每个字段的限制

Question

假设我们有这样一个 sql 查询，其中包含来自另一个 table 的连接数据。

SELECT 
    pr.num, pr.resort_id, p.src_mask
FROM 
    rutraveler.rt_photo_resort AS pr
    JOIN rutraveler.rt_photo AS p ON pr.photo_id = p.id
WHERE   pr.resort_id = '612' AND p.src_mask is not null
ORDER BY num
LIMIT 30

到目前为止，我们必须为几个 resort_id.
做几个查询如何更改查询，以便我们只有一个查询（WHERE resort_id in (612, 333, 111)，每个 resort_id?

结果不超过 30 项

Answer 1

使用 ROW_NUMBER 计算每个 resort_id 的行数。

SELECT resort_id, num, resort_id, src_mask 
FROM
(
  SELECT 
    pr.resort_id, pr.num, pr.resort_id, p.src_mask,
    ROW_NUMBER() OVER (PARTITION BY pr.resort_id ORDER BY num) AS rn
  FROM 
    rutraveler.rt_photo_resort AS pr
    JOIN rutraveler.rt_photo AS p ON pr.photo_id = p.id
  WHERE resort_id in (612, 333, 111) AND p.src_mask is not null
) data
WHERE rn <= 30
ORDER BY resort_id, num;

Answer 2

您可以将 CTE 与 ROW_NUMBER() 和 PARTITION BY

一起使用

WITH Results_CTE AS
(
    SELECT 
    pr.num, pr.resort_id, p.src_mask,ROW_NUMBER() over ( PARTITION BY pr.resort_id ORDER BY num) As Row_number
FROM 
    rutraveler.rt_photo_resort AS pr
    JOIN rutraveler.rt_photo AS p ON pr.photo_id = p.id
WHERE   pr.resort_id IN (A,B,C) AND p.src_mask is not null   
)

SELECT *
FROM Results_CTE
WHERE Row_number <= 30

每个字段的限制

Limit per some field

sql

postgresql

greatest-n-per-group