# CREATE VIEW
## description
    该语句用于创建一个逻辑视图
    语法：
        CREATE VIEW [IF NOT EXISTS]
        [db_name.]view_name
        (column1[ COMMENT "col comment"][, column2, ...])
        AS query_stmt
        
    说明：
        1. 视图为逻辑视图，没有物理存储。所有在视图上的查询相当于在视图对应的子查询上进行。
        2. query_stmt 为任意支持的 SQL
        
## example
    1. 在 example_db 上创建视图 example_view
    
        CREATE VIEW example_db.example_view (k1, k2, k3, v1)
        AS
        SELECT c1 as k1, k2, k3, SUM(v1) FROM example_table
        WHERE k1 = 20160112 GROUP BY k1,k2,k3;
        
    2. 创建一个包含 comment 的 view
    
        CREATE VIEW example_db.example_view
        (
            k1 COMMENT "first key",
            k2 COMMENT "second key",
            k3 COMMENT "third key",
            v1 COMMENT "first value"
        )
        COMMENT "my first view"
        AS
        SELECT c1 as k1, k2, k3, SUM(v1) FROM example_table
        WHERE k1 = 20160112 GROUP BY k1,k2,k3;
    
## keyword
    CREATE,VIEW