MySQL之视图外连接、内连接和子查询的使用

目录

一. 视图

        1.1 含义

        1.2 视图的基本语法

        1.3 视图的实操

二. 外连接、内连接和子查询的使用

        2.1 SQL脚本

        2.2 使用外连接、内连接和子查询进行解答

三. 思维导图

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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 
FROMt_mysql_student s,t_mysql_score sc 
WHEREs.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 TransferSource Server         : localhostSource Server Type    : MySQLSource Server Version : 80018Source Host           : localhost:3306Source Schema         : mybatis_ssmTarget Server Type    : MySQLTarget Server Version : 80018File Encoding         : 65001Date: 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

1. 涉及表：t_mysql_student ， t_mysql_score
2. 连接方式：内连接 
3. 行转列：流程控制函数    

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

1. 涉及表：t_mysql_student ， t_mysql_score
2. 连接方式：内连接 
3. 行转列：流程控制函数    

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

1. 涉及表：t_mysql_score
2. 连接方式：外连接 
3. 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： 

1. 涉及表：t_mysql_student ， t_mysql_score
2. 连接方式：内连接，外连接
3. 行转列：流程控制函数    
4. 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

SELECTs.*,( CASE WHEN t1.cid = '01' THEN t1.score END ) 语文,( CASE WHEN t2.cid = '02' THEN t2.score END ) 数学 
FROMt_mysql_student sINNER JOIN ( SELECT * FROM t_mysql_score WHERE cid = '01' ) t1 ON s.sid = t1.sidLEFT 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

1. 涉及表：t_mysql_student ， t_mysql_score
2. 查询方式：子查询
3. 行转列：流程控制函数    

SELECTs.*,( CASE WHEN sc.cid = '01' THEN sc.score END ) 语文,( CASE WHEN sc.cid = '02' THEN sc.score END ) 数学 
FROMt_mysql_student s,t_mysql_score sc 
WHEREs.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

1. 涉及表：t_mysql_student ， t_mysql_score
2. 查询方式：外连接
3. 聚合函数：四舍五入（ round() ），平均值（ avg() ）
4. 有聚合函数意味着分组（ GROUP BY ）

SELECTs.sid,s.sname,round(avg( sc.score ),2) 平均分 
FROMt_mysql_student s LEFT JOIN t_mysql_score sc ON s.sid = sc.sid 
GROUP BYs.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

1. 涉及表：t_mysql_student ， t_mysql_score
2. 查询方式：内连接
3. 最后建议进行一次分组，不然数据过多

SELECTs.sid,s.sname
FROMt_mysql_student s INNER JOIN t_mysql_score sc ON s.sid = sc.sid
GROUP BYs.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 )

1. 涉及表：t_mysql_student ， t_mysql_score
2. 查询方式：外连接
3. 聚合函数：计数（ count() ），求和（ sum() ）
4. 有聚合函数意味着分组（ GROUP BY ）

SELECTs.sid,s.sname,count(sc.score) 选课总数,sum(sc.score) 总成绩
FROMt_mysql_student s LEFT JOIN t_mysql_score sc ON s.sid = sc.sid 
GROUP BYs.sid,sc.sid

• 08）查询「李」姓老师的数量

SELECTcount(*) 
FROMt_mysql_teacher 
WHEREtname LIKE '李%'

三. 思维导图