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 操作

1.3 SQL数据

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

（2）查询同时存在" 01 "课程和" 02 "课程的情况

（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 )

（4）查询不存在" 01 "课程但存在" 02 "课程的情况

（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

（6）查询在t_mysql_score表存在成绩的学生信息

（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 )

三、思维导图

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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