Postgres 9.5 - 查询嵌套 JSON 元素的数组长度

Question

我有一个包含 JSON 字段的 Postgres 表，该字段标识与给定记录关联的图像。该字段的内容如下所示：

 {"photo-verification": 
   {"photos": [
     {"type": "photo-verification", "fileName": "4f35a880-e9a0-43f9-a31e-1bb8b765d04d", "mimeType": "image/jpeg", "createdBy": "jmlittm", "createdTs": "2016-06-20T20:25:39.706Z", "delFlag": false, "updatedBy": "jmlittm", "updatedTs": "2016-06-20T20:25:39.706Z"},
     {"type": "photo-verification", "fileName": "3a104d07-dc48-4f59-b83f-06cd35a21dae", "mimeType": "image/jpeg", "createdBy": "jmlittm", "createdTs": "2016-06-20T22:31:09.808Z", "delFlag": false, "updatedBy": "jmlittm", "updatedTs": "2016-06-20T22:31:09.808Z"}
     ]
   }
}

一条记录可以有 0 个或多个关联图像 - 如果没有图像，则整个字段将为空。我正在尝试编写一个查询来确定有多少图像与给定的记录相关联。最终结果应该是有多少记录有一张图像，有多少记录有不止一张。如果我查询 JSON 的顶级，如下所示：

select n.images->'photo-verification' from notes n;

我可以获得包含照片数组的内部 JSON，但如果我尝试深入挖掘，则不会得到任何结果。我的想法是我可以做一些类似的事情

select array_length(n.images->'photo-verification'->'photos', 1) from notes n;

或者

select json_array_length(n.images->'photo-verification'->'photos') from notes n;

但我最终得到了错误和提示，也许我应该考虑投射。

我刚刚开始深入研究 Postgres，所以我仍然试图围绕查询语言的一些细节进行思考。我将继续研究，但任何人可能提供的帮助或见解将不胜感激。

编辑：

因此，我认为我可以通过创建一个只有“照片”JSON 的视图并过滤掉所有空字段来简化问题：

CREATE VIEW photos as SELECT n.images->'photo-verification' as photo FROM notes.notes n where (n.images->'photo-verification')::text != '';

它起作用了，因为我现在有一个带有 JSON 列的视图，如下所示：

   {"photos": [
     {"type": "photo-verification", "fileName": "4f35a880-e9a0-43f9-a31e-1bb8b765d04d", "mimeType": "image/jpeg", "createdBy": "jmlittm", "createdTs": "2016-06-20T20:25:39.706Z", "delFlag": false, "updatedBy": "jmlittm", "updatedTs": "2016-06-20T20:25:39.706Z"},
     {"type": "photo-verification", "fileName": "3a104d07-dc48-4f59-b83f-06cd35a21dae", "mimeType": "image/jpeg", "createdBy": "jmlittm", "createdTs": "2016-06-20T22:31:09.808Z", "delFlag": false, "updatedBy": "jmlittm", "updatedTs": "2016-06-20T22:31:09.808Z"}
     ]
   }

但是，如果我尝试

select json_array_length(photo) from photos;

我得到：

ERROR: cannot get array length of a scalar

如果我尝试

select json_array_length(photo->'photos') from photos;

我得到一堆空白记录。

我一定在这里遗漏了什么......

Answer 1

create temp table t (id int, data json);
insert into t values 
(1, 
'{"photo-verification": 
   {"photos": [
     {"type": "photo-verification", "fileName": "4f35a880-e9a0-43f9-a31e-1bb8b765d04d", "mimeType": "image/jpeg", "createdBy": "jmlittm", "createdTs": "2016-06-20T20:25:39.706Z", "delFlag": false, "updatedBy": "jmlittm", "updatedTs": "2016-06-20T20:25:39.706Z"},
     {"type": "photo-verification", "fileName": "3a104d07-dc48-4f59-b83f-06cd35a21dae", "mimeType": "image/jpeg", "createdBy": "jmlittm", "createdTs": "2016-06-20T22:31:09.808Z", "delFlag": false, "updatedBy": "jmlittm", "updatedTs": "2016-06-20T22:31:09.808Z"}
     ]
   }
}'),
(2, 
'{"photo-verification": 
   {"photos": [
     {"type": "photo-verification", "fileName": "4f35a880-e9a0-43f9-a31e-1bb8b765d04d", "mimeType": "image/jpeg", "createdBy": "jmlittm", "createdTs": "2016-06-20T20:25:39.706Z", "delFlag": false, "updatedBy": "jmlittm", "updatedTs": "2016-06-20T20:25:39.706Z"}
     ]
   }
}')
;

select id, json_array_length(data->'photo-verification'->'photos')
from t;

或者，如果您对某个特定领域感兴趣：

select json_array_elements(data->'photo-verification'->'photos')->>'fileName' as fileName
from t;

Answer 2

似乎需要转换为 JSON：

select json_array_length((images->'photo-verification'->'photos')::json)
from notes;