如何在 PostgreSQL 中基于 order by 字段获得第二高到最低值的记录答案

【问题标题】：How to get the second highest to lowest value record on the based on order by field in PostgreSQL如何在 PostgreSQL 中基于 order by 字段获得第二高到最低值的记录
【发布时间】：2021-08-04 07:24:03
【问题描述】：

这里我有一个 JSON 字段，我想在每个 id 上选择第二高到最低的记录数据是

JSON 看起来像这样

{
    "user": [
        {
            "user_name": "Devang",
            "user_weight": 0.7676846955248864
        },
        {
            "user_name": "Meet",
            "user_weight": 1.1021
        },
        {
            "user_name": "Devang",
            "user_weight": 0.16163873153859706
        },
        {
            "user_name": "Rajan",
            "user_weight": 0.22163873153859706
        }
    ],
    "address": [
        {
            "address_name": "India"
        }
    ]
}

我执行的查询是

WITH cte AS (
SELECT id, ('{user,'||index-1||'}')::text[] as json_path, value->'user_weight'
FROM user_table, jsonb_array_elements(json_field->'user')
WITH ordinality arr(value, index) WHERE arr.value->>'user_name' IN ('Devang', 'Meet') order by id, value->'user_weight' DESC
) select * from cte;

获得这种记录

id  json_path   ?column?
1   {user,1}    1.1021
1   {user,0}    0.7676846955248864
1   {user,2}    0.16163873153859706
2   {user,0}    0.7676846955248864
2   {user,1}    0.07447325861051013

我想要这样的第二高到最低的记录

id  json_path   ?column?
1   {user,1}    1.1021 # this is the highest one in 1 id so I do not need this
1   {user,0}    0.7676846955248864
1   {user,2}    0.16163873153859706
2   {user,0}    0.7676846955248864   # this is the highest one in 2 ids so. I do not want
2   {user,1}    0.07447325861051013

输出是

id  json_path   ?column?
1   {user,0}    0.7676846955248864
1   {user,2}    0.16163873153859706
2   {user,1}    0.07447325861051013

看这里Demo

任何答案将不胜感激

【问题讨论】：

标签： sql postgresql common-table-expression jsonb

【解决方案1】：

有很多方法可以做到这一点，但如果你喜欢玩窗口函数，你可以RANK() 或DENSE_RANK() 按 id 的权重并消除外部查询中的第一个排名：

WITH cte AS (
  SELECT id, ('{user,'||index-1||'}')::text[] as json_path, (value->'user_weight')::text::numeric AS weight
  FROM user_table, jsonb_array_elements(json_field->'user')
  WITH ordinality arr(value, index) 
  WHERE arr.value->>'user_name' IN ('Devang', 'Meet') 
  ORDER BY id, value->'user_weight' DESC
) 
SELECT * FROM (
  SELECT cte.*, 
    RANK() OVER (PARTITION BY id ORDER BY id,weight DESC
                 RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS r
  FROM cte) j
WHERE r > 1;

演示：db<>fiddle

【讨论】：

【解决方案2】：

使用窗口函数和分区，我们可以像这样抓取并消除第一行：

WITH cte AS (
   SELECT id, ('{user,'||index-1||'}')::text[] as json_path, value->'user_weight', ROW_NUMBER() OVER(PARTITION BY id order by value->'user_weight' DESC ) as row_num
   FROM user_table, jsonb_array_elements(json_field->'user')
    WITH ordinality arr(value, index) WHERE arr.value->>'user_name' IN ('Devang', 'Meet') order by id, value->'user_weight' DESC
) 
select * from cte where row_num>1;

这里Demo

【讨论】：

不错。请记住，如果多条记录共享相同的 max(weight)，则此查询将失败。因此，它将仅跳过一条记录并显示其他记录。欢呼和快乐的编码