视图
1 #视图 2 /* 3 含义：虚拟表，和普通表一样使用 4 mysql5.1版本出现的新特性，是通过表动态生成的数据 5 6 比如：舞蹈班和普通班的对比 7 创建语法的关键字 是否实际占用物理空间 使用 8 9 视图 create view 只是保存了sql逻辑 增删改查：一般不能增删改 10 11 表 create table 保存了数据 增删改查：可以 12 13 14 15 16 */ 17 SHOW VARIABLES LIKE 'character_set_%'; 18 SHOW VARIABLES LIKE 'collation_%'; 19 SET character_set_database =utf8; 20 SET character_set_filesystem=utf8; 21 SET character_set_results =utf8; 22 SET character_set_server =utf8; 23 SET character_set_system =utf8; /*此处utf-8也可以*/ 24 SET collation_server = utf8_general_ci; 25 SET collation_database = utf8_general_ci; 26 27 ALTER TABLE major CONVERT TO CHARACTER SET utf8; 28 ALTER TABLE stuinfo CONVERT TO CHARACTER SET utf8; 29 #案例：查询姓张的学生名和专业名 30 SELECT stuname,majorName 31 FROM stuinfo s 32 INNER JOIN major m ON s.`majorid`=m.`majorName` 33 WHERE s.`stuname` LIKE '张%'; 34 SHOW CREATE TABLE major; 35 SHOW CREATE TABLE stuinfo; 36 37 CREATE VIEW v1 38 AS 39 SELECT stuname,majorName 40 FROM stuinfo s 41 INNER JOIN major m ON s.`majorid`=m.`majorName` 42 43 SELECT * FROM v1 WHERE stuname LIKE '张%'; 44 45 #一、创建视图 46 /* 47 语法： 48 create view 视图名 49 as 50 查询语句 51 52 */ 53 USE myemployees; 54 55 #案例讲解 56 #1.查询邮箱中包含a字符的员工名、部门名和工种信息 57 #①创建 58 CREATE VIEW myv1 59 AS 60 61 SELECT last_name,department_name,job_title 62 FROM employees e 63 JOIN departments d ON e.department_id = d.department_id 64 JOIN jobs j ON j.job_id = e.job_id; 65 66 #②使用 67 SELECT * FROM myv1 WHERE last_name LIKE '%a%'; 68 69 #2.查询各部门平均工资级别 70 #创建视图查看每个部门的平均工资 71 CREATE VIEW myv2 72 AS 73 SELECT AVG(salary) ag,department_id 74 FROM employees 75 GROUP BY department_id; 76 77 #②使用 78 SELECT myv2.ag, g.grade_level 79 FROM myv2 80 JOIN job_grades g 81 ON myv2.ag BETWEEN g.`lowest_sal` AND g.`highest_sal`; 82 83 84 85 #3.查询平均工资最低的部门信息 86 87 SELECT * FROM myv2 ORDER BY ag LIMIT 1; 88 89 #4.查询平均工资最低的部门名和工资 90 91 CREATE VIEW myv3 92 AS 93 SELECT * FROM myv2 ORDER BY ag LIMIT 1; 94 95 SELECT d.*,m.ag 96 FROM myv3 m 97 JOIN departments d 98 ON m.`department_id`=d.`department_id`; 99 100 101 102 #二、视图的修改 103 104 #方式一： 105 /* 106 create or replace view 107 as 108 查询语句; 109 110 111 */ 112 SELECT * FROM myv3; 113 114 CREATE OR REPLACE VIEW myv3 115 AS 116 SELECT AVG(salary),job_id 117 FROM employees 118 GROUP BY job_id; 119 120 #方式二： 121 /* 122 语法： 123 alter view 视图名 124 as 125 查询语句; 126 127 */ 128 ALTER VIEW myv3 129 AS 130 SELECT * FROM employees; 131 132 #三、删除视图 133 134 /* 135 136 语法:drop view 视图名,视图名...; 137 */ 138 DROP VIEW myv1,myv2,myv3; 139 140 #四、查看视图 141 DESC myv3; 142 SHOW CREATE VIEW myv3; 143 144 #五、视图的更细 145 CREATE OR REPLACE VIEW myv1 146 AS 147 SELECT last_name,email,salary*12*(1+IFNULL(commission_pet,0)) "annual salary" 148 FROM employees; 149 150 CREATE OR REPLACE VIEW myv1 151 AS 152 SELECT last_name,email 153 FROM employees; 154 155 SELECT * FROM myv1; 156 #1.插入 157 158 INSERT INTO myv1 VALUES('zhangfei','[email protected]'); 159 160 #2.修改 161 UPDATE myv1 SET last_name='zhangwuji' WHERE last_name='zhangfei'; 162 #错误代码： 1423 163 #Field of view 'myemployees.myv1' underlying table doesn't have a default value 164 # 造成该问题的原因是： 当向视图中插入数据时，同时也会向原表插入数据插入数据 ，而原表(employees)中存在多个字段不允许为空，所以无法插入 ，将这些不允许为空的字段修改为允许为空即可。 165 166 167 #3.删除 168 DELETE FROM myv1 WHERE last_name='zhangwuji'; 169 170 171 #具备以下特点的视图是不允许更新的 172 173 #①包含以下关键字的sql语句:分组函数、group by,distinct,having、union或有union all 174 175 CREATE OR REPLACE VIEW myv1 176 AS 177 SELECT MAX(salary) m,department_id 178 FROM employees 179 GROUP BY department_id; 180 181 SELECT * FROM myv1; 182 183 #更新 184 UPDATE myv1 SET m=9000 WHERE department_id=10; 185 186 #②常量视图 187 CREATE OR REPLACE VIEW myv2 188 AS 189 SELECT 'john' NAME; 190 191 SELECT * FROM myv2; 192 193 #更新 194 UPDATE myv2 SET NAME='lucy'; 195 196 #③select中包含子查询 197 CREATE OR REPLACE VIEW myv3 198 AS 199 200 SELECT department_id,(SELECT MAX(salary) FROM employees) 最高工资; 201 FROM departments 202 203 #更新 204 SELECT * FROM myv3; 205 UPDATE myv3 SET 最高工资=10000; 206 207 208 #④join 209 CREATE OR REPLACE VIEW myv4 210 AS 211 SELECT last_name,department_name 212 FROM employees e 213 JOIN departments d 214 ON e.department_id = d.department_id; 215 216 #更新 217 SELECT * FROM myv4; 218 UPDATE myv4 SET last_name = '张飞' WHERE last_name='Whalen'; 219 INSERT INTO myv4 VALUES('陈真','xxx'); 220 221 #⑤from 一个不能更新的视图 222 CREATE OR REPLACE VIEW myv5 223 AS 224 225 SELECT * FROM myv3; 226 227 #更新 228 229 SELECT * FROM myv5; 230 231 UPDATE myv5 SET 最高工资=10000 WHERE department_id=60; 232 233 234 #⑥where子句的子查询引用了from子句中的表 235 236 CREATE OR REPLACE VIEW myv6 237 AS 238 239 SELECT last_name,email,salary 240 FROM employees 241 WHERE employee_id IN( 242 SELECT manager_id 243 FROM employees 244 WHERE manager_id IS NOT NULL 245 246 ); 247 248 #更新 249 SELECT * FROM myv6; 250 251 UPDATE myv6 SET salary=10000 WHERE last_name = 'k_ing';
案例讲解
#【案例讲解】 #一、创建视图emp_v1,要求查询电话号码以'011'开头的员工姓名和工资、邮箱 CREATE OR REPLACE VIEW emp_v1 AS SELECT last_name,salary,email FROM employees WHERE phone_num LIKE '011%' #二、创建视图emp_v2,要去查询部门的员工最高工资高于12000的部门信息 CREATE OR REPLACE VIEW emp_v2 AS SELECT MAX(salary) max_dep,department_id FROM employees GROUP BY department_id HAVING MAX(salary)>12000 以前的 SELECT d.*,m.max_dep FROM departments d JOIN emp_v2 m ON m.department_id = d.department_id;
小结