查询所有张姓同学学号6_MySQL学习-6
练习一：简单查询
1.查询姓“猴”的学生名单
SELECT * FROM student
WHERE `姓名` LIKE '猴%';
2.查询姓名中最后一个字是“猴”的学生名单
SELECT * FROM student
WHERE `姓名` LIKE '%猴';
3.查询姓名中带“猴”的学生名单
SELECT * FROM student
WHERE `姓名` LIKE '%猴%';
4.查询姓“孟”老师的个数
SELECT * FROM teacher
WHERE `教师姓名` LIKE '孟%';
练习二：汇总分析
1.查询课程表编号为“002”的总成绩
SELECT SUM(成绩) FROM score
WHERE `课程号`=0002;
2.查询选了课程的学生人数
SELECT DISTINCT COUNT(DISTINCT `学号`) FROM score;
3.查询每科成绩最高和最低分
SELECT `课程号`,MAX(`成绩`),MIN(`成绩`) FROM score
GROUP BY `课程号`;
4.查询每门课程被选修的学生数
SELECT `课程号`,COUNT(DISTINCT `学号`) FROM score
GROUP BY `课程号`;
5.查询男生、女生人数
SELECT `性别`,COUNT(`姓名`) FROM student
GROUP BY `性别`;
6.查询平均成绩大于60分学生的学号和平均成绩
SELECT `学号`,AVG(成绩) AS "平均成绩" FROM score
GROUP BY `学号`
HAVING AVG(成绩) >60;
7.查询至少选修两门课程的学生学号
SELECT `学号`,COUNT(`课程号`) AS "课程数" FROM score
GROUP BY `学号`
HAVING COUNT(`课程号`) >1;
8.查询同名同姓学生名单并统计同名人数
SELECT COUNT(`姓名`) FROM student
GROUP BY `姓名`
HAVING COUNT(`姓名`) >1;
9.查询不及格的课程并按课程号从大到小排列
SELECT `课程号` FROM score
WHERE `成绩`<60
ORDER BY `课程号` DESC;
10.查询没门课程的平均成绩，结果按平均成绩升序排列，平均成绩相同时，按课程号降序排列
SELECT `课程号`,AVG(`成绩`) FROM score
GROUP BY `课程号`
ORDER BY AVG(`成绩`) ASC,`课程号` DESC;
11.检索课程编号为“0004”且分数小于60的学生学号，结果按分数降序排列
SELECT `学号`,`成绩`FROM score
WHERE `课程号`=0004 AND `成绩`<60
ORDER BY `成绩` DESC;
12.统计每门课程的学生选修人数（超过2人的课程才统计）
要求输出课程号和选修人数，查询结果按人数降序排序，若人数相同，按课程号升序排序
SELECT `课程号`,COUNT(`学号`)FROM score
GROUP BY `课程号`
HAVING COUNT(`学号`)>2
ORDER BY COUNT(`学号`) DESC,`课程号` ASC;
13.查询两门以上不及格课程的同学的学号，以及不及格课程的平均成绩
SELECT `学号`,AVG(`成绩`)FROM score
WHERE `成绩`<60
GROUP BY `学号`
HAVING COUNT(`课程号`)>1;
练习三：复杂查询
1.查询所有课程成绩小于80分学生的学号、姓名
SELECT `学号`,`姓名` FROM student
WHERE `学号` in (SELECT `学号` FROM score WHERE `成绩`<80 GROUP BY `学号` );
2.查询没有学全所有课程的学生的学号，姓名
SELECT `学号`,`姓名` FROM student
WHERE `学号` in
(
SELECT `学号` FROM score
GROUP BY `学号`
HAVING COUNT(`课程号`)<
(
SELECT COUNT(`课程号`) FROM course
)
);
3.查询只选修了两门课程的全部学生的学号和姓名
SELECT `学号`,`姓名` FROM student
WHERE `学号` in
(
SELECT `学号` FROM score
GROUP BY `学号`
HAVING COUNT(`课程号`)=2
);
4.1990年出生的学生名单
SELECT * FROM student
WHERE YEAR(`出生日期`)=1990;
5.按课程号分组取成绩最大值所在行的数据
SELECT * FROM score AS a
WHERE `成绩`=
(
SELECT MAX(`成绩`) FROM score AS b
WHERE b.`课程号` =a.`课程号`
GROUP BY `课程号`
);
6.按课程号分组取成绩最小值所在行的数据
SELECT * FROM score AS a
WHERE `成绩`=
(
SELECT MIN(`成绩`) FROM score AS b
WHERE b.`课程号` =a.`课程号`
GROUP BY `课程号`
);
7.查询每科成绩前两名的记录
(SELECT * FROM score WHERE `课程号`=0001 ORDER BY `成绩` DESC LIMIT 2)
UNION ALL
(SELECT * FROM score WHERE `课程号`=0002 ORDER BY `成绩` DESC LIMIT 2)
UNION ALL
(SELECT * FROM score WHERE `课程号`=0003 ORDER BY `成绩` DESC LIMIT 2);
练习四：多表查询
1.查询所有学生的学号、姓名、选课数、总成绩
SELECT a.`学号`,a.`姓名`,COUNT(b.`课程号`),SUM(b.`成绩`)
FROM student AS a
LEFT JOIN
score as b
ON a.`学号`=b.`学号`
GROUP BY a.`学号`;2.查询平均成绩大于85的所有学生的学号、姓名和平均成绩
SELECT a.`学号`,a.`姓名`,AVG(b.`成绩`)
FROM student AS a
LEFT JOIN
score as b
ON a.`学号`=b.`学号`
GROUP BY a.`学号`
HAVING AVG(b.`成绩`) >85;
3.查询学生的选课情况：学号，姓名，课程号，课程名称
SELECT a.`学号`,a.`姓名`,c.`课程号`,c.`课程名称`
FROM student AS a
LEFT JOIN
score as b
ON a.`学号`=b.`学号`
INNER JOIN course AS c
ON b.`课程号`=c.`课程号`;
4.查询每门课程的及格人数和不及格人数
SELECT `课程号`,
SUM(
CASE WHEN `成绩`>=60 THEN 1
ELSE 0
END) as "及格人数",
SUM(
CASE WHEN `成绩`<60 THEN 1
ELSE 0
END) as "不及格人数"
FROM score
GROUP BY `课程号`;
5.使用分段[100-85],[85-70],[70-60],[<60]来统计各科成绩，分别统计：各分段人数，课程号和课程名称
SELECT a.`课程号`,b.`课程名称`,
SUM(CASE WHEN `成绩`BETWEEN 100 AND 85 THEN 1 ELSE 0 END) as "[100-85]",
SUM(CASE WHEN `成绩` >=70 AND `成绩`<85 THEN 1 ELSE 0 END) as "[85-70]",
SUM(CASE WHEN `成绩` >60 AND `成绩`<70 THEN 1 ELSE 0 END) as "[70-60]",
SUM(CASE WHEN `成绩` <60 THEN 1 ELSE 0 END) as "[<60]"
FROM score AS a LEFT JOIN course AS b ON a.`课程号`=b.`课程号`
GROUP BY a.`课程号`,b.`课程名称`;
6.查询课程编号为0003且课程成绩在80分以上的学生的学号和姓名
SELECT a.`学号`,b.`姓名` FROM score AS a LEFT JOIN student as b ON a.`学号`=b.`学号`
WHERE a.`课程号`=0003 AND a.`成绩`>80;
7.行列转换
select `学号`,
MAX(case `课程号` when 0001 then `成绩` else 0 end ) as 课程号0001,
MAX(case `课程号` when 0002 then `成绩` else 0 end ) as 课程号0002,
MAX(case `课程号` when 0003 then `成绩` else 0 end ) as 课程号0003
FROM score
GROUP BY `学号`
笔记一：提高SQL效率
1.select字句中尽量避免使用*
2.where子句比较符号左侧避免函数，可换到等号右边去
3.尽量避免使用in和not in，用between代替
4.尽量避免使用or，用union代替
5.使用limit子句限制返回的数据行数