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Dynamic columns feature allows to store different sets of columns for each row in the table.

It is targeted at a case when one needs to store entities that may have many different attributes (like size, color, weight, etc). If the set of possible attributes is very large, or not known, one can't create a regular column for each attribute. In this case, dynamic columns can be used.

Introduction

Dynamic columns works by storing the columns in a blob and having a small set of functions to manipulate it.

The table can be created like so:

create table assets (
  item_name     varchar(40), -- Name is a common attribute for all items

  dynamic_cols  blob,        -- A blob column that will be used as a storage 
                             -- for dynamic columns.

  primary key(item_name)     -- not required, but it's a good practice to 
                             -- have a primary key
);

Then, one can insert data like so:

insert into assets values 
  ('MariaDB T-shirt', COLUMN_CREATE('color', 'blue', 'size', 'XL')),
  ('Thinkpad Laptop', COLUMN_CREATE('color', 'black', 'price', '500'));

note: the example uses MariaDB 10.0.1. In MariaDB 5.3, only numbers could be used. See [#mariadb-5.3] for details

And query it like so:

select item_name, COLUMN_GET(dynamic_cols, 'color' as char) as color from assets;
+-----------------+-------+
| item_name       | color |
+-----------------+-------+
| MariaDB T-shirt | blue  |
| Thinkpad Laptop | black |
+-----------------+-------+