**1. 視圖的特點** * 通過隱藏子查詢、連接查詢、函數來簡化查詢的邏輯結構； * 只保存定義，不存儲數據，相當于是建立了索引； * 如果刪除或者更改基礎表，因為視圖是基于表建立的，則查詢視圖將失敗； * <mark>視圖是只讀，不能插入或裝載數據；</mark> * 視圖在查詢和修改操作基本和表一樣，只是不能插入數據。 **2. 應用場景** * 將特定的列提供給用戶，保護數據隱私； * 用于查詢語句復雜的場景； **3. 示例：創建關于people表的視圖** （1）數據`people.txt` ```xml 001,tom,23,2019-03-16 002,jack,12,2019-03-13 003,robin,14,2018-08-13 004,justin,34,2018-10-12 005,jarry,24,2017-11-11 006,jasper,24,2017-12-12 ``` （2）創建people表并導入數據 ```sql create table if not exists people( id int, name string, age int, peo_date date ) row format delimited fields terminated by ','; load data local inpath "/hdatas/people.txt" into table people; ``` （3）為people創建多個視圖 ```sql #### 創建整張表的視圖 #### create view if not exists view_001 as select * from people; #### 視圖可以像表那樣進行查詢 #### 0: jdbc:hive2://hadoop101:10000> select * from view_001; +--------------+----------------+---------------+--------------------+--+ | view_001.id | view_001.name | view_001.age | view_001.peo_date | +--------------+----------------+---------------+--------------------+--+ | 1 | tom | 23 | 2019-03-16 | | 2 | jack | 12 | 2019-03-13 | | 3 | robin | 14 | 2018-08-13 | | 4 | justin | 34 | 2018-10-12 | | 5 | jarry | 24 | 2017-11-11 | | 6 | jasper | 24 | 2017-12-12 | +--------------+----------------+---------------+--------------------+--+ #### 只為name一列創建視圖 #### create view if not exists view_002 as select name from people; -- 只查詢到name一列的數據，做到保護其它列的數據隱私 0: jdbc:hive2://hadoop101:10000> select * from view_002; +----------------+--+ | view_002.name | +----------------+--+ | tom | | jack | | robin | | justin | | jarry | | jasper | +----------------+--+ #### 查詢當前數據庫有多少個視圖 #### -- 低于hive2.2.0版本 0: jdbc:hive2://hadoop101:10000> show tables; -- hive2.2.0+版本 0: jdbc:hive2://hadoop101:10000> show views; #### 查看視圖定義 #### 0: jdbc:hive2://hadoop101:10000> show create table view_002; +----------------------------------------------------+--+ | createtab_stmt | +----------------------------------------------------+--+ | CREATE VIEW `view_002` AS select `people`.`name` from `hivedb2`.`people` | +----------------------------------------------------+--+ #### 刪除視圖 #### 0: jdbc:hive2://hadoop101:10000> drop view if exists view_001; #### 修改視圖的屬性，比如comment #### 0: jdbc:hive2://hadoop101:10000> alter view view_002 set tblproperties('comment'='This is a view'); #### 更改視圖定義 #### alter view view_002 as select age from people; 0: jdbc:hive2://hadoop101:10000> select * from view_002; +---------------+--+ | view_002.age | +---------------+--+ | 23 | | 12 | | 14 | | 34 | | 24 | | 24 | +---------------+--+ ```