OECD implement category tree
Here is a list of themes and datasets. The attached file describes the hierarchy of the current .Stat themes, the IDParentTheme column shows the parent of each theme. The Datasets worksheet show which dataset is attached to each theme.
The Order column on the Themes worksheet defines the order of the theme within its parent attachment. When an IDTheme is the same as its IDParentTheme it is a top-level Theme.
Warning: avoid having a big list of datasets at the root level, related to tables or figures ("tabela" something...) – prefer to have them in a dedicated category (maybe the "overview" category), or else do not take them into account.