Oracle left join 查询 结果偏少

select t.instanceid,

t1.related_ne6,

t1.related_ne5,

t1.school_name,

t.cover_type,

'LTE' as net_type,

t1.resname,

t1.lac,

t1.ci,

t1.cell_id,

'重庆' as area,

'' as elses

from temp_school_zhc t1

left join uia_date_eutrancell t

on t1.ci||t1.cell_id=t.enodeb_id||t.eutrancell_id

/* t.enodeb_id = t1.ci

and t.eutrancell_id = t1.cell_id*/

where t.uia_time_id = 20150526

and t1.instanceid is null;

--and t1.ci||t1.cell_id is null;

--and t1.ci||t1.cell_id=160712

temp_school_zhc 表中instanceid为空的有2405条，可是查询结果之后只有2155条。left join之后为什么查询出来的数据偏少，原因为 where子句将left join之后的结果过滤掉了一部分。更改为如下之后查询的结果变为2405条了。

select t.instanceid,

t1.related_ne6,

t1.related_ne5,

t1.school_name,

t.cover_type,

'LTE' as net_type,

t1.resname,

t1.lac,

t1.ci,

t1.cell_id,

'重庆' as area,

'' as elses

from (select * from temp_school_zhc t where t.instanceid is null) t1

left join (select * from uia_date_eutrancell t where t.uia_time_id = 20150526) t

on t1.ci||t1.cell_id=t.enodeb_id||t.eutrancell_id

/* t.enodeb_id = t1.ci

and t.eutrancell_id = t1.cell_id*/

-- where t.uia_time_id = 20150526

--and t1.ci||t1.cell_id is null;

--and t1.ci||t1.cell_id=332111

虽然按照oracle的方式，where语句从下往上执行，可是执行的结果还是按照人的思维从上往下执行。