sql - 选择具有最大 JSON 键的对象

Question

给定一个名为 test 的表，其定义如下

 Column | Type | Modifiers
--------+------+-----------
 a      | json |

其中每一行都有一些格式类似于

的JSON

{
  "1": "abc",
  "2": "def",
  "3": "ghi" 
}

如何进行查询以选择具有最大键的值？ (在此示例中，您将选择 "ghi"，因为它是键，3 是最大的)

---

我将其概念化的方式，理想情况下它是这样的查询:

SELECT a->max(json_object_keys(a)) from test;

但显然那是行不通的。这可能吗？

Answer 1

让我们尝试逐步解决这个问题:

tometzky=> select * from json_object_keys('{"1": "abc","2": "def","3": "ghi"}'::json) as k;
 k 
---
 1
 2
 3
(3 rows)

tometzky=> select max(k) from json_object_keys('{"1": "abc","2": "def","3": "ghi"}'::json) as k;
 max 
-----
 3
(1 row)

tometzky=> create temporary table test (a json);
tometzky=> insert into test values('{
  "1": "abc",
  "2": "def",
  "3": "ghi" 
}');
tometzky=> insert into test values('{
  "100": "jkl",
  "20": "kmo",
  "3": "pqr" 
}');

tometzky=> select (select max(k) from json_object_keys(a) as k) from test;
 max 
-----
 3
 3
(2 rows)

tometzky=> select (select max(k::int) from json_object_keys(a) as k) from test;
 max 
-----
   3
 100
(2 rows)

所以答案是:

tometzky=> select a->(select max(k::int)::text from json_object_keys(a) as k) from test;
 ?column? 
----------
 "ghi"
 "jkl"
(2 rows)

我假设“具有最大键的值”是针对每一行的，否则它看起来不像是一个定义明确的问题。