how to import data from other sources (sql database, excel) ?

Thanks in advance,

Depending on the data your easiest option is most likely going to be export to a text file in the Outline format and then copy and paste as outline. 

Outline Format = 

    + Hyperlink Path
    - Note Line
    - Second Line.
    Child Thought
        + Hyperline Path
        -Note Line

Other more complex options which would also allow you to include attachments and note formatting would be xml (TheBrain 8) and JSON (TheBrain 9). You can search the forum for topics on each of the respective formats required for these, although TheBrain 9 doesn't have as much official documentation on it (but there is an excellent topic that can be found in the Beta forum I believe).
macOS 10.13
TheBrain 9.0.250

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