查询效率

Question

此查询的效率如何。它在子查询中从同一个表中选择两次。

SELECT DISTINCT country, rowCount FROM 
(
    SELECT *, (
        SELECT COUNT(id) FROM tbl WHERE hide != 1 AND country = y.country
    ) AS rowCount FROM tbl y

) AS x
HAVING rowCount >= 1
ORDER BY x.country ASC

我已经将表索引到需要的位置，并且查询在“0.000秒”内执行（HeidiSQL说）所以它一定很好，但有什么可以改善查询的结构吗？

Answer 1

似乎可以将其重写为单个查询：

SELECT country, COUNT(id) rowCount
FROM tbl
WHERE hide <> 1
GROUP BY country
HAVING rowCount >= 1

游乐场：http://sqlfiddle.com/#!2/28978/1