如何匹配列数，如果大于 2，则更新 postgresql 中的表

Question

输入表
https://i.stack.imgur.com/eH6W3.jpg
我想要的输出表
https://i.stack.imgur.com/RsCb9.jpg
我试过这种方式

update tb set Exception = 'Y'
select Card_No from tb
where Card_No in ( select Card_No from (select Card_No count(Left(Card_No,6)) from tb
group by Card_No having count(*)>=2)a);

上面的查询给了我这样的操作
https://i.stack.imgur.com/vJ47z.jpg

Answer 1

你可以这样做：

update mytable 
set exception = case when cnt > 1 then 'Y' else 'N' end
from (
    select substring(card_no, 1, 6) sub_card_no, count(*) cnt 
    from mytable
    group by 1
) t
where t.sub_card_no = substring(mytable.card_no, 1, 6)

使用窗口函数可能会更有效率：

update mytable 
set exception = case when cnt > 1 then 'Y' else 'N' end
from (
    select card_no, count(*) over(partition by substring(card_no, 1, 6)) cnt 
    from mytable
) t
where t.card_no = mytable.card_no