How to find the average value of specified field data in mysql

Question

Table price, field category, value 55, average field pi

category，pi
55 100
55 105
56 130
55 95
55 80
59 115
55 101
55 98

How to calculate the average value of field pi (data with category 55)

Answer 1

select avg(pi) as pi_avg from price where category=55