如何从选择查询返回值？

Question

我想计算两个城市之间的距离。我有以下功能：

CREATE OR REPLACE FUNCTION calc_distance_for_cities(fromCityId bigint, toCityId bigint) RETURNS integer LANGUAGE plpgsql as $$ 

    DECLARE 
      fromCityCoords Geometry; 
      toCityCoords Geometry; 

    BEGIN 
      SELECT * FROM geo_cities WHERE geo_cities.id = fromCityId RETURNING coords INTO fromCityCoords; 
      SELECT * FROM geo_cities WHERE geo_cities.id = toCityId RETURNING coords INTO toCityCoords; 
      SELECT ST_Distance(fromCityCoords, toCityCoords, true) 
    END; 

    $$ 

我在做什么错？

Answer 1

TL; DR您“做错了”两件事：returns integer应该是returns float并且最后select ..应该是return ..。

详情：

RETURN从PL/pgSQL函数返回一个值 - 你需要使用它，而不是最后SELECT。并且st_distance(..)根据http://postgis.refractions.net/docs/ST_Distance.html返回float值，因此您需要在您的第一行中使用returns float。

试试这个：

CREATE OR REPLACE FUNCTION calc_distance_for_cities(fromCityId bigint, toCityId bigint) RETURNS float LANGUAGE plpgsql as $$ 

    DECLARE 
      fromCityCoords Geometry; 
      toCityCoords Geometry; 

    BEGIN 
      SELECT coords INTO fromCityCoords FROM geo_cities WHERE geo_cities.id = fromCityId; 
      SELECT coords INTO toCityCoords FROM geo_cities WHERE geo_cities.id = toCityId; 
      RETURN ST_Distance(fromCityCoords, toCityCoords, true); 
    END; 

    $$; 

而且，这个功能其实并不需要PL/pgSQL的语言，可以轻松地在普通的SQL实现，与CTE（https://www.postgresql.org/docs/current/static/queries-with.html）：

create or replace function calc_distance_for_cities(int8, int8) returns float as $$ 
    with _from(city_coords) as (
    select coords from geo_cities where geo_cities.id = $1 
), _to(city_coords) as (
    select coords from geo_cities where geo_cities.id = $2 
) 
    select st_distance(_from.city_coords, _to.city_coords) 
    from _from, _to; 
$$ language sql; 

或者甚至没有CTE：

create or replace function calc_distance_for_cities(int8, int8) returns float as $$ 
    select st_distance(
    (select coords from geo_cities where geo_cities.id = $1), 
    (select coords from geo_cities where geo_cities.id = $2) 
); 
$$ language sql;