mysql问题积累示例

1.mysql 查询字段为空用is null。

select * from drqq where advice_id is null;

2.修改mysql的字段为空用null
update drqq set
advice_id=null where id='9';

3.mysql查询不为null的数据
select * from drlist
where advice_id is not null;

4.mysql的单行注释用#

5.mysql将null转换为0
select
name,ifnull(sum(visitstate=0),0) as pv0 from work

6.mysql将null转换掉

model.Dqq =
Convert.IsDBNull(dr["dqq"]).ToString();
model.Visitstate =
Convert.ToInt32(Convert.IsDBNull(dr["visitstate"]));
//去除null

7.mysql创建多表视图
drop view
work;//删除视图

create view work
as
select
advice.name,advice.relation,advice.type,advice.dltstate,drqq.dqq,patient.visitstate
from advice
left join drqq on drqq.advice_id=advice.id
left join patient on patient.dr_id=drqq.id;//创建视图

8.mysql视图查询

select name,count(distinct dqq) as dsum,count(visitstate) as
psum,ifnull(sum(visitstate=0),0) as pv0,ifnull(sum(visitstate=1),0)
as pv1 from work
where relation=3 and type=4 and dltstate=1
group by name;