Question

是否可以在SQL查询的limit子句中使用select count（）？

原始查询：

SELECT *
FROM flight_delays
LIMIT 10;

但动态的东西是这样的：

SELECT
  *
FROM
  flight_delays
LIMIT
  (
    select
      count(foo)
    from
      bar
    group by
      bar.bing
  )

Answer 1

您可以这样做：

SELECT fd.*
FROM (SELECT fd.*, ROW_NUMBER() OVER () as seqnum
      FROM flight_delays
     ) fd CROSS JOIN
     (select count(foo) as cnt
      from bar
     ) x
WHERE seqnum <= x.cnt;

Answer 2

您可以创建一个函数，该函数将首先计算LIMIT的值，然后使用该LIMIT执行查询。

CREATE OR REPLACE FUNCTION flight_delays_limited()
  RETURNS setof flight_delays
  $$
  DECLARE
    limitSize INTEGER = 0;
  BEGIN
  --custom logic for calculating the limit
  select
      count(foo) into limitSize
    from
      bar
    group by
      bar.bing;
  --end custom logic

  --return result from select with calculated limit
  RETURN QUERY
  SELECT * FROM flight_delays
    LIMIT limitSize;
  end;
$$ language plpgsql;

您可以这样调用该函数：

SELECT * FROM flight_delays_limited();

在限制子句中使用计数

2 个答案: