MySQL之视图&外连接、内连接和子查询的使用

一、视图

1.1 含义

虚拟表，和普通表一样使用

1.2 操作

• 创建视图

create view 视图名 as?

• 修改视图

方式一：

create or replace view 视图名 as?

【查看视图相关字段】

方式二：

alter view 视图名 as? ?

【查看的SQL语句】

• 查看视图

方式一：

desc 视图名【看视图字段】

方式二：

show create view 视图名

• 删除视图

drop view 视图名

1.3 SQL数据

/*
?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、连接查询案例

（1）查询" 01 "课程比" 02 "课程成绩高的学生的信息及课程分数

? ? ? ? 方式一：

SELECT
?? ?*?
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

????????方式二：

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

（2）查询同时存在" 01 "课程和" 02 "课程的情况

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

?

（3）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 )

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

（4）查询不存在" 01 "课程但存在" 02 "课程的情况

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'

（5）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
?? ?s.sid,
?? ?s.sname,
?? ?ROUND(AVG(sc.score),2) 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>= 60

（6）查询在t_mysql_score表存在成绩的学生信息

??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

（7）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 )

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,s.sname

三、思维导图