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最小化这一个。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";

如果有人给我指的是一些可以帮助我练习和理解MySQL命令的精彩网站或文章，我也将不胜感激。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

嗯，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，因为你想在三个不同的表上COUNT。

修改

如果你想要一个三个数字的总和，那就是答案：

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

Answer 2

SELECT 'states', COUNT(*) FROM states WHERE id = 1
UNION
SELECT 'cities', COUNT(*) FROM cities WHERE id = 5
UNION
SELECT 'areas' , COUNT(*) FROM areas WHERE id = 3

编辑：我更喜欢@Silver Light的答案，但是由于他无法想到另一种方法，我想我会把这个作为参考。