Could you explain your question a bit more? I don't yet understand your question fully.
Do you wish to find the your defined PI AF Element categories from one PI AF database via PI SQL conmmander please? If so, please use the PI SQL commander to link the PI AF database which you want. Then, you could see a table, named "Category", under "Asset -> Tables". This table is used to store all categories you defined.
If you search this information on the PI OLEDB Enterprise Users Guide, you will find:
“Elements, element attributes, element templates, and element template attributes can be placed into one or more categories, which helps with organization and searching. Categorization is captured by ElementCategory, ElementAttributeCategory, ElementTemplateCategory, and ElementTemplateAttributeCategory tables which refer to the global category table as well as to the element, element attribute, element template, and element template attribute tables (M-N relationship decompositions).”
Hence, all categories are stored on the Category table as Xi has already pointed out. Nevertheless, for instance, the ElementAttributeCategory table which has two columns (CategoryID and ElementAttributeID) is the table responsible for linking an ElementAttribute with its proper category.
Hope this helps!!