一開始我寫的sql類似於
select COUNT(*) from (select max(inspect_date) as newest_inspect_date,conclusion from single_card where conclusion='合格' and…… group by instrument_card_ID) as a;
大體意思就是從single_card表裏查詢,如果instrument_card_ID相同,則只選inspect_date最新的一項。還要進行篩選要求conclusion爲合格。 最後計算查詢的所有項的數目
上述寫法問題在於,該種寫法是先進行conclusion爲合格的篩選,之後再選取instrument_card_ID相同組中的inspect_date最新項
如果有instrument_card_ID相同組中,最新的一項是不合格(conclusion),在此之前有合格項(conclusion),這樣查詢也會選取conclusion合格項中的inspect_date最新項,而不是inspect_date最新項的conclusion合格項
改寫爲:
select COUNT(a.conclusion) from (select conclusion,ROW_NUMBER() over (PARTITION by instrument_card_ID order by inspect_date desc) as sel_row_num from single_card where …… )a where a.conclusion = '合格' and a.sel_row_num=1;
PARTITION by後面是分組列
order by後面是排序列
where a.conclusion = '合格' and a.sel_row_num=1 則可篩選