Supertable

Create a Supertable

CREATE STABLE [IF NOT EXISTS] stb_name (create_definition [, create_definition] ...) TAGS (create_definition [, create_definition] ...) [table_options]
 
create_definition:
    col_name column_definition
 
column_definition:
    type_name [PRIMARY KEY] [ENCODE 'encode_type'] [COMPRESS 'compress_type'] [LEVEL 'level_type']

table_options:
    table_option ...

table_option: {
    COMMENT 'string_value'
  | SMA(col_name [, col_name] ...)  
}

More explanations

1. Each supertable can have a maximum of 4096 columns, including tags. The minimum number of columns is 3: a timestamp column used as the key, one tag column, and one data column.
2. Since version 3.3.0.0, besides the timestamp, you can specify another column as primary key using PRIMARY KEY keyword, the column specified using primary key must be type of integer or varchar.
3. The TAGS keyword defines the tag columns for the supertable. The following restrictions apply to tag columns:
   • A tag column can use the TIMESTAMP data type, but the values in the column must be fixed numbers. Timestamps including formulae, such as "now + 10s", cannot be stored in a tag column.
   • The name of a tag column cannot be the same as the name of any other column.
   • The name of a tag column cannot be a reserved keyword.
   • Each supertable must contain between 1 and 128 tags. The total length of the TAGS keyword cannot exceed 16 KB.
4. Regarding how to use ENCODE and COMPRESS, please refer to Encode and Compress for Column.
5. For more information about table parameters, see Create a Table.

View a Supertable

View All Supertables

SHOW STABLES [LIKE tb_name_wildcard];

The preceding SQL statement shows all supertables in the current TDengine database.

View the CREATE Statement for a Supertable

SHOW CREATE STABLE stb_name;

The preceding SQL statement can be used in migration scenarios. It returns the CREATE statement that was used to create the specified supertable. You can then use the returned statement to create an identical supertable on another TDengine database.

View the Supertable Schema

DESCRIBE [db_name.]stb_name;

View tag information for all child tables in the supertable

SHOW TABLE TAGS FROM table_name [FROM db_name];
SHOW TABLE TAGS FROM [db_name.]table_name;

taos> SHOW TABLE TAGS FROM st1;
             tbname             |     id      |         loc          |
======================================================================
 st1s1                          |           1 | beijing              |
 st1s2                          |           2 | shanghai             |
 st1s3                          |           3 | guangzhou            |
Query OK, 3 rows in database (0.004455s)

The first column of the returned result set is the subtable name, and the subsequent columns are the tag columns.

If you already know the name of the tag column, you can use the following statement to get the value of the specified tag column.

taos> SELECT DISTINCT TBNAME, id FROM st1;
             tbname             |     id      |
===============================================
 st1s1                          |           1 |
 st1s2                          |           2 |
 st1s3                          |           3 |
Query OK, 3 rows in database (0.002891s)

It should be noted that DISTINCT and TBNAME in the SELECT statement are essential, and TDengine will optimize the statement according to them, so that it can return the tag value correctly and quickly even when there is no data or a lot of data.

View the tag information of a subtable

taos> SHOW TAGS FROM st1s1;
   table_name    |     db_name     |   stable_name   |    tag_name     |    tag_type     |    tag_value    |
============================================================================================================
 st1s1           | test            | st1             | id              | INT             | 1               |
 st1s1           | test            | st1             | loc             | VARCHAR(20)     | beijing         |
Query OK, 2 rows in database (0.003684s)

Similarly, you can also use the SELECT statement to query the value of the specified tag column.

taos> SELECT DISTINCT TBNAME, id, loc FROM st1s1;
     tbname      |     id      |       loc       |
==================================================
 st1s1           |           1 | beijing         |
Query OK, 1 rows in database (0.001884s)

Drop STable

DROP STABLE [IF EXISTS] [db_name.]stb_name

Note: Deleting a supertable will delete all subtables created from the supertable, including all data within those subtables.

Note：Dropping a supertable doesn't release the disk space occupied by the table, instead all the rows in the table are marked as deleted, so these data will not occur when querying. The disk space will be released when the system automatically performs compact operation or the user performs compact manually.

Modify a Supertable

ALTER STABLE [db_name.]tb_name alter_table_clause
 
alter_table_clause: {
    alter_table_options
  | ADD COLUMN col_name column_type
  | DROP COLUMN col_name
  | MODIFY COLUMN col_name column_type
  | ADD TAG tag_name tag_type
  | DROP TAG tag_name
  | MODIFY TAG tag_name tag_type
  | RENAME TAG old_tag_name new_tag_name
}
 
alter_table_options:
    alter_table_option ...
 
alter_table_option: {
    COMMENT 'string_value'
}

More explanations

Modifications to the table schema of a supertable take effect on all subtables within the supertable. You cannot modify the table schema of subtables individually. When you modify the tag schema of a supertable, the modifications automatically take effect on all of its subtables.

• ADD COLUMN: adds a column to the supertable.
• DROP COLUMN: deletes a column from the supertable.
• MODIFY COLUMN: changes the length of a BINARY or NCHAR column. Note that you can only specify a length greater than the current length.
• ADD TAG: adds a tag to the supertable.
• DROP TAG: deletes a tag from the supertable. When you delete a tag from a supertable, it is automatically deleted from all subtables within the supertable.
• MODIFY TAG: modifies the definition of a tag in the supertable. You can use this keyword to change the length of a BINARY or NCHAR tag column. Note that you can only specify a length greater than the current length.
• RENAME TAG: renames a specified tag in the supertable. When you rename a tag in a supertable, it is automatically renamed in all subtables within the supertable.
• Like odinary tables, the primary key of a supertable cannot be modified or added or deleted using ADD/DROP COLUMN.

Add a Column

ALTER STABLE stb_name ADD COLUMN col_name column_type;

Delete a Column

ALTER STABLE stb_name DROP COLUMN col_name;

Modify the Data Length

ALTER STABLE stb_name MODIFY COLUMN col_name data_type(length);

The preceding SQL statement changes the length of a BINARY or NCHAR data column. Note that you can only specify a length greater than the current length.

Add A Tag

ALTER STABLE stb_name ADD TAG tag_name tag_type;

The preceding SQL statement adds a tag of the specified type to the supertable. A supertable cannot contain more than 128 tags. The total length of all tags in a supertable cannot exceed 16 KB.

Remove A Tag

ALTER STABLE stb_name DROP TAG tag_name;

The preceding SQL statement deletes a tag from the supertable. When you delete a tag from a supertable, it is automatically deleted from all subtables within the supertable.

Change A Tag

ALTER STABLE stb_name RENAME TAG old_tag_name new_tag_name;

The preceding SQL statement renames a tag in the supertable. When you rename a tag in a supertable, it is automatically renamed in all subtables within the supertable.

Change Tag Length

ALTER STABLE stb_name MODIFY TAG tag_name data_type(length);

The preceding SQL statement changes the length of a BINARY or NCHAR tag column. Note that you can only specify a length greater than the current length. (Available in 2.1.3.0 and later versions)

View a Supertable

You can run projection and aggregate SELECT queries on supertables, and you can filter by tag or column by using the WHERE keyword.

If you do not include an ORDER BY clause, results are returned by subtable. These results are not ordered. You can include an ORDER BY clause in your query to strictly order the results.

note

All tag operations except for updating the value of a tag must be performed on the supertable and not on individual subtables. If you add a tag to an existing supertable, the tag is automatically added with a null value to all subtables within the supertable.