Create an external table|V4.3.1| docs|Distributed Database

Create an external table

Last Updated：2025-12-04 07:03:00 Updated

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

Privileges required

To create an external table, the current user must have the CREATE privilege. For more information about how to view the privileges of the current user, see View user privileges.

Create an external table

The following example shows the syntax for creating an external table:

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 in the syntax are described as follows:

col_name col_type [AS (metadata$filecol{N})]: specifies the columns of the external table. You can use AS (metadata$filecol{N}) to manually specify column mapping.

The column types 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 and detailed descriptions, see Data types.

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

For example, in the following example, C1 int AS (metadata$filecol2) indicates that the C1 column in the ext_t2 external table corresponds to the second column in the file; C2 int AS (metadata$filecol4) indicates that the C2 column in the ext_t2 external table corresponds 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 want to manually specify column mapping, automatic column mapping will fail and you must manually specify the mapping for all columns.
LOCATION = '<string>': specifies the path where the external file is stored. Typically, the data files of an external table are stored in a dedicated directory. The directory can contain subdirectories. When you create an external table, the system automatically collects all files in the directory.

The following 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 the following example, local_file_path is an absolute path, but in practice, it can also be a relative path.
    
    Notice
    
    In the local location format, local_file_path must be a directory rather than a file. If you want to specify a single file, you need to specify the parent directory of the file in the LOCATION parameter and specify the file itself by setting the PATTERN parameter.
  - In scenarios that use the local location format, when you configure the file path that OceanBase Database can access by using the secure_file_priv system variable, you must set secure_file_priv to a directory that is the upper directory of local_file_path, namely, local_file_path must be a subdirectory of secure_file_priv.
    
    The secure_file_priv system variable specifies the path that OceanBase Database can access when importing data to or exporting data from a file. For more information about secure_file_priv, 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 the system tables of the database in an encrypted form.
  
  Notice
  
  If you use an object storage path, separate the parameters of the path with &. Make sure that the values of the parameters you enter contain only uppercase and lowercase English letters, digits, and /-_$+=. If you enter characters other than the preceding ones, the setting may fail.
FORMAT: specifies the format of the external file.
- TYPE: specifies the type of the external file. Only CSV files are supported.
- LINE_DELIMITER: specifies the line delimiter of the file. If you do not specify this parameter, the default value LINE_DELIMITER='\n' takes effect.
- FIELD_DELIMITER: specifies the field delimiter of the file. If you do not specify this parameter, the default value FIELD_DELIMITER='\t' takes effect.
- ESCAPE: specifies the escape character of the file. For example, ESCAPE ='*' specifies to use an asterisk (*) as the escape character, replacing the default escape character (). If you do not specify this parameter, the default value ESCAPE ='\' takes effect.
- FIELD_OPTIONALLY_ENCLOSED_BY: specifies the characters that enclose the field values in the file. For example, ESCAPE = '"' specifies to enclose the field values in double quotation marks. If you do not specify this parameter, the default value takes effect.
- ENCODING: specifies the character set encoding format of the file. For more information about the character sets supported in MySQL mode of OceanBase Database, see Character sets. If you do not specify this parameter, the default value UTF8MB4 takes effect.
- NULL_IF: specifies the strings that are to be treated as NULL values. If you do not specify this parameter, the default value takes effect.
- SKIP_HEADER: specifies the number of lines to skip in the file header. If you do not specify this parameter, the file header is not skipped by default.
- SKIP_BLANK_LINES: specifies whether to skip blank lines. If you do not specify this parameter, the default value FALSE takes effect.
- TRIM_SPACE: specifies whether to remove leading and trailing spaces from fields in the file. If you do not specify this parameter, the default value FALSE takes effect.
- EMPTY_FIELD_AS_NULL: specifies whether to treat empty strings as NULL values. If you do not specify this parameter, the default value FALSE takes effect.
PATTERN: specifies the regular pattern string to filter files in the directory specified by LOCATION. For each file in the directory specified by LOCATION, if the file name matches the pattern string, the external table can access the file. Otherwise, the external table skips the file. If you do not specify this parameter, the external table can access all files in the directory specified by LOCATION by default.

Assume that a file named data.csv is stored in the /home/admin/oceanbase/ directory on your local machine, and 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 tenant to which the external table belongs by using the local Unix socket.

The following example shows how to establish such a connection:
```
obclient -S /home/admin/oceanbase/run/sql.sock -uroot@sys -p********
```
For more information about how to connect to OceanBase Database using a local Unix socket, see secure_file_priv.
Configure the path /home/admin/oceanbase/ to allow access to the database.
```
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 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 based on the file format. Then, the system converts the data into internal data types of OceanBase Database and returns the data rows. Here is an example of querying 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

You can also combine an external table with a regular table for queries. Assume that a regular table named info exists in the current database, and the table contains the following data:

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

Here is an example of combining the external table ext_t3 with the regular table info for queries.

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 queries, 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.
The external file system manages the files accessed by an external table. If the external storage system is unavailable, an error is returned when you query the external table.

Next steps

When you create an external table, the system saves the file list that matches the PATTERN in the LOCATION specified in the LOCATION parameter in the system table of OceanBase Database. During a scan, the system accesses external files based on this file list. If other files are added to the external directory, you must perform an operation to update the external table to add the new files to the file list of the external table. For more information, see External file management.

After you create an external table, you can drop it. The statement for dropping an external table is the same as that for dropping a normal table. You can execute the DROP TABLE statement to drop an external table. For more information, see Drop a table.

Create an external table

Privileges required

Create an external table

Notice

Notice

Notice

Considerations

Next steps

References