MySQL查询从另一个表中获取单个项目

Question

I have two tables- 1. Feed 2. Seller

Feed table is like this

seller_id       item_id     end_date
=========       =======     =========
1               1           2017-02-12[A future date]
1               2           2017-03-12[A future date]
1               3           2017-01-12[A past date]
2               12          2017-02-12[A future date]
3               123         2017-01-10[A past date]

Seller table is like this

seller_id
=========
1
2
3

I like to get the result like this where end_date is greater than or equal to Current Date and I need only a single item_id for each seller_id-

seller_id       item_id
=========       =======
1               1 or 2
2               12

And my SQL Query is like-

SELECT S.seller_id, R.item_id FROM Feed F, Seller S WHERE S.seller_id=F.seller_id AND F.end_date>=CURDATE() GROUP BY S.seller_id LIMIT 1;

Answer 1

Check this..

SELECT S.seller_id, MAX(R.item_id) AS item_id 
    FROM Feed F, Seller S 
    WHERE S.seller_id=F.seller_id 
    AND F.end_date>=CURDATE() 
    GROUP BY S.seller_id;

Answer 2

You can use min() , or max() , or even any_value() aggregate function with a group by clause to select a item_id per seller:

SELECT S.seller_id, max(R.item_id)
FROM Feed F
INNER JOIN Seller S on S.seller_id=F.seller_id
WHERE F.end_date>=CURDATE()
GROUP BY S.seller_id

It depends on your exact business requirements which function to use.

Answer 3

You only need the table Feed, you will group by id_seller after having selected items with enddate greater than current date. Then select a unique item_id by some aggregate function of your choice ( max() here). Here is an exemple of your request :

SELECT
    F.seller_id,
    max(F.itemid)
FROM
    Feed F
WHERE
    F.end_date > CURRENT_DATE

GROUP BY
    F.seller_id