SQL 50 题（MySQL 版，包括建库建表、插入数据等完整过程，适合复习 SQL 知识点）

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SQL 50 题（MySQL 版，包括建库建表、插入数据等完整过程，适合复习 SQL 知识点）

目录

• 1.建库建表
• 2.插入数据
• 3.SQL 50 题
• • 3.1.SQL 01——查询"01"课程比"02"课程成绩高的学生的信及课程分数
  • 3.2.SQL 02——查询"01"课程比"02"课程成绩低的学生的信及课程分数
  • 3.3.SQL 03——查询平均成绩大于等于 60 分的同学的学生编号、学生姓名和平均成绩
  • 3.4.SQL 04——查询平均成绩小于60分的同学的学生编号和学生姓名和平均成绩（包括有成绩的和无成绩的）
  • 3.5.SQL 05——查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩
  • 3.6.SQL 06——查询"李"姓老师的数量
  • 3.7.SQL 07——查询学过"张三"老师授课的同学的信
  • 3.8.SQL 08——查询没学过"张三"老师授课的同学的信
  • 3.9.SQL 09——查询学过编号为"01"并且也学过编号为"02"的课程的同学的信
  • 3.10.SQL 10——查询学过编号为"01"但是没有学过编号为"02"的课程的同学的信
  • 3.11.SQL 11——查询没有学全所有课程的同学的信
  • 3.12.SQL 12——查询至少有一门课与学号为"01"的同学所学相同的同学的信
  • 3.13.SQL 13——查询和"01"号的同学学习的课程完全相同的其他同学的信
  • 3.14.SQL 14——查询没学过"张三"老师讲授的任一门课程的学生姓名
  • 3.15.SQL 15——查询两门及其以上不及格课程的同学的学号，姓名及其平均成绩
  • 3.16.SQL 16——检索"01"课程分数小于60，按分数降序排列的学生信
  • 3.17.SQL 17——按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩
  • 3.18.SQL 18——查询各科成绩最高分、最低分和平均分：以如下形式显示：课程ID，课程名称，最高分，最低分，平均分，及格率，中等率，优良率，优秀率。及格为 >= 60，中等为 70-80，优良为 80-90，优秀为 >= 90。
  • 3.19.SQL 19——按各科成绩进行排序，并显示排名，成绩重复时合并名次
  • 3.20.SQL 20——查询学生的总成绩并进行排名，总分重复时不保留名次空缺
  • 3.21.SQL 21——查询不同老师所教不同课程平均分，并从高到低显示
  • 3.22.SQL 22——查询所有课程的成绩第 2 名到第 3 名的学生信及该课程成绩
  • 3.23.SQL 23——统计各科成绩各分数段人数：课程编号，课程名称，[100-85]，[85-70]，[70-60]，[0-60]及所占百分比
  • 3.24.SQL 24——查询学生平均成绩及其名次
  • 3.25.SQL 25——查询各科成绩前三名的记录
  • 3.26.SQL 26——查询每门课程被选修的学生数
  • 3.27.SQL 27——查询出只有两门课程的全部学生的学号和姓名
  • 3.28.SQL 28——查询男生、女生人数
  • 3.29.SQL 29——查询名字中含有"风"字的学生信
  • 3.30.SQL 30——查询同姓名同性别学生名单，并统计同名人数
  • 3.31.SQL 31——查询 1990 年出生的学生名单
  • 3.32.SQL 32——查询每门课程的平均成绩，结果按平均成绩降序排列，平均成绩相同时，按课程编号升序排列
  • 3.33.SQL 33——查询平均成绩大于等于 85 的所有学生的学号、姓名和平均成绩
  • 3.34.SQL 34——查询课程名称为"数学"，且分数低于 60 的学生姓名和分数
  • 3.35.SQL 35——查询所有学生的课程及分数情况
  • 3.36.SQL 36——查询任何一门课程成绩在 70 分以上的姓名、课程名称和分数
  • 3.37.SQL 37——查询不及格的课程的学生姓名、课程名称和分数
  • 3.38.SQL 38——查询课程编号为 01 且课程成绩在 80 分以上的学生的学号和姓名
  • 3.39.SQL 39——求每门课程的学生人数
  • 3.40.SQL 40——查询选修"张三"老师所授课程的学生中，成绩最高的学生信及其成绩
  • 3.41.SQL 41——查询不同课程成绩相同的学生的学生编号、课程编号、学生成绩
  • 3.42.SQL 42——查询每门课程成绩最好的前两名
  • 3.43.SQL 43——统计每门课程的学生选修人数（超过 5 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列，若人数相同，按课程号升序排列
  • 3.44.SQL 44——查询至少选修两门课程的学生学号
  • 3.45.SQL 45——查询选修了全部课程的学生信
  • 3.46.SQL 46——查询各学生的年龄
  • 3.47.SQL 47——查询本周过生日的学生
  • 3.48.SQL 48——查询下周过生日的学生
  • 3.49.SQL 49——查询本月过生日的学生
  • 3.50.SQL 50——查询下月过生日的学生

① 本文整理了经典的 50 道 SQL 题目，文本分为建库建表、插入数据以及 SQL 50 题这三个部分。 ② 本文所使用的 MySQL 版本为 5.5，虽然版本有一点旧，但是对 SQL 知识点的复习没有太大的影响（除了一些旧版没有的函数）。 ③ 由于本文旨在对 SQL 基础知识进行复习，并且所涉及的数据量也十分的小，所以在编写 SQL 语句时，并未过多考虑 SQL 优化的方面。如果读者有其它的解法或者发现错误之处，可在评论区留言，笔者在看到后会及时更新！

1.建库建表

（1）建库：创建一个名为 sqlpractice 的数据库。 （2）建表：建立 student、course、teacher 和 score 这 4 张表。它们的字段以及之间的关系如下图所示。 （3）建库建表的完整 SQL 语句如下所示。

# 建库 create database sqlpractice; use sqlpractice; # 建立 Student 学生表 CREATE TABLE Student( s_id VARCHAR(20), s_name VARCHAR(20) NOT NULL, s_birth VARCHAR(20) NOT NULL, s_sex VARCHAR(10) NOT NULL, PRIMARY KEY(s_id) # 主键 ); # 建立 Course 课程表 CREATE TABLE Course( c_id VARCHAR(20), c_name VARCHAR(20) NOT NULL, t_id VARCHAR(20) NOT NULL, PRIMARY KEY(c_id) # 主键 ); # 建立 Teacher 教师表 CREATE TABLE Teacher( t_id VARCHAR(20), t_name VARCHAR(20) NOT NULL DEFAULT '', PRIMARY KEY(t_id) # 主键 ); # 建立 Score 分数表 CREATE TABLE Score( s_id VARCHAR(20), c_id VARCHAR(20), s_score INT(3), PRIMARY KEY(s_id, c_id) # 联合主键 ); # 添加外键 # 语法：ALTER TABLE 从表 ADD FOREIGN KEY(外键字段) REFERENCES 主表(主键字段) ALTER TABLE Course ADD FOREIGN KEY(t_id) REFERENCES Teacher(t_id) ALTER TABLE Score ADD FOREIGN KEY(s_id) REFERENCES Student(s_id) ALTER TABLE Score ADD FOREIGN KEY(c_id) REFERENCES Course(c_id) 2.插入数据

（1）向上面创建的 4 张表中插入测试数据的 SQL 语句如下所示（需要注意表之间的关系，以免插入数据失败）。

# 分别向四张表中插入数据 INSERT INTO Student VALUES('01', '赵雷', '1990-01-01', '男'); INSERT INTO Student VALUES('02', '钱电', '1990-12-21', '男'); INSERT INTO Student VALUES('03', '孙风', '1990-05-20', '男'); INSERT INTO Student VALUES('04', '李云', '1990-08-06', '男'); INSERT INTO Student VALUES('05', '周梅', '1991-12-01', '女'); INSERT INTO Student VALUES('06', '吴兰', '1992-03-01', '女'); INSERT INTO Student VALUES('07', '郑竹', '1989-07-01', '女'); INSERT INTO Student VALUES('08', '王菊', '1990-01-20', '女'); INSERT INTO Course VALUES('01', '语文', '02'); INSERT INTO Course VALUES('02', '数学', '01'); INSERT INTO Course VALUES('03', '英语', '03'); INSERT INTO Teacher VALUES('01', '张三'); INSERT INTO Teacher VALUES('02', '李四'); INSERT INTO Teacher VALUES('03', '王五'); INSERT INTO Score VALUES('01', '01', 80); INSERT INTO Score VALUES('01', '02', 90); INSERT INTO Score VALUES('01', '03', 99); INSERT INTO Score VALUES('02', '01', 70); INSERT INTO Score VALUES('02', '02', 60); INSERT INTO Score VALUES('02', '03', 80); INSERT INTO Score VALUES('03', '01', 80); INSERT INTO Score VALUES('03', '02', 80); INSERT INTO Score VALUES('03', '03', 80); INSERT INTO Score VALUES('04', '01', 50); INSERT INTO Score VALUES('04', '02', 30); INSERT INTO Score VALUES('04', '03', 20); INSERT INTO Score VALUES('05', '01', 76); INSERT INTO Score VALUES('05', '02', 87); INSERT INTO Score VALUES('06', '01', 31); INSERT INTO Score VALUES('06', '03', 34); INSERT INTO Score VALUES('07', '02', 89); INSERT INTO Score VALUES('07', '03', 98);

（2）检验插入数据是否成功

SELECT * FROM Student; SELECT * FROM Course; SELECT * FROM Teacher; SELECT * FROM Score;

Student 表

Course 表

Teacher 表

Score 表

3.SQL 50 题 3.1.SQL 01——查询"01"课程比"02"课程成绩高的学生的信及课程分数 # 本题需要比较"01"课程比"02"课程的成绩，故在 where 中将 score 表中的字段 s_score 使用 2 次（即分别对应"01"课程的成绩和"02"课程的成绩） # 所以可以使用为 s_score 表取别名的方式来多次使用 score 表中的字段 SELECT student.*, score1.s_score FROM student, score AS score1, score AS score2 WHERE student.s_id = score1.s_id AND score1.s_id = score2.s_id # student, score1, score2 表连接的条件是它们的 s_id 均相等 AND score1.c_id = '01' AND score2.c_id = '02' AND score1.s_score > score2.s_score;

3.2.SQL 02——查询"01"课程比"02"课程成绩低的学生的信及课程分数 SELECT student.*, score1.s_score FROM student, score AS score1, score AS score2 WHERE student.s_id = score1.s_id AND score1.s_id = score2.s_id # student, score1, score2 表连接的条件是它们的 s_id 均相等 AND score1.c_id = '01' AND score2.c_id = '02' AND score1.s_score < score2.s_score;

3.3.SQL 03——查询平均成绩大于等于 60 分的同学的学生编号、学生姓名和平均成绩 # 1.创建临时表 ss EXPLAIN SELECT student.s_id, student.s_name, ss.avg_score FROM student, (SELECT s_id, AVG( s_score ) AS avg_score FROM score GROUP BY s_id) AS ss WHERE student.s_id = ss.s_id AND ss.avg_score >= 60; # 2.先进行内连接，然后再分组 SELECT student.s_id, s_name, AVG(score.s_score) avg_score FROM student INNER JOIN score ON student.s_id = score.s_id GROUP BY student.s_id, s_name HAVING AVG(score.s_score) >= 60

3.4.SQL 04——查询平均成绩小于60分的同学的学生编号和学生姓名和平均成绩（包括有成绩的和无成绩的） # isnull(exper) 判断 exper 是否为空，是则返回 1，否则返回 0 # ifnull(exper1, exper2) 判断 exper1 是否为空，是则用 exper2 代替 # nullif(exper1, exper2) 如果 expr1 = expr2 成立，那么返回值为 NULL，否则返回值为 expr1。 SELECT student.s_id, s_name, AVG(score.s_score) avg_score FROM student LEFT OUTER JOIN score ON student.s_id = score.s_id GROUP BY student.s_id, s_name HAVING AVG(IFNULL(score.s_score,0)) < 60

3.5.SQL 05——查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩 SELECT student.s_id, student.s_name, COUNT(DISTINCT c_id) AS totalCourses, SUM(s_score) AS totalScores FROM student # 由于要查询所有的学生，故无论其是否有课程信都要查询，所以使用 LEFT OUTER JOIN LEFT OUTER JOIN score ON student.s_id = score.s_id GROUP BY student.s_id, student.s_name;

3.6.SQL 06——查询"李"姓老师的数量 SELECT COUNT(*) FROM teacher WHERE t_name LIKE '李%'

3.7.SQL 07——查询学过"张三"老师授课的同学的信 # 1.使用多表连接(score, course, teacher)找到上张三老师课的同学的 s_id，然后再根据 s_id 从 student 表中查询同学信 SELECT student.* FROM student WHERE s_id IN ( SELECT s_id FROM score, course, teacher WHERE teacher.t_name = '张三' AND teacher.t_id = course.t_id AND course.c_id = score.c_id ); # 2.多层嵌套子查询（当数据量较大时，一般不推荐使用子查询） # 在 student 表中根据上过张三老师教的课的学生 s_id 来查询他们的信 SELECT student.* FROM student WHERE student.s_id = ANY ( # 在 score 表中根据张三老师教的课程 c_id 来查找上这些课的学生 s_id SELECT DISTINCT s_id FROM score WHERE score.c_id = ( # 在 course 表中根据张三老师的 t_id 查询他所教的课程 c_id SELECT c_id FROM course WHERE course.t_id = ( # 在 teacher 表中查询张三老师的 t_id SELECT t_id FROM teacher WHERE t_name = '张三' ) ) )

3.8.SQL 08——查询没学过"张三"老师授课的同学的信 SELECT student.* FROM student WHERE student.s_id != ALL ( SELECT DISTINCT s_id FROM score WHERE score.c_id = ( SELECT c_id FROM course WHERE course.t_id = ( SELECT t_id FROM teacher WHERE t_name = '张三' ) ) )

3.9.SQL 09——查询学过编号为"01"并且也学过编号为"02"的课程的同学的信 SELECT student.* FROM student WHERE student.s_id = ANY ( SELECT s1.s_id FROM score AS s1, score AS s2 WHERE s1.s_id = s2.s_id AND s1.c_id = '01' AND s2.c_id = '02' )

3.10.SQL 10——查询学过编号为"01"但是没有学过编号为"02"的课程的同学的信 SELECT student.* FROM student WHERE student.s_id NOT IN ( SELECT s1.s_id FROM score AS s1, score AS s2 WHERE s1.s_id = s2.s_id AND s1.c_id = '01' AND s2.c_id = '02' )

3.11.SQL 11——查询没有学全所有课程的同学的信 # 下面的课程数量 3 也可以用 (SELECT count(*) FROM course) 来代替 SELECT * FROM student WHERE s_id IN (SELECT s_id FROM score GROUP BY s_id HAVING count(c_id) < 3)

3.12.SQL 12——查询至少有一门课与学号为"01"的同学所学相同的同学的信 # 不包括学号为 '01' 学生自己 SELECT * FROM student WHERE s_id IN (SELECT DISTINCT s_id FROM score WHERE c_id IN (SELECT c_id FROM score WHERE s_id = '01') AND s_id != '01')

3.13.SQL 13——查询和"01"号的同学学习的课程完全相同的其他同学的信 SELECT * FROM student WHERE s_id IN ( SELECT s_id FROM score WHERE c_id IN (SELECT DISTINCT c_id FROM score WHERE s_id = '01') AND s_id != '01' GROUP BY s_id HAVING # 下面的课程数量 3 也可以用 (SELECT count(*) FROM course) 来代替 count( c_id ) >= 3 )

3.14.SQL 14——查询没学过"张三"老师讲授的任一门课程的学生姓名 SELECT s_name FROM student WHERE s_id NOT IN ( SELECT s_id FROM score WHERE c_id IN ( # 查询由姓名为张三的老师所讲授的课程 id SELECT c_id FROM course WHERE # 姓名为张三的老师可能有多个，所以此处不能直接使用 = t_id IN (SELECT t_id FROM teacher WHERE t_name = '张三') ) )

3.15.SQL 15——查询两门及其以上不及格课程的同学的学号，姓名及其平均成绩 SELECT stu.s_id, stu.s_name, tmp_t.avg_score FROM student AS stu RIGHT JOIN ( SELECT s_id, avg(s_score) AS avg_score FROM score WHERE s_score < 60 GROUP BY s_id HAVING count(s_score) >= 2 ) AS tmp_t ON stu.s_id = tmp_t.s_id

3.16.SQL 16——检索"01"课程分数小于60，按分数降序排列的学生信 SELECT stu.* FROM student AS stu INNER JOIN score ON stu.s_id = score.s_id WHERE c_id = '01' AND s_score < 60 ORDER BY s_score DESC

3.17.SQL 17——按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩 SELECT s_id, max(CASE c_id WHEN '01' THEN s_score ELSE 0 END) AS '01', max(CASE c_id WHEN '02' THEN s_score ELSE 0 END) AS '02', max(CASE c_id WHEN '03' THEN s_score ELSE 0 END) AS '03', avg(s_score) AS avg_score FROM score GROUP BY s_id ORDER BY avg_score DESC

3.18.SQL 18——查询各科成绩最高分、最低分和平均分：以如下形式显示：课程ID，课程名称，最高分，最低分，平均分，及格率，中等率，优良率，优秀率。及格为 >= 60，中等为 70-80，优良为 80-90，优秀为 >= 90。 SELECT sc.c_id AS "课程ID", c.c_name AS '课程名称', MAX( sc.s_score ) AS "最高分", MIN( sc.s_score ) AS '最低分', AVG( sc.s_score ) AS '平均分', SUM(IF (sc.s_score BETWEEN 60 AND 70, 1, 0)) / COUNT(*) as '及格率', SUM(IF (sc.s_score BETWEEN 70 AND 80, 1, 0)) / COUNT(*) as '中等率', SUM(IF (sc.s_score BETWEEN 80 AND 90, 1, 0)) / COUNT(*) as '优良率', SUM(IF (sc.s_score >= 90, 1, 0)) / COUNT(*) as '优秀率' FROM score AS sc JOIN course AS c ON sc.c_id = c.c_id GROUP BY sc.c_id

3.19.SQL 19——按各科成绩进行排序，并显示排名，成绩重复时合并名次 SELECT sc1.c_id, sc1.s_id, sc1.s_score, count(sc2.s_score) + 1 AS rank FROM score AS sc1 LEFT JOIN score AS sc2 ON sc1.s_score < sc2.s_score AND sc1.c_id = sc2.c_id GROUP BY sc1.c_id, sc1.s_id, sc1.s_score ORDER BY sc1.c_id, rank

3.20.SQL 20——查询学生的总成绩并进行排名，总分重复时不保留名次空缺 SELECT student.s_id, s_name, sum(s_score) AS '总成绩' FROM student LEFT JOIN score ON student.s_id = score.s_id GROUP BY student.s_id, s_name ORDER BY sum(s_score) DESC

3.21.SQL 21——查询不同老师所教不同课程平均分，并从高到低显示 SELECT teacher.t_id, t_name, avg(s_score * 1.0) AS avg_score FROM teacher, course, score WHERE teacher.t_id = course.t_id AND course.c_id = score.c_id GROUP BY teacher.t_id, t_name, score.c_id ORDER BY avg(s_score * 1.0) DESC

3.22.SQL 22——查询所有课程的成绩第 2 名到第 3 名的学生信及该课程成绩 # 1.分别对每门课程进行查询，然后再合并查询结果，但是如果课程太多，该方法就不太合适 SELECT t1.* FROM ( SELECT st.*, c.c_id, c.c_name, sc.s_score FROM student st LEFT JOIN score sc ON sc.s_id = st.s_id INNER JOIN course c ON c.c_id = sc.c_id AND c.c_id = "01" ORDER BY sc.s_score DESC LIMIT 1, 2 ) as t1 UNION ALL SELECT t2.* FROM ( SELECT st.*, c.c_id, c.c_name, sc.s_score FROM student st LEFT JOIN score sc ON sc.s_id = st.s_id INNER JOIN course c ON c.c_id = sc.c_id AND c.c_id = "02" ORDER BY sc.s_score DESC LIMIT 1, 2 ) as t2 UNION ALL SELECT t3.* FROM ( SELECT st.*, c.c_id, c.c_name, sc.s_score FROM student st LEFT JOIN score sc ON sc.s_id = st.s_id INNER JOIN course c ON c.c_id = sc.c_id AND c.c_id = "03" ORDER BY sc.s_score DESC LIMIT 1, 2 ) as t3 # 2.一次性查询，需要注意的是 row_number() 在 MySQL 8.0 中才支持 SELECT c_id, student.*, s_score FROM student INNER JOIN ( SELECT s_id, s_score, c_id, row_number() over (PARTITION BY c_id ORDER BY s_score DESC) AS rank FROM score ) AS tmp_t ON tmp_t.s_id = student.s_id WHERE tmp_t.rank IN (2, 3)

3.23.SQL 23——统计各科成绩各分数段人数：课程编号，课程名称，[100-85]，[85-70]，[70-60]，[0-60]及所占百分比 SELECT score.c_id, course.c_name, sum(CASE WHEN s_score BETWEEN 0 AND 60 THEN 1 ELSE 0 END) AS '[0-60]人数', sum(CASE WHEN s_score BETWEEN 61 AND 70 THEN 1 ELSE 0 END) AS '[61-70]人数', sum(CASE WHEN s_score BETWEEN 71 AND 85 THEN 1 ELSE 0 END) AS '[71-85]人数', sum(CASE WHEN s_score BETWEEN 86 AND 100 THEN 1 ELSE 0 END) AS '[86-100]人数', sum(CASE WHEN s_score BETWEEN 0 AND 60 THEN 1 ELSE 0 END) / count( * ) AS '[0-60]人数所占百分比', sum(CASE WHEN s_score BETWEEN 61 AND 70 THEN 1 ELSE 0 END) / count( * ) AS '[61-70]人数所占百分比', sum(CASE WHEN s_score BETWEEN 71 AND 85 THEN 1 ELSE 0 END) / count( * ) AS '[71-85]人数所占百分比', sum(CASE WHEN s_score BETWEEN 86 AND 100 THEN 1 ELSE 0 END) / count( * ) AS '[86-100]人数所占百分比' FROM score LEFT JOIN course ON score.c_id = course.c_id GROUP BY score.c_id, course.c_name

3.24.SQL 24——查询学生平均成绩及其名次 SELECT student.s_id, avg(s_score) AS '平均成绩' FROM student, score WHERE student.s_id = score.s_id GROUP BY student.s_id ORDER BY avg(s_score) DESC

3.25.SQL 25——查询各科成绩前三名的记录 # 1.分别对每科进行查询，然后再合并查询结果，但是如果课程太多，该方法就不太合适 (SELECT c_id, s_score FROM score WHERE c_id = '01' ORDER BY s_score DESC LIMIT 3) UNION ALL (SELECT c_id, s_score FROM score WHERE c_id = '02' ORDER BY s_score DESC LIMIT 3) UNION ALL (SELECT c_id, s_score FROM score WHERE c_id = '03' ORDER BY s_score DESC LIMIT 3) # 2.一次性查询出结果 SELECT DISTINCT tmp_t.c_id, tmp_t.s_score FROM ( SELECT DISTINCT student.*, sc1.c_id, sc1.s_score, count(DISTINCT sc2.s_score) + 1 AS rank FROM score AS sc1 LEFT JOIN score AS sc2 ON sc1.c_id = sc2.c_id AND sc1.s_score < sc2.s_score LEFT JOIN student ON sc1.s_id = student.s_id GROUP BY sc1.c_id, sc1.s_id ORDER BY sc1.c_id, sc1.s_score DESC ) AS tmp_t WHERE tmp_t.rank BETWEEN 1 AND 3

3.26.SQL 26——查询每门课程被选修的学生数 SELECT c_id, count( s_id ) AS '选修该门课程的学生数' FROM score GROUP BY c_id

3.27.SQL 27——查询出只有两门课程的全部学生的学号和姓名 SELECT student.s_id, student.s_name FROM student, score WHERE student.s_id = score.s_id GROUP BY s_id HAVING count(c_id) = 2

3.28.SQL 28——查询男生、女生人数 SELECT sum(CASE WHEN s_sex = '男' THEN 1 ELSE NULL END) AS '男生人数', sum(CASE WHEN s_sex = '女' THEN 1 ELSE NULL END) AS '女生人数' FROM student

3.29.SQL 29——查询名字中含有"风"字的学生信 SELECT * FROM student WHERE s_name LIKE '%风%'

在 MySQL 中，LIKE 操作符用于在文本字段中搜索特定的模式。如果需要在文本字段中匹配通配符本身，可以使用反斜杠字符转义通配符。例如，如果您要在一个名为 ’mytable’ 的表中查找包含下划线字符的字符串，可以使用以下查询： SELECT * FROM mytable WHERE mycolumn LIKE '%\\_%' ESCAPE '\\'; 在上面的查询中，ESCAPE 关键字指定了转义字符为反斜杠，因此我们在通配符前添加了一个反斜杠字符。这将告诉 MySQL 仅匹配下划线字符本身，而不是作为通配符进行匹配。

3.30.SQL 30——查询同姓名同性别学生名单，并统计同名人数 SELECT stu1.s_name, tmp_tt AS '同名人数' FROM student AS stu1 LEFT JOIN ( SELECT s_name, s_sex, count(*) AS cnt FROM student GROUP BY s_name, s_sex ) AS tmp_t ON stu1.s_name = tmp_t.s_name AND stu1.s_sex = tmp_t.s_sex WHERE tmp_tt > 1

3.31.SQL 31——查询 1990 年出生的学生名单 SELECT * FROM student WHERE s_birth LIKE '1990%'

3.32.SQL 32——查询每门课程的平均成绩，结果按平均成绩降序排列，平均成绩相同时，按课程编号升序排列 SELECT score.c_id, avg( s_score ) AS avg_score FROM score GROUP BY c_id ORDER BY avg( s_score ) DESC, c_id ASC

3.33.SQL 33——查询平均成绩大于等于 85 的所有学生的学号、姓名和平均成绩 SELECT student.s_id, student.s_name, avg(s_score) AS '平均成绩' FROM student INNER JOIN score ON student.s_id = score.s_id GROUP BY score.s_id, student.s_id, student.s_name HAVING avg(score.s_score) > 85

3.34.SQL 34——查询课程名称为"数学"，且分数低于 60 的学生姓名和分数 SELECT s_name, s_score FROM student, score WHERE student.s_id = score.s_id AND c_id IN (SELECT c_id FROM course WHERE c_name = '数学') AND s_score < 60

3.35.SQL 35——查询所有学生的课程及分数情况 SELECT student.s_id, student.s_name, course.c_name, score.s_score FROM student, course, score WHERE student.s_id = score.s_id AND score.c_id = course.c_id ORDER BY s_id

3.36.SQL 36——查询任何一门课程成绩在 70 分以上的姓名、课程名称和分数 SELECT student.s_name, course.c_name, score.s_score FROM student, score, course WHERE student.s_id = score.s_id AND score.c_id = course.c_id AND s_score > 70

3.37.SQL 37——查询不及格的课程的学生姓名、课程名称和分数 SELECT student.s_name, course.c_name, score.s_score FROM student, score, course WHERE student.s_id = score.s_id AND score.c_id = course.c_id AND s_score < 60

3.38.SQL 38——查询课程编号为 01 且课程成绩在 80 分以上的学生的学号和姓名 SELECT student.s_id, s_name FROM student, score WHERE student.s_id = score.s_id AND c_id = '01' AND s_score >= 80

3.39.SQL 39——求每门课程的学生人数 SELECT c_name, count( s_id ) AS '学生人数' FROM score, course WHERE score.c_id = course.c_id GROUP BY score.c_id, c_name

3.40.SQL 40——查询选修"张三"老师所授课程的学生中，成绩最高的学生信及其成绩 # 这里默认的是一门老师只教授一门课程 SELECT student.*, score.s_score FROM student, score WHERE student.s_id = score.s_id AND c_id IN (SELECT c_id FROM course WHERE t_id IN (SELECT t_id FROM teacher WHERE t_name = '张三')) ORDER BY s_score DESC LIMIT 1

3.41.SQL 41——查询不同课程成绩相同的学生的学生编号、课程编号、学生成绩 SELECT sc1.s_id, sc1.c_id, sc2.c_id, sc1.s_score, sc2.s_score FROM score AS sc1, score AS sc2 WHERE sc1.s_id = sc2.s_id AND sc1.s_score = sc2.s_score AND sc1.c_id != sc2.c_id

3.42.SQL 42——查询每门课程成绩最好的前两名 SELECT sc1.c_id, sc1.s_id, count( sc2.s_score ) + 1 AS rank FROM score AS sc1 LEFT JOIN score AS sc2 ON sc1.c_id = sc2.c_id AND sc1.s_score < sc2.s_score GROUP BY sc1.c_id, sc1.s_score, sc1.s_id HAVING count( sc2.s_score ) < 2 ORDER BY sc1.c_id, rank

3.43.SQL 43——统计每门课程的学生选修人数（超过 5 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列，若人数相同，按课程号升序排列 SELECT c_id, count(*) AS '选修人数' FROM score GROUP BY c_id HAVING count( * ) > 5 ORDER BY '选修人数' DESC, c_id ASC

3.44.SQL 44——查询至少选修两门课程的学生学号 SELECT s_id FROM score GROUP BY s_id HAVING count(c_id) >= 2

3.45.SQL 45——查询选修了全部课程的学生信 SELECT * FROM student WHERE # SELECT count(*) FROM course) 查询的是总课程的数量 s_id IN (SELECT s_id FROM score GROUP BY s_id HAVING count(c_id) = (SELECT count(*) FROM course))

3.46.SQL 46——查询各学生的年龄 # 1.按照年份来计算 SELECT s_id, s_name, (YEAR(now()) - YEAR(s_birth)) AS age FROM student /* 2.使用 timestampdiff() (1) TIMESTAMPDIFF(): 第一个参数设置时间单位，可以精确到年(YEAR)、天(DAY)、小时(HOUR)，分钟(MINUTE)和秒(SECOND)。对于比较 的两个时间，时间小的放在前面，时间大的放在后面。 (3) datediff(): 返回值是相差的天数，无法定位到小时、分钟和秒。 */ SELECT s_id, s_name, timestampdiff(YEAR, s_birth, now()) AS age FROM student

3.47.SQL 47——查询本周过生日的学生 # week(时间): 默认从 0 开始，表示星期天为一个星期的第一天，国外算法 # week(时间, 1): 从 1 开始，表示星期一为一个星期的第一天，国内算法 SELECT s_id, s_name FROM student WHERE WEEK (s_birth) = WEEK (now(), 1)

3.48.SQL 48——查询下周过生日的学生 SELECT s_id, s_name FROM student WHERE WEEK (s_birth) = WEEK (now(), 1) + 1

3.49.SQL 49——查询本月过生日的学生 SELECT s_id, s_name FROM student WHERE MONTH (s_birth) = MONTH (now())

3.50.SQL 50——查询下月过生日的学生 SELECT s_id, s_name FROM student WHERE MONTH (s_birth) = MONTH (now()) + 1

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