Question

我必须从包含空间信息的json文件中提取数据。该文件的内容是

create table field(data json);

COPY field(data) FROM '/home/guest-pc5/field.json';

我创建了一个表，然后将该文件的内容复制到其中

SELECT json_array_elements(data->'vertices') from field;
 {"lat":46.744628268759314,"lon":6.569952920654968}
 {"lat":46.74441692818192,"lon":6.570487107359068}
 {"lat":46.74426116111054,"lon":6.570355867853787}
 {"lat":46.74447250168793,"lon":6.569821681149689}
(4 rows)

我现在可以查询我的数据

了

    select * from json_to_recordset('[{"lat":46.744628268759314,"lon":6.569952920654968},{"lat":46.74441692818192,"lon":6.570487107359068},{"lat":46.74426116111054,"lon":6.570355867853787},{"lat":46.74447250168793,"lon":6.569821681149689}]') as (lat numeric, lon numeric);
    ERROR:  function json_to_recordset(unknown) does not exist
    LINE 1: select * from json_to_recordset('[{"lat":46.744628268759314,...
                      ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.

问题在于我无法像那样使用它。我想只捕获“lat”和“lon”的值，将它们放在字段表中

我试图使用函数json_to_recordset但没有成功

FnMut

Answer 1

您可以使用json manipulator operator ->>从json_array_elements输出中获取所需的值作为文本。为方便起见，您可以在json_array_elements子句中调用FROM（这是对lateral的set-returning function次调用）：

SELECT
    f.data AS original_json,
    CAST((e.element->>'lat') AS numeric) AS lat,
    CAST((e.element->>'lon') AS numeric) AS lon
FROM
    field AS f,
    json_array_elements(f.data->'vertices') AS e(element);

通过它，您可以简单地创建一个表（或将INSERT用于现有表）：

CREATE TABLE coordinates AS
SELECT
    f.data AS original_json,
    CAST((e.element->>'lat') AS numeric) AS lat,
    CAST((e.element->>'lon') AS numeric) AS lon
FROM
    field AS f,
    json_array_elements(f.data->'vertices') AS e(element);

OBS：LATERAL是隐式的，因为LATERAL关键字对于集合返回函数调用是可选的，但你可以使它真正明确，如：

FROM
    field f
    CROSS JOIN LATERAL json_array_elements(f.data->'vertices') AS e(element);

此外，LATERAL仅为9.3+，尽管您使用json_array_elements时肯定高于此值（仅限9.3+）。

在postgres中查询json

1 个答案: