tl;dr
UPDATE my_table
SET
-- ↓ possible values are 1 ~ 190
country_id = FLOOR(1+ rand()*190),
-- ↓ you also can use RAND and CASE instead of ELT()
favorite_fluit = ELT(FLOOR(1+ rand()*3), 'orange', 'apple', 'strawberry');
-- ↓ 50%
WHERE RAND() < 0.5
;
ref: