哪个最快？计数子查询或分组依据

Question

哪个最快/最有效？两者都有效。

解决方案 1. 按 id 分组：

SELECT
    *, 
    COUNT(*) AS pages 
FROM
    notis 
WHERE
    cid = 20 
GROUP BY
    n.id
ORDER BY
    nid DESC 

解决方案 2. 在子查询中计数：

SELECT
    *,
    (select count(*) FROM notis WHERE cid=20) AS count
FROM
    notis
WHERE
    cid = 20
ORDER BY
    nid DESC

Answer 1

解决方案 1 是最佳且快速的： 因为解决方案 2 有子查询

但是您必须像这样更改您的查询：

  SELECT *, COUNT(*) AS pages 
  FROM notis 
  WHERE cid = 20 
  GROUP BY nid
  ORDER BY nid DESC 

您更改了 GROUP BY 和 ORDER BY 的位置。

我不知道您是在寻找 count(*) 还是想要按每个 nid 计数。如果 count(*) 则考虑不使用 GROUP BY

 SELECT *, COUNT(*) AS pages 
 FROM notis 
 WHERE cid = 20 
 ORDER BY nid DESC