查找每个工作日的每个room_id的最长连续时间

Question

我正在尝试查找每个工作日每个room_id的最长连续时间。我想将工作日转换为其整数表示形式（0代表星期日，1代表星期一，等等）。

到目前为止我所拥有的：

create or replace function iweekday(weekday  varchar(9))
returns int as $$
DECLARE iw INT;
begin
select  iw= CASE weekday
                        WHEN 'Monday' THEN 1
                        WHEN 'Tuesday' THEN 2
                        WHEN 'Wednesday' THEN 3
                        WHEN 'Thursday' THEN 4
                        WHEN 'Friday' THEN 5
                        WHEN 'Saturday' THEN 6  
                        WHEN 'Sunday' THEN 0
                    END;
end;
$$ language plpgsql;


create or replace function fourpointnine()
returns table (room_id character(7), iw int, start_time int,end_time int) as $$
DECLARE iw INT;
begin
with recursive cte as(
    select  l.room_id, l.weekday, l.start_time, l.end_time
    from "learningactivity" l
    union all 
    select l.room_id, l.weekday, cte.start_time, l.end_time
    from cte join "learningactivity" l on l.room_id=cte.room_id and l.weekday=cte.weekday and cte.end_time=l.start_time
),  cte2 as(
    select *,  row_number() over (partition by cte.room_id  order by EXTRACT(EPOCH FROM cte.end_time - cte.start_time)/3600 desc ) as rn
    from cte
)


select distinct cte2.room_id, iweekday(cte2.weekday), cte2.start_time, cte2.end_time
from cte2
where rn=1
group by cte2.room_id, iweekday(cte2.weekday), cte2.start_time, cte2.end_time;
end;
$$ language plpgsql;

但是我收到此错误：

ERROR:  query has no destination for result data
HINT:  If you want to discard the results of a SELECT, use PERFORM instead.
CONTEXT:  PL/pgSQL function iweekday(character varying) line 4 at SQL statement

如何正确执行？

Answer 1

立即错误可以用RETURN QUERY修复。参见：

• How to return result of a SELECT inside a function in PostgreSQL?

但是不要打扰，从一开始，PL / pgSQL函数就显得过于刻板。

您的助手功能通过这种方式更简单，更便宜：

CREATE OR REPLACE FUNCTION iweekday(weekday text)
  RETURNS int LANGUAGE sql IMMUTABLE PARALLEL SAFE AS
$func$
SELECT CASE weekday
         WHEN 'Monday'    THEN 1
         WHEN 'Tuesday'   THEN 2
         WHEN 'Wednesday' THEN 3
         WHEN 'Thursday'  THEN 4
         WHEN 'Friday'    THEN 5
         WHEN 'Saturday'  THEN 6  
         WHEN 'Sunday'    THEN 0
        END
$func$;

（[PARALLEL SAFE仅用于Postgres 9.6或更高版本。]

最重要的是，它可以是inlined。相关：

Difference between language sql and language plpgsql in PostgreSQL functions
• 并且您的主要功能可以替换为该查询：

WITH RECURSIVE cte AS (
   SELECT l.room_id, l.weekday, l.start_time, l.end_time
   FROM   learningactivity l

   UNION ALL 
   SELECT l.room_id, l.weekday, c.start_time, l.end_time
   FROM   cte c
   JOIN   learningactivity l ON l.room_id = c.room_id
                            AND l.weekday = c.weekday
                            AND l.start_time = c.end_time
  )
SELECT DISTINCT ON (1, 2)
       c.room_id, iweekday(c.weekday) AS iw, c.start_time, c.end_time
FROM   cte c
ORDER  BY 1, 2, c.start_time - c.end_time;

如果您坚持使用功能：

CREATE OR REPLACE FUNCTION fourpointnine()
  RETURNS TABLE (room_id text, iw int, start_time int, end_time int)
  LANGUAGE sql AS
$func$
    -- paste query from above
$func$;

请勿使用数据类型character(N)。参见：

Any downsides of using data type "text" for storing strings?