Default category got deleted. How to get it back as ID=1?

You need 3 steps:

1.Creating a term in wp_terms with ID is 1

2.Insert a term_taxanomy into wp_term_taxonomy with term_id is 1, and taxonomy must be category

3.Insert a term_relationships into wp_term_relationships with object_id is 1 and term_taxonomy_id is 1
Run below code in PHPMYADMIN to re-create this category!

INSERT INTO wp_terms (term_id, `name`, slug) VALUES (1, 'Uncategorized', 'uncategorized');
INSERT INTO wp_term_taxonomy (term_taxonomy_id, term_id, taxonomy, parent) VALUES (1, 1, 'category', 0);
INSERT INTO wp_term_relationships (object_id, term_taxonomy_id, term_order) VALUES (1, 1, 0);