The Dynamic columns feature was introduced in MariaDB 5.3.
COLUMN_DELETE(dyncol_blob, column_nr, column_nr...); COLUMN_DELETE(dyncol_blob, column_name, column_name...);
Deletes a dynamic column with the specified name. Multiple names can be given. The return value is a dynamic column blob after the modification.
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https://mariadb.com/kb/en/column_delete/