大数据之

　　视图
　　视图（view)作为数据库系统的一个重要概念，它实际上就是通过选择已存在的表进行转换、裁剪、计算、过滤等操作生成的一个虚拟表，视图可以快速进行数据转换、简化查询逻辑等功能。建立视图CREATE VIEW [IF NOT EXISTS] [db_name.]view_name [(column_name [COMMENT column_comment], ...) ]   [COMMENT view_comment]   [TBLPROPERTIES (property_name = property_value, ...)]   AS SELECT ...;
　　示例CREATE VIEW my_view(col_name COMMENT ＂col_name＂)   COMMENT ＂view_comment＂   AS   SELECT  col_name   FROM my_tbl   WHERE sex=＂male＂;删除视图DROP VIEW [IF EXISTS] [db_name.]view_name;修改视图ALTER VIEW [db_name.]view_name AS select_statement; --用新的select列重新创建视图 ALTER VIEW [db_name.]view_name SET TBLPROPERTIES (＂COMMENT＂=＂view_tbl＂); --修改视图表描述物化视图
　　普通视图是一张虚拟表，而物化视图相当于把数据进行cache操作，注意物化视图只能支持事务表。建立物化视图CREATE MATERIALIZED VIEW [IF NOT EXISTS] [db_name.]materialized_view_name   [DISABLE REWRITE]   [COMMENT materialized_view_comment]   [PARTITIONED ON (col_name, ...)]   [CLUSTERED ON (col_name, ...) | DISTRIBUTED ON (col_name, ...) SORTED ON (col_name, ...)]   [     [ROW FORMAT row_format]     [STORED AS file_format]       | STORED BY ＂storage.handler.class.name＂ [WITH SERDEPROPERTIES (...)]   ]   [LOCATION hdfs_path]   [TBLPROPERTIES (property_name=property_value, ...)] AS SELECT ...;
　　语法结构上和创建视图一致，增加了物化表的一些设置同创建表基本一样。删除物化视图DROP MATERIALIZED VIEW [db_name.]materialized_view_name;修改物化视图确定是否开启重写ALTER MATERIALIZED VIEW [db_name.]materialized_view_name ENABLE|DISABLE REWRITE;