1 #檢視
  2 /*
  3 含義：虛擬表，和普通表一樣使用
  4 mysql5.1版本出現的新特性，是通過表動態生成的資料
  5 
  6 比如：舞蹈班和普通班的對比
  7 
  8 
  9     建立語法的關鍵字    是否實際佔用物理空間    使用
 10 檢視    create view        只是儲存了sql邏輯    增刪改查，一般不能增刪改
 11 表    create table        儲存了資料        增刪改查
 12 
 13 */
 14 
 15 #案例：查詢姓張的學生名和專業名
 16 SELECT stuname,majorname
 
 17 FROM stuinfo s
 18 INNER JOIN major m ON s.majorid = m.id
 19 WHERE s.stuname LIKE '張%';
 20 
 21 CREATE VIEW v1 
 22 AS 
 23 SELECT stuname,majorname
 24 FROM stuinfo s
 25 INNER JOIN major m ON s.majorid = m.id;
 26 
 27 SELECT * FROM v1 WHERE s.stuname LIKE '張%';
 28 
 29 #一、建立檢視
 30 /*
 31 create view 檢視名
 
 32 as
 33 查詢語句;
 34 
 35 */
 36 USE myemployees;
 37 #1.查詢姓名中包含a字元的員工名、部門名和工種資訊
 38 #(1)建立
 39 CREATE VIEW myv1
 40 AS
 41 SELECT last_name,department_name,job_title
 42 FROM employees e
 43 JOIN departments d ON e.department_id=d.department_id
 44 JOIN jobs j ON e.job_id=j.job_id;
 45 #(2)使用
 46 SELECT
 
 * FROM myv1
 47 WHERE last_name LIKE '%a%';
 48 
 49 #2.查詢各部門的平均工資級別
 50 CREATE VIEW myv2
 51 AS
 52 SELECT AVG(salary) avs,department_id
 53 FROM employees
 54 GROUP BY department_id;
 55 
 56 SELECT grade_level,department_id FROM job_grades j
 57 JOIN myv2 m ON m.avs BETWEEN j.lowest_sal AND j.highest_sal;
 58 
 59 #3.查詢平均工資最低的部門資訊
 60 SELECT * FROM departments d
 61 JOIN myv2 m ON d.department_id=m.department_id
 62 ORDER BY  m.avs ASC LIMIT 1;
 63 #4.查詢平均工資最低的部門名和工資
 64 CREATE VIEW myv3
 65 AS
 66 SELECT * FROM myv2 ORDER BY avs LIMIT 1;
 67 
 68 SELECT department_name,avs FROM
 69 departments d JOIN myv3 m ON d.department_id=m.department_id;
 70 
 71 #二、檢視的修改
 72 #方式一：
 73 /*
 74 create or replace view 檢視名
 75 as
 76 查詢語句;
 77 */
 78 
 79 SELECT * FROM myv3;
 80 
 81 CREATE OR REPLACE VIEW myv3
 82 AS
 83 SELECT AVG(salary),job_id
 84 FROM employees
 85 GROUP BY job_id;
 86 
 87 #方式二：
 88 /*
 89 語法：
 90 alter view 檢視名
 91 as
 92 查詢語句;
 93 */
 94 ALTER VIEW myv3
 95 AS
 96 SELECT * FROM employees;
 97 
 98 #三、刪除檢視
 99 /*
100 語法：drop view 檢視名,檢視名,...;
101 */
102 DROP VIEW myv1,myv2,myv3;
103 
104 #四、檢視檢視
105 DESC myv3;
106 
107 SHOW CREATE VIEW myv3; -- 在mysql客戶端裡使用比較方便
108 
109 #五、檢視的更新
110 CREATE OR REPLACE VIEW myv1
111 AS 
112 SELECT last_name,email
113 FROM employees;
114 
115 SELECT * FROM myv1;
116 SELECT * FROM employees;
117 #1.插入
118 INSERT INTO myv1 VALUES('張飛','[email protected]');
119 #2.修改
120 UPDATE myv1 SET last_name='張無忌' WHERE last_name='張飛';
121 #3.刪除
122 DELETE FROM myv1 WHERE last_name='張無忌';
123 
124 #具備以下特點的檢視不允許更新
125 #(1)包含以下關鍵字的sql語句：分組函式、distinct、group by、having、union或者union all
126 CREATE OR REPLACE VIEW myv1
127 AS
128 SELECT MAX(salary) m,department_id
129 FROM employees
130 GROUP BY department_id;
131 
132 SELECT * FROM myv1;
133 #更新
134 UPDATE myv1 SET m=9000 WHERE department_id=10; -- 不允許更新
135 
136 #(2)常量檢視
137 CREATE OR REPLACE VIEW myv2
138 AS
139 SELECT 'john' NAME;
140 
141 SELECT * FROM myv2;
142 #更新
143 UPDATE myv2 SET NAME='lucy'; -- 常量檢視不允許更新
144 
145 #(3)select中包含子查詢
146 CREATE OR REPLACE VIEW myv3
147 AS
148 SELECT (SELECT MAX(salary) FROM employees) 最高工資;
149 
150 #更新
151 SELECT * FROM myv3;
152 UPDATE myv3 SET 最高工資=10000; -- 包含子查詢的select語句不允許更新
153 
154 #(4)join
155 CREATE OR REPLACE VIEW myv4
156 AS
157 SELECT last_name,department_name
158 FROM employees e 
159 JOIN departments d ON e.department_id=d.department_id;
160 
161 #更新
162 SELECT * FROM myv4;
163 UPDATE myv4 SET last_name='張飛' WHERE last_name='Whalen';
164 INSERT INTO myv4 VALUES('陳真','xxxx'); -- 有join的不能插入(更新)
165 
166 #(5)from一個不能更新的檢視
167 CREATE OR REPLACE VIEW myv5
168 AS
169 SELECT * FROM myv3;
170 
171 #更新
172 SELECT * FROM myv5;
173 UPDATE myv5 SET 最高工資=10000 WHERE department_id=60; -- from一個不能更新的檢視的，不能更新
174 
175 #(6)where子句的子查詢引用了from子句中的表
176 CREATE OR REPLACE VIEW myv6
177 AS
178 SELECT last_name,email,salary
179 FROM employees
180 WHERE employee_id IN (
181     SELECT manager_id FROM employees
182     WHERE manager_id IS NOT NULL
183     );
184 
185 SELECT * FROM myv6;
186 #更新
187 UPDATE myv6 SET salary=10000 WHERE last_name='K_ing'; -- 不允許更新