Question

所以我有一张下表，

product_id      sender_id      timestamp      ...other columns...
1               2              1222
1               2              3423
1               2              1231
2               2              890
3               4              234
2               3              234234

我想获取sender_id = 2的行，但是我想对product_id进行计数和分组，并按时间戳降序进行排序。这意味着我需要以下结果

product_id      sender_id      timestamp      count      ...other columns...
1               2              3423           3
2               2              890            1

我尝试了以下查询：

SELECT product_id, sender_id, timestamp, count(product_id), ...other columns...
FROM table
WHERE sender_id = 2
GROUP BY product_id

但是我收到以下错误Error in query: ERROR: column "table.sender_id" must appear in the GROUP BY clause or be used in an aggregate function

似乎我无法选择不在GROUP BY中的列。我在网上发现的另一种方法是加入

SELECT product_id, sender_id, timestamp, count, ...other columns...
FROM table
JOIN (
    SELECT product_id, COUNT(product_id) AS count
    FROM table
    GROUP BY (product_id)
) table1 ON table.product_id = table1.product_id
WHERE sender_id = 2
GROUP BY product_id

但是这样做只是列出所有行，而不进行分组或计数。我的猜测是ON部分只是再次扩展表。

Answer 1

尝试使用product_id, sender_id

分组

select product_id, sender_id, count(product_id), max(timestamp) maxtm 
from t
where sender_id = 2
group by product_id, sender_id
order by maxtm desc

如果您还想要其他列：

select t.*, t1.product_count
from t
inner join (
    select product_id, sender_id, count(product_id) product_count, max(timestamp) maxtm 
    from t
    where sender_id = 2
    group by product_id, sender_id
) t1
on t.product_id = t1.product_id and t.sender_id = t1.sender_id and t.timestamp = t1.maxtm
order by t1.maxtm desc

Answer 2

只需使用数据进行锻炼即可

CREATE TABLE products (product_id INTEGER,
sender_id INTEGER,
time_stamp INTEGER)

INSERT INTO products VALUES
(1,2,1222),
(1,2,3423),
(1,2,1231),
(2,2,890),
(3,4,234),
(2,3,234234)

SELECT  product_id,sender_id,string_agg(time_stamp::text,','),count(product_id) 
FROM products
WHERE sender_id=2
GROUP BY product_id,sender_id

这里您具有不同的time_stamp，因此您需要应用一些汇总或仅在select语句中删除该列。

如果您在select语句中删除time_stamp，则将非常容易，如下所示：

SELECT  product_id,sender_id,count(product_id) 
FROM products
WHERE sender_id=2
GROUP BY product_id,sender_id

选择其他不计的列，按分组

2 个答案: