SQL查询中的列数

Question

I want to put COUNT(item_id) in this statement:

SELECT * FROM `notifications` WHERE `uid` = '3' AND `seen` = '0' AND id IN (
SELECT MAX(id), COUNT(item_id)
FROM `notifications`
GROUP BY item_id
)  ORDER BY id DESC

But This error occurred: Operand should contain 1 column(s).

Table:

[id] [uid] [item_id] [seen]
 1     3       69       0
 2     3       69       0
 3     3       70       0
 4     3       69       0
 5     3       70       0
 6     3       69       0

Expected output: (Order BY id DESC) where 69 is the last record.

[item_id] [num]
    69      4
    70      2

Answer 1

An educated guess says that you want a JOIN :

SELECT n.*, nmax.cnt
FROM notifications n JOIN
     (SELECT item_id,  MAX(id) as max_id, COUNT(item_id) as cnt
      FROM notifications
      GROUP BY item_id
     ) nmax
     ON n.item_id = nmax.item_id AND nmax.id = nmax.max_id
WHERE n.uid = 3 AND n.seen = 0  -- removed the single quotes because these are probably numbers
ORDER BY n.id DESC;

It is unclear whether you want the filtering conditions in the subquery as well.

Answer 2

Given your sample data and expected results, there's no need for a subquery:

select item_id, count(*)
from notifications
group by item_id
where uid = 3 and seen = 0
order by max(id) desc;

• Sample Demo