MySQL之子查询、连接查询（内&外）以及分页查询（实操）

前言

续上篇博主MySQL之视图&索引&执行计划这篇给大家讲解MySQL之子查询、连接查询(内&外)以及分页查询

一、SQL脚本

/*
 Navicat Premium Data Transfer

 Source Server         : localhost
 Source Server Type    : MySQL
 Source Server Version : 80018
 Source Host           : localhost:3306
 Source Schema         : mybatis_ssm

 Target Server Type    : MySQL
 Target Server Version : 80018
 File Encoding         : 65001

 Date: 04/07/2023 23:53:33
*/

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for t_mysql_course
-- ----------------------------
DROP TABLE IF EXISTS `t_mysql_course`;
CREATE TABLE `t_mysql_course`  (
  `cid` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '课程编号',
  `cname` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '课程名称',
  `tid` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '教师编号',
  PRIMARY KEY (`cid`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci COMMENT = '课程信息表' ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of t_mysql_course
-- ----------------------------
INSERT INTO `t_mysql_course` VALUES ('01', '语文', '02');
INSERT INTO `t_mysql_course` VALUES ('02', '数学', '01');
INSERT INTO `t_mysql_course` VALUES ('03', '英语', '03');

-- ----------------------------
-- Table structure for t_mysql_score
-- ----------------------------
DROP TABLE IF EXISTS `t_mysql_score`;
CREATE TABLE `t_mysql_score`  (
  `sid` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '学生编号，外键',
  `cid` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '课程编号，外键',
  `score` float NULL DEFAULT 0 COMMENT '成绩',
  INDEX `sid`(`sid`) USING BTREE,
  INDEX `cid`(`cid`) USING BTREE,
  CONSTRAINT `t_mysql_score_ibfk_1` FOREIGN KEY (`sid`) REFERENCES `t_mysql_student` (`sid`) ON DELETE RESTRICT ON UPDATE RESTRICT,
  CONSTRAINT `t_mysql_score_ibfk_2` FOREIGN KEY (`cid`) REFERENCES `t_mysql_course` (`cid`) ON DELETE RESTRICT ON UPDATE RESTRICT
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci COMMENT = '成绩信息表' ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of t_mysql_score
-- ----------------------------
INSERT INTO `t_mysql_score` VALUES ('01', '01', 80);
INSERT INTO `t_mysql_score` VALUES ('01', '02', 90);
INSERT INTO `t_mysql_score` VALUES ('01', '03', 99);
INSERT INTO `t_mysql_score` VALUES ('02', '01', 70);
INSERT INTO `t_mysql_score` VALUES ('02', '02', 60);
INSERT INTO `t_mysql_score` VALUES ('02', '03', 80);
INSERT INTO `t_mysql_score` VALUES ('03', '01', 80);
INSERT INTO `t_mysql_score` VALUES ('03', '02', 80);
INSERT INTO `t_mysql_score` VALUES ('03', '03', 80);
INSERT INTO `t_mysql_score` VALUES ('04', '01', 50);
INSERT INTO `t_mysql_score` VALUES ('04', '02', 30);
INSERT INTO `t_mysql_score` VALUES ('04', '03', 20);
INSERT INTO `t_mysql_score` VALUES ('05', '01', 76);
INSERT INTO `t_mysql_score` VALUES ('05', '02', 87);
INSERT INTO `t_mysql_score` VALUES ('06', '01', 31);
INSERT INTO `t_mysql_score` VALUES ('06', '03', 34);
INSERT INTO `t_mysql_score` VALUES ('07', '02', 89);
INSERT INTO `t_mysql_score` VALUES ('07', '03', 98);

-- ----------------------------
-- Table structure for t_mysql_student
-- ----------------------------
DROP TABLE IF EXISTS `t_mysql_student`;
CREATE TABLE `t_mysql_student`  (
  `sid` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '学生编号',
  `sname` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '学生名称',
  `sage` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '学生年龄',
  `ssex` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '学生性别',
  PRIMARY KEY (`sid`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci COMMENT = '学生信息表' ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of t_mysql_student
-- ----------------------------
INSERT INTO `t_mysql_student` VALUES ('01', '赵雷', '1990-01-01', '男');
INSERT INTO `t_mysql_student` VALUES ('02', '钱电', '1990-12-21', '男');
INSERT INTO `t_mysql_student` VALUES ('03', '孙风', '1990-12-20', '男');
INSERT INTO `t_mysql_student` VALUES ('04', '李云', '1990-12-06', '男');
INSERT INTO `t_mysql_student` VALUES ('05', '周梅', '1991-12-01', '女');
INSERT INTO `t_mysql_student` VALUES ('06', '吴兰', '1992-01-01', '女');
INSERT INTO `t_mysql_student` VALUES ('07', '郑竹', '1989-01-01', '女');
INSERT INTO `t_mysql_student` VALUES ('09', '张三', '2017-12-20', '女');
INSERT INTO `t_mysql_student` VALUES ('10', '李四', '2017-12-25', '女');
INSERT INTO `t_mysql_student` VALUES ('11', '李四', '2012-06-06', '女');
INSERT INTO `t_mysql_student` VALUES ('12', '赵六', '2013-06-13', '女');
INSERT INTO `t_mysql_student` VALUES ('13', '孙七', '2014-06-01', '女');

-- ----------------------------
-- Table structure for t_mysql_teacher
-- ----------------------------
DROP TABLE IF EXISTS `t_mysql_teacher`;
CREATE TABLE `t_mysql_teacher`  (
  `tid` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '教师编号',
  `tname` varchar(20) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL COMMENT '教师名称',
  PRIMARY KEY (`tid`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci COMMENT = '教师信息表' ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of t_mysql_teacher
-- ----------------------------
INSERT INTO `t_mysql_teacher` VALUES ('01', '张三');
INSERT INTO `t_mysql_teacher` VALUES ('02', '李四');
INSERT INTO `t_mysql_teacher` VALUES ('03', '王五');

SET FOREIGN_KEY_CHECKS = 1;

二、实操以及实现思路

• 09）查询学过「张三」老师授课的同学的信息

SELECT
	s.*,
	c.cname,
	t.tname,
	sc.score 
FROM
	t_mysql_teacher t,
	t_mysql_course c,
	t_mysql_student s,
	t_mysql_score sc 
WHERE
	t.tid = c.tid 
	AND c.cid = sc.cid 
	AND sc.sid = s.sid 
	AND t.tname = '张三'

• 10）查询没有学全所有课程的同学的信息
  分析：没有学全，学全了有多少门，统计一共有多少门学科
  统计每一个学生学了多少门

SELECT
	s.sid,
	s.sname,
	count( sc.score ) n 
FROM
	t_mysql_student s
	LEFT JOIN t_mysql_score sc ON s.sid = sc.sid 
GROUP BY
	s.sid,
	s.sname 
HAVING
	n <(
	SELECT
		count( 1 ) 
	FROM
	t_mysql_course)
	

• 11）查询没学过"张三"老师讲授的任一门课程的学生姓名
  分析：子查询
  张三教了哪些课程
  找学过张三的课程有哪些人

SELECT
	s.sid,
	s.sname 
FROM
	t_mysql_score sc,
	t_mysql_student s 
WHERE
	s.sid = sc.sid 
	AND sc.cid NOT IN ( SELECT cid FROM t_mysql_course c, t_mysql_teacher t WHERE c.tid = t.tid AND t.tname = '张三' ) 
GROUP BY
	s.sid,
	s.sname

• 12）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
  分析：聚合函数 内连接 要先筛选再分组

SELECT
	s.sid,
	s.sname,
	avg( sc.score ) 
FROM
	t_mysql_student s,
	t_mysql_score sc 
WHERE
	s.sid = sc.sid 
	AND sc.score < 60 
GROUP BY
	s.sid,
	s.sname

• 13）检索" 01 "课程分数小于 60，按分数降序排列的学生信息

SELECT
	s.*,
	sc.score 
FROM
	t_mysql_student s,
	t_mysql_score sc 
WHERE
	s.sid = sc.sid 
	AND sc.cid = '01' 
	AND sc.score < 60 
ORDER BY
	sc.score DESC

• 14）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
  分析：聚合函数 GROUP BY
  从高到低 ORDER BY DESC
  所有学生的所有课程的成绩
  姓名 语文 数学 英语 平均成绩

SELECT
	s.sid,
	s.sname,
	sum((case when sc.cid='01' then sc.score end))语文,
	sum((case when sc.cid='02' then sc.score end))数学,
	sum((case when sc.cid='03' then sc.score END))英语,
	round(avg(sc.score),2) 平均分
 FROM
	t_mysql_score sc
	RIGHT JOIN t_mysql_student s ON sc.sid = s.sid 
GROUP BY
	s.sid,
	s.sname

	SELECT
	s.sid,
	s.sname,
	sum(if(sc.cid='01',sc.score,0))语文,
	sum(if(sc.cid='02',sc.score,0))数学,
	sum(if(sc.cid='03',sc.score,0))英语,
	round(avg(sc.score),2) 平均分
 FROM
	t_mysql_score sc
	RIGHT JOIN t_mysql_student s ON sc.sid = s.sid 
GROUP BY
	s.sid,
	s.sname

• 15）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 name，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
  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
  分析：聚合函数 GROUP BY
  大量使用函数

SELECT
	c.cid,c.cname,
	count(sc.score) 人数,
	max(sc.score) 最高分,
	min(sc.score) 最低分,
	round(avg(sc.score),2) 平均分,
	concat(round(sum(if(sc.score>=60,1,0))/(select count(1) from t_mysql_student)*100,2),'%') 及格率,
	concat(round(sum(if(sc.score>=70 and sc.score<=80,1,0))/(select count(1) from t_mysql_student)*100,2),'%') 中等率,
	concat(round(sum(if(sc.score>80 and sc.score<=90,1,0))/(select count(1) from t_mysql_student)*100,2),'%') 优良率,
	concat(round(sum(if(sc.score>=90,1,0))/(select count(1) from t_mysql_student)*100,2),'%') 优秀率
FROM
 t_mysql_score sc
	LEFT JOIN t_mysql_course c ON sc.cid = c.cid
group by
c.cid,c.cname