Syntax
------

COLUMN_ADD(dyncol_blob, column_nr, value [as type], [column_nr, value [as
type]]...);
COLUMN_ADD(dyncol_blob, column_name, value [as type], [column_name, value [as
type]]...);

Description
-----------

Adds or updates dynamic columns.

* dyncol_blob must be either a valid dynamic columns blob (for example,
COLUMN_CREATE returns such blob), or an empty string.
* column_name specifies the name of the column to be added. If dyncol_blob
already has a column with this name, it will be overwritten.
* value specifies the new value for the column.  Passing a NULL value will
cause the column to be deleted.
* as type is optional. See #datatypes section for a discussion about types.

The return value is a dynamic column blob after the modifications.

Examples
--------

UPDATE t1 SET dyncol_blob=COLUMN_ADD(dyncol_blob, "column_name", "value")
WHERE id=1;

Note: COLUMN_ADD() is a regular function (just like CONCAT()), hence, in order
to update the value in the table you have to use the UPDATE ... SET
dynamic_col=COLUMN_ADD(dynamic_col, ....) pattern.

URL: https://mariadb.com/kb/en/column_add/