create table holdings (
institution_id int,
symbol varchar(32),
shares int,
quarter varchar(32)
);
insert into holdings values
(1, 'AAPL', 0, '2020_Q4'),
(2, 'TSLA', 200000, '2020_Q4'),
(1, 'AAPL', 100000, '2020_Q3');
select * from (
select
holdings.*,
lead(shares) over (partition by institution_id, symbol order by quarter desc) previous_quarter
from holdings
where symbol = 'AAPL'
) copmarsion
where shares < coalesce(previous_quarter, 0);