','助教','电子工程系');
--添加课程表
INSERT INTO course VALUES('3-105','计算机导论','825');
INSERT INTO course VALUES('3-245','操作系统','804');
INSERT INTO course VALUES('6-166','数字电路','856');
INSERT INTO course VALUES('9-888','高等数学','831');
--添加成绩表
INSERT INTO score VALUES('103','3-245','86');
INSERT INTO score VALUES('105','3-245','75');
INSERT INTO score VALUES('109','3-245','68');
INSERT INTO score VALUES('103','3-105','92');
INSERT INTO score VALUES('105','3-105','88');
INSERT INTO score VALUES('109','3-105','76');
INSERT INTO score VALUES('103','6-166','85');
INSERT INTO score VALUES('105','6-166','79');
INSERT INTO score VALUES('109','6-166','81');
2、查询练习
a.查询表中所有或者部分字段的数据
SELECT * FROM 表名;(所有字段)
SELECT 字段1,字段2,字段3... FROM 表名
SELECT s_no,s_name,s_sex from student;
--返回结果
+------+--------+-------+
| s_no | s_name | s_sex |
+------+--------+-------+
| 101 | 曾华 | 男 |
| 102 | 匡明 | 男 |
| 103 | 王丽 | 女 |
| 104 | 李军 | 男 |
| 105 | 王芳 | 女 |
| 106 | 陆军 | 男 |
| 107 | 王尼玛 | 男 |
| 108 | 张全蛋 | 男 |
| 109 | 赵铁柱 | 男 |
+------+--------+-------+
b.查询表中不重复的字段
SELECT DISTINCT 字段名 FROM 表名;(使用关键字DISTINCT)
SELECT DISTINCT t_depart FROM teacher;
--返回结果
+------------+
| t_depart |
+------------+
| 计算机系 |
| 电子工程系 |
+------------+
c.查询值在某一范围内的记录
SELECT 字段名 FROM 表名 WHERE 值的条件;
值的条件:
- 逻辑运算符
- BETWEEN AND
- IN 值的取值集合
SELECT * FROM score WHERE grade BETWEEN 60 AND 80;
--between and 包含边界
--result
+------+-------+-------+
| s_no | c_no | grade |
+------+-------+-------+
| 105 | 3-245 | 75 |
| 105 | 6-166 | 79 |
| 109 | 3-105 | 76 |
| 109 | 3-245 | 68 |
+------+-------+-------+
也可以
SELECT * FROM score WHERE grade < 60 AND grade >80;
SELECT * FROM score WHERE grade in (76,79);
d.按所选字段值升序或者降序排列
SELECT 字段名 FROM 表名 ORDER BY 选中的字段 DESC(ASC);
SELECT * FROM student ORDER BY s_class DESC;
--result
+------+--------+-------+---------------------+---------+
| s_no | s_name | s_sex | s_birth | s_class |
+------+--------+-------+---------------------+---------+
| 101 | 曾华 | 男 | 1977-09-01 00:00:00 | 95033 |
| 103 | 王丽 | 女 | 1976-01-23 00:00:00 | 95033 |
| 104 | 李军 | 男 | 1976-02-20 00:00:00 | 95033 |
| 107 | 王尼玛 | 男 | 1976-02-20 00:00:00 | 95033 |
| 102 | 匡明 | 男 | 1975-10-02 00:00:00 | 95031 |
| 105 | 王芳 | 女 | 1975-02-10 00:00:00 | 95031 |
| 106 | 陆军 | 男 | 1974-06-03 00:00:00 | 95031 |
| 108 | 张全蛋 | 男 | 1975-02-10 00:00:00 | 95031 |
| 109 | 赵铁柱 | 男 | 1974-06-03 00:00:00 | 95031 |
+------+--------+-------+---------------------+---------+
以字段1升序,字段2降序
SELECT 字段名 FROM 表名 ORDER BY 字段1 ASC, 字段2 DESC;
SELECT * FROM score ORDER BY c_no ASC, grade DESC;
--result
+------+-------+-------+
| s_no | c_no | grade |
+------+-------+-------+
| 103 | 3-105 | 92 |
| 105 | 3-105 | 88 |
| 109 | 3-105 | 76 |
| 103 | 3-245 | 86 |
| 105 | 3-245 | 75 |
| 109 | 3-245 | 68 |
| 103 | 6-166 | 85 |
| 109 | 6-166 | 81 |
| 105 | 6-166 | 79 |
+------+-------+-------+
e.查询符合某个字段的记录数目
SELECT COUNT(*) FROM 表名 WHRER 条件;
查询班级为”95031”的学生人数;
SELECT COUNT(*) FROM student WHERE s_class='95031';
--result
+----------+
| COUNT(*) |
+----------+
| 5 |
+----------+
f.子查询
查询score表中成绩最高分的学生学号
SELECT s_no FROM score WHERE grade=(SELECT MAX(grade) FROM score);
--result
+------+
| s_no |
+------+
| 103 |
+------+
上述语句select max(grade) from score 可能会出现一个问题,即最大值有两个及以上,为了避免这种现象,我们可以用limit x,y限制查询记录的个数。
如:
SELECT s_no, c-no FROM score ORDER BY grade LIMIT 0,1;
--result
+------+-------+
| s_no | c_no |
+------+-------+
| 109 | 3-245 |
+------+-------+
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