Menu by itself is a taxonomy in WordPress. It means that you can find all menus in
wp_terms table, by running following query:
FROM wp_terms AS t
LEFT JOIN wp_term_taxonomy AS tt ON tt.term_id = t.term_id
WHERE tt.taxonomy = 'nav_menu';
Menu item is custom post type in WordPress. They are stored in
wp_posts table. You can find all of them by using this query:
WHERE post_type = 'nav_menu_item';
Relations between menus and menu items are stored in
wp_term_relationships table. To find all items of specific menu you can use this query:
FROM wp_posts AS p
LEFT JOIN wp_term_relationships AS tr ON tr.object_id = p.ID
LEFT JOIN wp_term_taxonomy AS tt ON tt.term_taxonomy_id = tr.term_taxonomy_id
WHERE p.post_type = 'nav_menu_item'
AND tt.term_id = /*your menu ID*/;
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