用分析函式優化標量子查詢

原語句如下

select ii.*,

case
when (select count(1)
from ii
where ii.id > 0
and ii.flag = 2
and ii.i_code = ii.i_code
and ii.c_id not in
(select c_id
from c
where ig_name like '%停用%')) > 1 then
2else
1end as mulinv
from ii
where (ii.id > 0 and itemdesc like :1 and ii.isphantom <> :2)
and ii.c_id = :3
order by ii.i_code, ii.i_name, ii. d_id

經詢問 c.c_id是主鍵，這樣就可以改寫為left join

select *

from (select ii.*,
case
/*用分析函式代替標量自聯接*/
when (sum(case when flag = 2 and c.c_id is null then 1 end) over(partition by ii.i_code)) > 1 then
2else
1end as mulinv
from ii
/*因c.cid為主鍵，所以可改為left join而不必擔心主查詢資料會翻倍*/
left join c on (c.c_id = ii.c_id and c.ig_name like '%停用%')
/*為了保證分析函式視窗內資料與原標量範圍一致，這兒的過濾條件要保持一致*/
where ii.id > 0) 
/*提取出原標量所需資料後再應用其它的過濾條件*/
where itemdesc like :1
and ii.isphantom <> :2
and ii.c_id = :3
order by ii.i_code, ii.i_name, ii. d_id;

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