Question

我使用COPY命令获取数据副本。 COPY看起来比sstables更简单。但看起来它无法导入空字符串。原始表中为空的列在导入时为空。下面重现的步骤。

CREATE TABLE empty_example (id bigint PRIMARY KEY, empty_column text, null_column text);
INSERT INTO empty_example (id, empty_column) VALUES ( 1, '');
SELECT * from empty_example ;
 id | empty_column | null_column
----+--------------+-------------
  1 |              |        null
COPY empty_example TO 'empty_example.csv';
TRUNCATE empty_example ;
COPY empty_example FROM  'empty_example.csv';
SELECT * from empty_example ;
 id | empty_column | null_column
----+--------------+-------------
  1 |         null |        null

我尝试使用WITH选项，但无法解决问题。是否可以使用COPY保留空/空字符串区别？

Answer 1

您使用的是哪个版本的Cassandra？从Cassandra 3.4开始，COPY命令有许多选项来处理空字符串或空字符串：

cqlsh:system_schema> help COPY

        COPY [cqlsh only]

          COPY x FROM: Imports CSV data into a Cassandra table
          COPY x TO: Exports data from a Cassandra table in CSV format.

        COPY <table_name> [ ( column [, ...] ) ]
             FROM ( '<file_pattern_1, file_pattern_2, ... file_pattern_n>' | STDIN )
             [ WITH <option>='value' [AND ...] ];

        File patterns are either file names or valid python glob expressions, e.g. *.csv or folder/*.csv.

        COPY <table_name> [ ( column [, ...] ) ]
             TO ( '<filename>' | STDOUT )
             [ WITH <option>='value' [AND ...] ];

        Available common COPY options and defaults:

          DELIMITER=','           - character that appears between records
          QUOTE='"'               - quoting character to be used to quote fields
          ESCAPE='\'              - character to appear before the QUOTE char when quoted
          HEADER=false            - whether to ignore the first line
          NULL=''                 - string that represents a null value

如您所见，默认情况下，选项 NULL =''表示空字符串被视为 null 值。要更改此行为，请将 NULL ='null'或您想要的任何字符设置为空值...

cassandra copy在reimport

1 个答案: