一个selfjoin我有一个visits
表看起来像这样:选择的第一个孩子记录在TSQL
id identity(1,1) not null,
visit_date datetime not null,
patient_id int not null,
flag bit not null
对于每一个记录,我需要找到一个匹配的记录是同时或更早,具有相同的patient_id,并具有flag
设置为1。我现在正在做的是:
select parent.id as parent_id,
(
select top 1
child.id as child_id
from
visits as child
where
child.visit_date <= parent.visit_date
and child.patient_id = parent.patient_id
and child.flag = 1
order by
visit_date desc
) as child_id
from
visits as parent
所以,此查询工作正常,但它运行太慢了 - 我怀疑这是因为子查询。是否有可能将其重写为联接查询?