MySQL之视图&外连接、内连接和子查询的使用

[一. 视图](#一. 视图)

[1.1 含义](#1.1 含义)

[1.2 视图的基本语法](#1.2 视图的基本语法)

[1.3 视图的实操](#1.3 视图的实操)

[二. 外连接、内连接和子查询的使用](#二. 外连接、内连接和子查询的使用)

[2.1 SQL脚本](#2.1 SQL脚本)

[2.2 使用外连接、内连接和子查询进行解答](#2.2 使用外连接、内连接和子查询进行解答)

[三. 思维导图](#三. 思维导图)

一. 视图

1.1 含义

视图（view）是一种虚拟 存在的表，是一个逻辑表 ，视图中的数据并不在数据库中实际存在。行和列数据来自我们定义视图的查询中使用的表 ，并且是在使用视图时动态生成 的。作为一个select语句保存在数据字典中的，通俗的讲，视图只保存了查询的SQL逻辑，不保存查询结果。所以我们在创建视图的时候，主要的工作就落在创建这条SQL查询语句上。

1.2 视图的基本语法

创建视图

create view 视图名
as
查询语句;
修改视图

#方式一
create or replace view 视图名
as
查询语句;

#方式二
alert view 视图名
as
查询语句;
查看视图

#查看视图相关字段
DESC 视图名;

#查看视图相关语句
SHOW CREATE VIEW 视图名;
删除视图

drop view 视图名,视图名,...;

1.3 视图的实操

创建视图

CREATE VIEW v_student_score AS SELECT
s.*,
sc.cid,
sc.score
FROM
t_mysql_student s,
t_mysql_score sc
WHERE
s.sid = sc.sid;

查看视图

查看视图相关字段：

DESC v_student_score;

查看视图创建的相关语句：

SHOW CREATE VIEW v_student_score;

删除视图

DROP VIEW v_student_score;

可以看见我们之前创建的视图已经没有了：

二. 外连接、内连接和子查询的使用

2.1 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;

2.2 使用外连接、内连接和子查询进行解答

01）查询" 01 "课程比" 02 "课程成绩高的学生的信息及课程分数

**涉及表：**t_mysql_student ， t_mysql_score

**连接方式：**内连接

**行转列：**流程控制函数

SELECT
	s.*,
	( CASE WHEN t1.cid = '01' THEN t1.score END ) 语文,
	( CASE WHEN t2.cid = '02' THEN t2.score END ) 数学 
FROM
	t_mysql_student s,
	( SELECT * FROM t_mysql_score WHERE cid = '01' ) t1,
	( SELECT * FROM t_mysql_score WHERE cid = '02' ) t2 
WHERE
	s.sid = t1.sid 
	AND t1.sid = t2.sid 
	AND t1.score > t2.score

02）查询同时存在" 01 "课程和" 02 "课程的情况

**涉及表：**t_mysql_student ， t_mysql_score

**连接方式：**内连接

**行转列：**流程控制函数

SELECT
	s.*,
	( CASE WHEN t1.cid = '01' THEN t1.score END ) 语文,
	( CASE WHEN t2.cid = '02' THEN t2.score END ) 数学 
FROM
	t_mysql_student s,
	( SELECT * FROM t_mysql_score WHERE cid = '01' ) t1,
	( SELECT * FROM t_mysql_score WHERE cid = '02' ) t2 
WHERE
	s.sid = t1.sid 
	AND t1.sid = t2.sid

03）查询存在" 01 "课程但可能不存在" 02 "课程的情况(不存在时显示为 null )

**涉及表：**t_mysql_score

**连接方式：**外连接

以01课表为主

SELECT *  FROM
	( SELECT * FROM t_mysql_score WHERE cid = '01' ) t1
LEFT JOIN 
	( SELECT * FROM t_mysql_score WHERE cid = '02' ) t2 
ON t1.sid = t2.sid

推荐：上面的这种查询方式看着不太友好，所以还有另外的一种，加上学生的信息 ：

**涉及表：**t_mysql_student ， t_mysql_score

**连接方式：**内连接，外连接

**行转列：**流程控制函数

可以更加直接的看到那个学生哪门课程没有学习，一目了然

SELECT
	s.*,
	( CASE WHEN t1.cid = '01' THEN t1.score END ) 语文,
	( CASE WHEN t2.cid = '02' THEN t2.score END ) 数学 
FROM
	t_mysql_student s
	INNER JOIN ( SELECT * FROM t_mysql_score WHERE cid = '01' ) t1 ON s.sid = t1.sid
	LEFT JOIN ( SELECT * FROM t_mysql_score WHERE cid = '02' ) t2 ON t1.sid = t2.sid

04）查询不存在" 01 "课程但存在" 02 "课程的情况

**涉及表：**t_mysql_student ， t_mysql_score

**查询方式：**子查询

**行转列：**流程控制函数

SELECT
	s.*,
	( CASE WHEN sc.cid = '01' THEN sc.score END ) 语文,
	( CASE WHEN sc.cid = '02' THEN sc.score END ) 数学 
FROM
	t_mysql_student s,
	t_mysql_score sc 
WHERE
	s.sid = sc.sid 
	AND s.sid NOT IN ( SELECT sid FROM t_mysql_score WHERE cid = '01' ) 
	AND sc.cid = '02'

05）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

**涉及表：**t_mysql_student ， t_mysql_score

**查询方式：**外连接

聚合函数： 四舍五入（ round() ），平均值（ avg()）

有聚合函数意味着分组（ GROUP BY ）

SELECT
	s.sid,
	s.sname,
	round(avg( sc.score ),2) 平均分 
FROM
	t_mysql_student s LEFT JOIN 
	t_mysql_score sc ON s.sid = sc.sid 
GROUP BY
	s.sid,
	s.sname 
HAVING
	平均分 >= 60

06）查询在t_mysql_score表存在成绩的学生信息

**涉及表：**t_mysql_student ， t_mysql_score

**查询方式：**内连接

最后建议进行一次分组，不然数据过多

SELECT
	s.sid,
	s.sname
FROM
	t_mysql_student s INNER JOIN 
	t_mysql_score sc ON s.sid = sc.sid
GROUP BY
	s.sid,
	s.sname

07）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(没成绩的显示为 null )

**涉及表：**t_mysql_student ， t_mysql_score

**查询方式：**外连接

聚合函数： 计数（ count() ），求和（sum()）

有聚合函数意味着分组（ GROUP BY ）

SELECT
	s.sid,
	s.sname,
	count(sc.score) 选课总数,
	sum(sc.score) 总成绩
FROM
	t_mysql_student s LEFT JOIN 
	t_mysql_score sc ON s.sid = sc.sid 
GROUP BY
	s.sid,
	sc.sid

08）查询「李」姓老师的数量

SELECT
	count(*) 
FROM
	t_mysql_teacher 
WHERE
	tname LIKE '李%'