-- 1、查询"01"课程比"02"课程成绩高的学生的信息及课程分数
SELECT
  a.*,
  b.s_score AS 01_score,
  c.s_score AS 02_score
FROM
  student a
  JOIN score b ON a.s_id = b.s_id AND b.c_id = '01'
  LEFT JOIN score c ON a.s_id = c.s_id AND c.c_id = '02' OR c.c_id = NULL
WHERE b.s_score > c.s_score;

SELECT
  st.*,
  sc1.s_score AS 01_score,
  sc2.s_score AS 02_score
FROM score AS sc1, score AS sc2, student AS st
WHERE sc1.c_id = '01' AND (sc2.c_id = '02' OR sc2.c_id = NULL) AND sc1.s_id = sc2.s_id AND sc1.s_score > sc2.s_score AND
      sc1.s_id = st.s_id;

-- 2、查询"01"课程比"02"课程成绩低的学生的信息及课程分数

SELECT
  a.*,
  b.s_score AS 01_score,
  c.s_score AS 02_score
FROM
  student a LEFT JOIN score b ON a.s_id = b.s_id AND b.c_id = '01' OR b.c_id = NULL
  JOIN score c ON a.s_id = c.s_id AND c.c_id = '02'
WHERE b.s_score < c.s_score;

SELECT
  st.*,
  sc1.s_score AS 01_score,
  sc2.s_score AS 02_score
FROM score AS sc1, score AS sc2, student AS st
WHERE sc1.c_id = '01' AND (sc2.c_id = '02' OR sc2.c_id = NULL) AND sc1.s_id = sc2.s_id AND sc1.s_score < sc2.s_score AND
      sc1.s_id = st.s_id;

-- 3、查询平均成绩大于等于60分的同学的学生编号和学生姓名和平均成绩
SELECT
  b.s_id,
  b.s_name,
  ROUND(AVG(a.s_score), 2) AS avg_score
FROM
  student b
  JOIN score a ON b.s_id = a.s_id
GROUP BY b.s_id, b.s_name
HAVING ROUND(AVG(a.s_score), 2) >= 60;

SELECT
  st.*,
  round(avg(s_score), 2) AS avg_score
FROM score AS sc, student AS st
WHERE sc.s_id = st.s_id
GROUP BY s_id
HAVING avg(s_score) >= 60;

-- 4、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)

SELECT
  b.s_id,
  b.s_name,
  ROUND(AVG(a.s_score), 2) AS avg_score
FROM
  student b
  LEFT JOIN score a ON b.s_id = a.s_id
GROUP BY b.s_id, b.s_name
HAVING ROUND(AVG(a.s_score), 2) < 60
UNION
SELECT
  a.s_id,
  a.s_name,
  0 AS avg_score
FROM
  student a
WHERE a.s_id NOT IN (
  SELECT DISTINCT s_id
  FROM score);

-- 5、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
  a.s_id,
  a.s_name,
  count(b.c_id)  AS sum_course,
  sum(b.s_score) AS sum_score
FROM
  student a
  LEFT JOIN score b ON a.s_id = b.s_id
GROUP BY a.s_id, a.s_name;


SELECT
  st.s_id,
  st.s_name,
  count(sc.c_id),
  sum(sc.s_score)
FROM student AS st LEFT JOIN score AS sc ON st.s_id = sc.s_id
GROUP BY st.s_id;

-- 6、查询"李"姓老师的数量
SELECT count(*)
FROM teacher
WHERE t_name LIKE '李%';

-- 7、查询学过"张三"老师授课的同学的信息
SELECT a.*
FROM
  student a
  JOIN score b ON a.s_id = b.s_id
WHERE b.c_id IN (
  SELECT c_id
  FROM course
  WHERE t_id = (
    SELECT t_id
    FROM teacher
    WHERE t_name = '张三'));

SELECT *
FROM student AS st
WHERE st.s_id IN (SELECT sc.s_id
                  FROM score AS sc
                  WHERE sc.c_id IN (SELECT co.c_id
                                    FROM course AS co
                                    WHERE co.t_id IN (SELECT t_id
                                                      FROM teacher AS te
                                                      WHERE te.t_name = '张三')));

-- 8、查询没学过"张三"老师授课的同学的信息
SELECT *
FROM
  student c
WHERE c.s_id NOT IN (
  SELECT a.s_id
  FROM student a
    JOIN score b ON a.s_id = b.s_id
  WHERE b.c_id IN (
    SELECT c_id
    FROM course
    WHERE t_id = (
      SELECT t_id
      FROM teacher
      WHERE t_name = '张三')));

SELECT *
FROM student AS st
WHERE st.s_id NOT IN (SELECT sc.s_id
                      FROM score AS sc
                      WHERE sc.c_id IN (SELECT co.c_id
                                        FROM course AS co
                                        WHERE co.t_id IN (SELECT t_id
                                                          FROM teacher AS te
                                                          WHERE te.t_name = '张三')));
-- 9、查询学过编号为"01"并且也学过编号为"02"的课程的同学的信息

SELECT a.*
FROM
  student a, score b, score c
WHERE a.s_id = b.s_id AND a.s_id = c.s_id AND b.c_id = '01' AND c.c_id = '02';

SELECT st.*
FROM score sc_1, score sc_2, student st
WHERE sc_1.s_id = sc_2.s_id AND sc_1.c_id = '01' AND sc_2.c_id = '02' AND sc_1.s_id = st.s_id;

-- 10、查询学过编号为"01"但是没有学过编号为"02"的课程的同学的信息

SELECT a.*
FROM
  student a
WHERE a.s_id IN (SELECT s_id
                 FROM score
                 WHERE c_id = '01') AND a.s_id NOT IN (SELECT s_id
                                                       FROM score);

SELECT st.*
FROM score sc_1, score sc_2, student st
WHERE sc_1.s_id = sc_2.s_id AND sc_1.c_id = '01' AND sc_2.c_id = '02' AND sc_1.s_id = st.s_id;

-- 11、查询没有学全所有课程的同学的信息
SELECT s.*
FROM
  student s
WHERE s.s_id IN (
  SELECT s_id
  FROM score
  WHERE s_id NOT IN (
    SELECT a.s_id
    FROM score a
      JOIN score b ON a.s_id = b.s_id AND b.c_id = '02'
      JOIN score c ON a.s_id = c.s_id AND c.c_id = '03'
    WHERE a.c_id = '01'));

SELECT st.*
FROM student st
WHERE st.s_id IN (
  SELECT sc.s_id
  FROM score sc
  WHERE sc.s_id NOT IN (
    SELECT sc.s_id
    FROM score sc
    GROUP BY sc.s_id
    HAVING count(*) >= (SELECT count(*)
                        FROM course)));

-- 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息
SELECT *
FROM student
WHERE s_id IN (
  SELECT DISTINCT a.s_id
  FROM score a
  WHERE a.c_id IN (SELECT a.c_id
                   FROM score a
                   WHERE a.s_id = '01')
);


SELECT st.*
FROM student st
WHERE st.s_id IN (SELECT DISTINCT s_id
                  FROM score
                  WHERE c_id IN (SELECT sc.c_id
                                 FROM score sc
                                 WHERE sc.s_id = '01'));

-- 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息
SELECT a.*
FROM student a
WHERE a.s_id IN (
  SELECT DISTINCT s_id
  FROM score
  WHERE s_id != '01' AND c_id IN (SELECT c_id
                                  FROM score
                                  WHERE s_id = '01')
  GROUP BY s_id
  HAVING count(1) = (SELECT count(1)
                     FROM score
                     WHERE s_id = '01'));


SELECT st.*
FROM student st
WHERE st.s_id != '01' AND st.s_id IN (
  SELECT s_id
  FROM score
  WHERE c_id IN (SELECT c_id
                 FROM score
                 WHERE s_id = '01')
  GROUP BY s_id
  HAVING count(1) = (SELECT count(1)
                     FROM score
                     WHERE s_id = '01'));

-- 14、查询没学过"张三"老师讲授的任一门课程的学生姓名
SELECT a.s_name
FROM student a
WHERE a.s_id NOT IN (
  SELECT s_id
  FROM score
  WHERE c_id =
        (SELECT c_id
         FROM course
         WHERE t_id = (
           SELECT t_id
           FROM teacher
           WHERE t_name = '张三'))
  GROUP BY s_id);

SELECT s_name
FROM student
WHERE s_id NOT IN (
  SELECT s_id
  FROM score
  WHERE c_id = (SELECT c_id
                FROM teacher te, course co
                WHERE te.t_name = '张三' AND co.t_id = te.t_id));

实际开发中，经常性的从数据库读取数据，但是数据库中的数据并不全是业务逻辑实际需要的，这时，有两个选择，一个是在构建SQL查询语句时就写where子句进行过滤，另一个是将数据悉数读取出来，在客户机利用代码对返回数据进行过滤。

如果数据量较小，此两者方式差距不大。尽管关系型数据库的逻辑处理性能偏弱，但在数据行数较多的情况下，在SQL层查询时过滤往往能有效缩短客户机的请求时间。

经自己测试，应用层利用代码处理数据会极大影响应用的性能，并且造成代码所处的那处业务逻辑不具备可伸缩性，服务器也不得不发送多余的数据，导致网络宽带的浪费。