This topic describes how to create a mapping table for a data table, query all mapping tables of a data table in an instance, query information about a mapping table, and query data in a table in the SQL mode.

Enter the SQL mode

Run the sql command to enter the SQL mode.

Create a mapping table

To query data in a data table, you must create a mapping table for the data table.

The following sample code shows how to create a mapping table for the data table named mytable:

CREATE TABLE mytable(
     `uid` VARCHAR(1024),
     `pid` BIGINT(20),
     `b` DOUBLE,
     `c` BOOL,
     `d` MEDIUMTEXT,
     PRIMARY KEY(`uid`,`pid`)
);

Query mapping tables

Query the mapping tables of a data table in an instance.

Run the SHOW TABLES; command to query the mapping tables of a data table in an instance.

The following result is returned:
+----------------------+
| Tables_in_myinstance |
+----------------------+
| mytable              |
+----------------------+
| mytstable            |
+----------------------+
| mytstable::meta      |
+----------------------+

In the preceding command output, mytable is the data table for which mapping tables are created, mytstable is a time series data table, and mytstable::meta is a time series metadata table.

Query information about a mapping table

Query information about a mapping table.

  • Command syntax
    DESCRIBE table_name;
  • Examples

    The following sample code shows how to query the information about the mapping table named mytable:

    DESCRIBE mytable;
    The following result is returned:
    +-------+---------------+------+-----+-------+
    | Field | Type          | Null | Key | Extra |
    +-------+---------------+------+-----+-------+
    | uid   | varchar(1024) | NO   | PRI |       |
    +-------+---------------+------+-----+-------+
    | pid   | bigint(20)    | NO   | PRI |       |
    +-------+---------------+------+-----+-------+
    | b     | double        | YES  |     |       |
    +-------+---------------+------+-----+-------+
    | c     | tinyint(1)    | YES  |     |       |
    +-------+---------------+------+-----+-------+
    | d     | mediumtext    | YES  |     |       |
    +-------+---------------+------+-----+-------+

Delete a mapping table

If changes are made to the attribute columns of a data table, you can delete the mapping table of the data table and create a new mapping table.

  • Command syntax
    DROP MAPPING TABLE table_name;
  • Example

    The following sample code shows how to delete the mapping table named mytable:

    DROP MAPPING TABLE mytable;

Query data in a table

Query data in a table by executing the SELECT statement.

The following sample code shows how to query all data in the table named mytable:
SELECT * FROM mytable;

Exit the SQL mode

Run the exit; command to exit the SQL mode.