mysql 在查询和更新sql中都可以进行判断，下面根据我这两条sql看下case when和ifnull的用法：

• case when

update t_tradefee set margin_profit = (case when market_price is null 
then (2.3-basic_fee)*remain_amount 
else  (2.3-market_price)*remain_amount+margin_profit END),
market_price = 2.3 where substring(cardno,1,3)='001' and trade_type = 1

更新过程中的判断，如果 market_price 为空则set margin_profit的值为 (2.3-basic_fee)*remain_amount ，否则 set margin_profit的值为 2.3-market_price)*remain_amount+margin_profit

用法：case when… else… end

• ifnull

select day(op_time) as day,sum(price) as price,
IFNULL((select sum(price) from t_margin_profit where op_time<'2016-1-1 00:00:00' ),0) as old
 from t_margin_profit 
 where year(op_time)='2016' and month(op_time)='1' GROUP BY day(op_time)