Create an external table|V4.2.2| docs|Distributed Database

Create an external table

Last Updated：2025-12-02 02:50:56 Updated

You can execute the CREATE EXTERNAL TABLE statement to create an external table. When you create an external table, you must specify the path to the data file and the format of the data file to read data from external files.

Privilege requirements

To create an external table, the current user must have the CREATE privilege. To view the privileges of the current user, see View user privileges.

Create an external table

The SQL statement for creating an external table is as follows:

CREATE EXTERNAL TABLE table_name
( col_name col_type [AS (metadata$filecol{N})] 
  [ , col_name col_type [AS (metadata$filecol{N})] ]
  [ , ... ] )
LOCATION = '<string>'
FORMAT = (
  TYPE = 'CSV'
  LINE_DELIMITER = '<string>' | <expr>
  FIELD_DELIMITER = '<string>' | <expr>
  ESCAPE = '<character>' | <expr>
  FIELD_OPTIONALLY_ENCLOSED_BY = '<character>' | <expr>
  ENCODING = 'charset'
  NULL_IF = ('<string>' | <expr>, '<string>' | <expr> ...)
  SKIP_HEADER = <int>
  SKIP_BLANK_LINES = { TRUE | FALSE }
  TRIM_SPACE = { TRUE | FALSE }
  EMPTY_FIELD_AS_NULL = { TRUE | FALSE }
)
[ PATTERN = '<regex_pattern>' ]

The parameters are described as follows:

col_name col_type [AS (metadata$filecol{N})]: defines a column. You can use AS (metadata$filecol{N}) to manually define column mapping.

The columns supported by external tables are the same as those supported by regular tables. For more information about the data types supported in MySQL mode of OceanBase Database, see Overview.

By default, the columns in an external file are automatically and sequentially mapped to the columns defined in the external table. In other words, the first column in the external table is mapped to the first column in the external file, the second column in the external table is mapped to the second column in the external file, and so on.

For example, in the following example, the C1 column in the ext_t1 external table is automatically mapped to the first column in the external file; and the C2 column is automatically mapped to the second column in the external file.
```
CREATE EXTERNAL TABLE ext_t1 (
  C1 int,
  C2 int
  )
  LOCATION = 'oss://$ACCESS_ID:$ACCESS_KEY@$HOST/tpch_1g_data/lineitem/'
  FORMAT = (
  TYPE = 'CSV'
  FIELD_DELIMITER = '|'
  );
```
If the columns in the external file are in a different order from those in the external table, you can use a pseudocolumn in the format of metadata$filecol{N} to specify that the Nth column in the external table is mapped to the Nth column in the external file. Note that the columns in the file are numbered starting from 1.

For example, in the following example, C1 int AS (metadata$filecol2) indicates that the C1 column in the ext_t2 external table is mapped to the second column in the file; and C2 int AS (metadata$filecol4) indicates that the C2 column in the ext_t2 external table is mapped to the fourth column in the external file.
```
CREATE EXTERNAL TABLE ext_t2 (
            C1 int AS (metadata$filecol2),
            C2 int AS (metadata$filecol4)
  )
  LOCATION = 'oss://$ACCESS_ID:$ACCESS_KEY@$HOST/tpch_1g_data/lineitem/'
  FORMAT = (
  TYPE = 'CSV'
  FIELD_DELIMITER = '|'
  );
```
Notice

If you need to manually define column mapping, the automatic column mapping feature will fail and you will need to manually define the mapping for all columns.
LOCATION = '<string>': specifies the path where the external file is stored. Generally, the data files of an external table are stored in a dedicated directory, which can contain subdirectories. When you create an external table, the system automatically collects all files in the directory.

Two formats are supported:
- Local location format: LOCATION = '[file://] local_file_path'
  - local_file_path: can be a relative path or an absolute path. If you enter a relative path, the current directory must be the installation directory of OceanBase Database. In an absolute path, the drive letter and colon must be specified.
    
    Notice
    
    In the local location format, local_file_path must be the path of a directory and cannot be the path of a file. If you want to specify a single file, you can specify the path of its parent directory in the LOCATION parameter and use the PATTERN parameter to specify the file.
  - For the local location format, if you use the system variable secure_file_priv to specify the path where OceanBase Database can access, secure_file_priv must be the path of a directory that is the parent directory of local_file_path. In other words, local_file_path must be a subdirectory of secure_file_priv.
    
    The system variable secure_file_priv is used to specify the path that OceanBase Database can access when you import data to or export data from a file. For more information about the secure_file_priv variable, see secure_file_priv.
- Remote location format: LOCATION = '{oss|cos}://$ACCESS_ID:$ACCESS_KEY@$HOST/remote_file_path'
  
  $ACCESS_ID, $ACCESS_KEY, and $HOST are the access information required to access Alibaba Cloud OSS or Tencent Cloud COS. These sensitive access information is stored in encrypted form in the system tables of the database.
  
  Notice
  
  When you use an object storage path, separate the parameters in the object storage path with the & symbol. Make sure that the values of the parameters you entered contain only uppercase and lowercase letters, numbers, and /-_$+=. If you enter any other characters, the setting may fail.
FORMAT: specifies the format of external files.
- TYPE: specifies the type of external files. Only CSV files are supported.
- LINE_DELIMITER: specifies the line delimiter of a file. If this parameter is not specified, the default value LINE_DELIMITER='\n' is used.
- FIELD_DELIMITER: specifies the column delimiter of a file. If this parameter is not specified, the default value FIELD_DELIMITER='\t' is used.
- ESCAPE: specifies the escape character of a file. For example, ESCAPE ='*' indicates that the asterisk (*) is the escape character, which replaces the default escape character (). If this parameter is not specified, the default value ESCAPE ='\' is used.
- FIELD_OPTIONALLY_ENCLOSED_BY: specifies the characters that enclose field values in a file. For example, ESCAPE = '"' indicates that field values are enclosed in double quotation marks. If this parameter is not specified, the default value is an empty string.
- ENCODING: specifies the character set encoding format of a file. For more information about the character sets supported in MySQL mode of OceanBase Database, see Character sets. If this parameter is not specified, the default value UTF8MB4 is used.
- NULL_IF: specifies the strings that are treated as NULL values. If this parameter is not specified, the default value is an empty string.
- SKIP_HEADER: specifies the number of lines to skip in the header. If this parameter is not specified, the default is to skip no lines.
- SKIP_BLANK_LINES: specifies whether to skip blank lines. If this parameter is not specified, the default is FALSE.
- TRIM_SPACE: specifies whether to remove leading and trailing spaces from fields in the file. If this parameter is not specified, the default is FALSE.
- EMPTY_FIELD_AS_NULL: specifies whether an empty string is treated as NULL. If this parameter is not specified, the default is FALSE.
PATTERN: specifies a regular pattern string to filter files in the directory specified in LOCATION. For each file in the directory specified in LOCATION, if the file matches the pattern string, the external table can access the file. Otherwise, the external table will skip the file. If this parameter is not specified, the external table can access all files in the directory specified in LOCATION by default.

Assume that a file named data.csv exists in the /home/admin/oceanbase/ directory on the local server. The file contains the following data.

1,"lin",98
2,"hei",90
3,"ali",95

On the OBServer node, the tenant administrator connects to the MySQL tenant of the cluster through the local Unix socket.

Here is an example of a connection:
```
obclient -S /home/admin/oceanbase/run/sql.sock -uroot@sys -p********
```
The specific procedure for connecting to OceanBase Database through the local Unix socket is described in secure_file_priv.
Configure the path /home/admin/oceanbase/ that OceanBase Database can access.
```
SET GLOBAL secure_file_priv = "/home/admin/oceanbase/";
```
After the command is executed, you need to restart the session for the modification to take effect.

Reconnect to the database and create an external table named ext_t3.

CREATE EXTERNAL TABLE ext_t3(id int, name char(10),score int)
LOCATION = '/home/admin/oceanbase/'
FORMAT = (
TYPE = 'CSV'
FIELD_DELIMITER = ','
FIELD_OPTIONALLY_ENCLOSED_BY ='"'
)
PATTERN = 'data.csv';

After the external table is created, you can use the SHOW CREATE TABLE statement to view the table definition, just like you do with a regular table.

SHOW CREATE TABLE ext_t3;

The query result is as follows:

+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table  | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ext_t3 | CREATE EXTERNAL TABLE `ext_t3` (
  `id` int(11) GENERATED ALWAYS AS (metadata$filecol1),
  `name` char(10) GENERATED ALWAYS AS (metadata$filecol2),
  `score` int(11) GENERATED ALWAYS AS (metadata$filecol3)
)
LOCATION='file:///home/admin/oceanbase/'
PATTERN='data.csv'
FORMAT (
  TYPE = 'CSV',
  FIELD_DELIMITER = ',',
  FIELD_OPTIONALLY_ENCLOSED_BY = '"',
  ENCODING = 'utf8mb4'
)DEFAULT CHARSET = utf8mb4 ROW_FORMAT = DYNAMIC COMPRESSION = 'zstd_1.3.8' REPLICA_NUM = 1 BLOCK_SIZE = 16384 USE_BLOOM_FILTER = FALSE TABLET_SIZE = 134217728 PCTFREE = 0 |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set

You can also access the external table, just like you do with a regular table. When you query an external table, the system reads the external file through the driver of the external table and parses the file according to its format. Then, the system converts the data into internal data types of OceanBase Database and returns the data rows. Here is an example of how to query the external table lineitem that is created just now.

SELECT * FROM ext_t3;

The query result is as follows:

+----+------+-------+
| id | name | score |
+----+------+-------+
|  1 | lin  |    98 |
|  2 | hei  |    90 |
|  3 | ali  |    95 |
+----+------+-------+
3 rows in set

In addition, external tables can be queried together with regular tables. Assume that the current database contains a regular table named info, whose data is as follows:

+------+--------+------+
| name | sex    | age  |
+------+--------+------+
| lin  | male   |    8 |
| hei  | male   |    9 |
| li   | female |    8 |
+------+--------+------+
3 rows in set

Here is an example of how to query the external table ext_t3 together with the regular table info.

SELECT info.* FROM info, ext_t3 WHERE info.name = ext_t3.name AND ext_t3.score > 90;

The query result is as follows:

+------+--------+------+
| name | sex    | age  |
+------+--------+------+
| lin  | male   |    8 |
| li   | female |    8 |
+------+--------+------+
2 rows in set

For more information about query operations, see Read data.

Considerations

An external table can only be queried, and you cannot perform DML operations on it.
When you query an external table, if the external file accessed by the table is deleted, the system does not return an error, but instead returns an empty result set.
If the external storage system fails, an error is returned when you query the external table, because the file managed by the external storage system cannot be accessed.

Next steps

When you create an external table, the system saves the file list that matches the specified PATTERN and is located at the path specified in the LOCATION clause to a system table of OceanBase Database. The system uses this file list to locate external files when the system scans the external table. If you add new files to the external directory, you must perform an operation to add the new files to the file list of the external table. For more information, see Manage external files of an external table.

You can use the DROP TABLE statement to drop an external table, just like you drop a regular table. For more information, see Drop a table.

Create an external table

Privilege requirements

Create an external table

Notice

Notice

Notice

Considerations

Next steps

References