database – I want a more concise data form than a table

Instead of opting for a complex nested array, I decided that it might be better to keep it than a simple set of two-level tables.

                $ x = array (
101 => [ (int $parent_id), (bool $completed)  ],
102 => [ (int $parent_id), (bool $completed)  ],
103 => [ (int $parent_id), (bool $completed)  ],
104 => [ (int $parent_id), (bool $completed)  ],
...
)

I realized that I was looking for something simpler, like what python offers with its data type "list". I think it's as simple as I could get with the storage of a disordered list in PHP / MySQL. Someone has something better?