哪个最快？计入子查询或分组依据

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.计入子查询：

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是最好和最快的： 因为解决方案2有子查询

但你必须改变你的查询：

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

你改变了GROUP BY和ORDER BY的位置。

我不知道您是否在寻找计数（*），或者您希望每nid计算一次。如果count（*）则考虑不使用GROUP BY

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