不等連線的標量子查詢改寫
前面的例子中，標量子詢查中都是等值的關聯條件，能直接改為left join 或者彙總後再用left join，但遇到不等連線怎麼辦呢？
select a.licenceid,
a.data_source,
a.street,
(select
min(contdata)
from ct
where ct.licenceid = a.licenceid
and ct.data_source = a.data_source
and trunc(contdate) >= a.opendaledate) as mincontdata,
(select
min(buydate)
from ct
where ct.licenceid = a.licenceid
and ct.data_source = a.data_source
and trunc(buydate) >= a.opendaledate) as minbuydate
from a;
要想把兩個量合併，必須要先與a表關聯，這樣才能在取最值前過濾
select a.rowid as rid,
min(case
when trunc(ct.contdate) >= a.opendaledate) then
ct.contdate end)as mincontdate,
min(case
when trunc(ct.buydate) >= a.opendaledate) then
ct.buydate end)as minbuydate
from ct
inner
join a on(ct.licenceid = a.licenceid
and ct.data_source = a.data_source)
group
by a.rowid
到此標量部分搞定，再左聯就可以了
with ct2 as(
select a.rowid as rid,
min(case
when trunc(ct.contdate) >= a.opendaledate) then
ct.contdate end)as mincontdate,
min(case
when trunc(ct.buydate) >= a.opendaledate) then
ct.buydate end)as minbuydate
from ct
inner
join a on(ct.licenceid = a.licenceid
and ct.data_source = a.data_source)
group
by a.rowid)
select a.licenceid,
a.data_source,
a.street,
ct2.mincontdate as mincontdate,
ct2.minbuydate as minbuydate
from a
left
join ct2 on(ct2.rid = a.rowid);
選自《oracle 查詢優化改寫技巧與案例》 有教無類 落落 著
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