mysql - 我如何将这个 MySQL 查询最小化为一个？

Question

我有这三个查询，我想将其最小化为一个。我该怎么做？

    $query  = "SELECT COUNT(*) FROM states WHERE id = 1";
    $query2 = "SELECT COUNT(*) FROM cities WHERE id = 5";
    $query3 = "SELECT COUNT(*) FROM areas WHERE id = 3";

另外，如果有人指出一些可以帮助我练习和理解 MySQL 命令的很棒的网站或文章，我将不胜感激。非常感谢。

编辑:我希望脚本返回一个结果(变量)中合并的行数。

Answer 1

好吧，(我知道)在一个查询中执行此操作的唯一方法是:

SELECT 
(SELECT COUNT(*) FROM states WHERE id = 1) AS state_count,
(SELECT COUNT(*) FROM cities WHERE id = 5) AS city_count,
(SELECT COUNT(*) FROM areas WHERE id = 3) AS area_count

这几乎不能称为“优化”，因为还有 3 个表扫描，但无法避免这些扫描，因为您想在三个不同的表上进行 COUNT。

编辑

如果您想要三个数字的总和，那么这就是答案:

SELECT 
(
    (SELECT COUNT(*) FROM states WHERE id = 1) 
    + (SELECT COUNT(*) FROM cities WHERE id = 5) 
    + (SELECT COUNT(*) FROM areas WHERE id = 3)
) AS sum