实际价格是卖出价减去买入价乘以卖出数量。

select a.`security`,
case when b.side is not null
then (b.price-a.price) * b.quantity
else '' end "realized_p&l"
from tbl a
left join tbl b
on a.`security`=b.`security` and b.side='SELL'
where a.side='BUY' 

result: 
security    realized_p&l
MSFT    -1000
AAPL