Hive中怎樣創建內部數據表和外部數據表？

數據表是存儲數據的基本單位，Hive數據表主要分為內部表(又叫托管表)和外部表，以內部表和外部表為基礎可以創建分區表或分桶表，即內/外部分區表或內/外部分桶表。接下來，針對內部表和外部表進行詳細講解。

默認情況下，內部表和外部表的數據都存儲在Hive配置文件中參數hive.metastore.warehouse.dir指定的路徑。它們的區別在于刪除內部表時，內部表的元數據和數據會一同刪除;而刪除外部表時，只刪除外部表的元數據，不會刪除數據。外部表相對來說更加安全，數據組織更加靈活并且方便共享源數據文件。

CREATE TABLE IF NOT EXISTS
hive_database.managed_table(
staff_id INT COMMENT "This is staffid",
staff_name STRING COMMENT "This is staffname",
salary FLOAT COMMENT "This is staff salary",
hobby ARRAY<STRING> CONMENT "This is staff hobby",
deduct(hps MAP<STRING, FLOAT> COMMENT "This is staff deduction",
address STRUCT<street:STRING,city:STRING> COMMENT "This is staff address"
)
ROW FORMAT DELIMITED
FIELDS TERMINATED BY ','
COLLECTION ITEMS TERMINATED BY
MAP KEYS TERMINMTED BY  ':'
LINES TERMINATED BY '\n'
STORED AS textfile
TBLPROPERTIES("comment"="This is a managed table");

上述命令中，指定ROW FORMAT DELIMITED子句使用Hive內置的Serde，自定義字段(FIELDS)分隔符為“，”;自定義集合元素(COLLECTION ITEMS)的分隔符為“_”;自定義MAP(MAP KEYS)的鍵值對分隔符為“：”;自定義行(LINES)分隔符為\n。

(2)創建外部表external_table的命令如下。

CREATE TABLE IF NOT EXISTS
hive_database.managed_table(
staff_id INT COMMENT "This is staffid",
staff_name STRING COMMENT "This is staffname",
salary FLOAT COMMENT "This is staff salary",
hobby ARRAY<STRING> CONMENT "This is staff hobby",
deduct(hps MAP<STRING, FLOAT> COMMENT "This is staff deduction",
address STRUCT<street:STRING,city:STRING> COMMENT "This is staff address"
)
ROW FORMAT DELIMITED
FIELDS TERMINATED BY ','
COLLECTION ITEMS TERMINATED BY
MAP KEYS TERMINMTED BY  ':'
LINES TERMINATED BY '\n'
STORED AS textfile
TBLPROPERTIES("comment"="This is a managed table");

上述命令中，通過在CREATETABLE句式中指定EXTERNAL子句創建外部表。創建外部表時通常配合LOCATION子句指定數據的存儲位置，便于數據的維護與管理。