Find attribute_id
SELECT * FROM eav_attribute where attribute_code = 'is_anchor'
Update all of them with anchor_id from above (usually is ID 51)
UPDATE `catalog_category_entity_int` set value = 1 where attribute_id = 51
FWIW: I (@rondy) am not the creator of the content shared here, which is an excerpt from Edmond Lau's book. I simply copied and pasted it from another location and saved it as a personal note, before it gained popularity on news.ycombinator.com. Unfortunately, I cannot recall the exact origin of the original source, nor was I able to find the author's name, so I am can't provide the appropriate credits.