Question

如何创建外部表格，只设置文件中的几列？

Ex：在存档中，我有六列，A，B，C，D，E，F。但在我的表中我只想要A，C，F。

有可能吗？

Answer 1

我不知道有哪种方法可以选择性地为外部表包含HDFS文件中的列。根据您的使用情况，基于外部表定义视图以仅包含所需的列可能就足够了。例如，给出以下外部表的愚蠢示例：

hive> CREATE EXTERNAL TABLE ext_table (
    >   A STRING,
    >   B STRING,
    >   C STRING,
    >   D STRING,
    >   E STRING,
    >   F STRING
    > )
    > ROW FORMAT DELIMITED FIELDS TERMINATED BY ','
    > STORED AS TEXTFILE
    > LOCATION '/tmp/ext_table';
OK
Time taken: 0.401 seconds
hive> SELECT * FROM ext_table;
OK
row_1_col_A row_1_col_B     row_1_col_C     row_1_col_D     row_1_col_E     row_1_col_F
row_2_col_A row_2_col_B     row_2_col_C     row_2_col_D     row_2_col_E     row_2_col_F
row_3_col_A row_3_col_B     row_3_col_C     row_3_col_D     row_3_col_E     row_3_col_F
Time taken: 0.222 seconds, Fetched: 3 row(s)

然后创建一个视图以仅包含所需的列：

hive> CREATE VIEW filtered_ext_table AS SELECT A, C, F FROM ext_table;
OK
Time taken: 0.749 seconds
hive> DESCRIBE filtered_ext_table; 
OK
a                           string                              
c                           string                              
f                           string                              
Time taken: 0.266 seconds, Fetched: 3 row(s)
hive> SELECT * FROM filtered_ext_table;
OK
row_1_col_A row_1_col_C     row_1_col_F
row_2_col_A row_2_col_C     row_2_col_F
row_3_col_A row_3_col_C     row_3_col_F
Time taken: 0.301 seconds, Fetched: 3 row(s)

实现所需功能的另一种方法是，您需要能够修改支持外部表的HDFS文件 - 如果您感兴趣的列都在每行的开头附近，那么您可以定义外部表仅捕获前3列（不考虑文件中实际存在多少列）。例如，使用与上面相同的数据文件：

hive> DROP TABLE IF EXISTS ext_table;
OK
Time taken: 1.438 seconds
hive> CREATE EXTERNAL TABLE ext_table (
    >   A STRING,
    >   B STRING,
    >   C STRING
    > )
    > ROW FORMAT DELIMITED FIELDS TERMINATED BY ','
    > STORED AS TEXTFILE
    > LOCATION '/tmp/ext_table';
OK
Time taken: 0.734 seconds
hive> SELECT * FROM ext_table;
OK
row_1_col_A row_1_col_B     row_1_col_C
row_2_col_A row_2_col_B     row_2_col_C
row_3_col_A row_3_col_B     row_3_col_C
Time taken: 0.727 seconds, Fetched: 3 row(s)

Answer 2

我找到了答案here

create table tmpdc_ticket(
    SERVICE_ID CHAR(144),
    SERVICE_TYPE CHAR(50),
    CUSTOMER_NAME CHAR(200),
    TELEPHONE_NO CHAR(144),
    ACCOUNT_NUMBER CHAR(144),
    FAULT_STATUS CHAR(50),
    BUSINESS_GROUP CHAR(100)
)
organization external(
    type    oracle_loader
    default directory sample_directory
    access parameters(
        records delimited by newline
        nologfile
        skip 1
        fields terminated by '|'
        missing field values are null
            (DUMMY_1,
             DUMMY_2,
             SERVICE_ID CHAR(144),
             SERVICE_TYPE CHAR(50),
             CUSTOMER_NAME CHAR(200),
             TELEPHONE_NO CHAR(144),
             ACCOUNT_NUMBER CHAR(144),
             FAULT_STATUS CHAR(50),
             BUSINESS_GROUP CHAR(100)
        )
    )
    location(sample_directory:'sample_file.txt')
)
reject limit 1
noparallel
nomonitoring;

外部表（HIVE）仅从文件中选择几列

2 个答案: