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 TransferSource Server : localhost
Source Server Type : MySQL
Source Server Version : 80018
Source Host : localhost:3306
Source Schema : mybatis_ssmTarget Server Type : MySQL
Target Server Version : 80018
File 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;
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
*
FROMt_mysql_student s,
( SELECT * FROM t_mysql_score WHERE cid = '01' ) t1,
( SELECT * FROM t_mysql_score WHERE cid = '02' ) t2WHERE
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
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