Question

我有四张桌子

公司（字段：id，name）
cities（fields：id，name）
类别（字段：id，名称）
city_category_company（字段：company_id，city_id，category_id）

我需要计算城市页面上每个类别的公司数量。

现在，我是如此丑陋的循环

$q = mysqli_query($db, "SELECT * FROM `categories`");
while ($r = mysqli_fetch_assoc($q)) {
    $r2 = mysqli_fetch_assoc(
        mysqli_query($db, "SELECT COUNT(`company_id`) AS `num` FROM `city_category_company` WHERE `city_id` = '{$city['id']}' AND `category_id` = '{$r['id']}'")
    );
}

但是如果我有很多类别，那么在一个页面（50或更多）上对数据库的查询就会变得非常多。请帮我优化查询

Answer 1

试试这个：

SELECT c.id, c.name, COUNT(ccc.company_id) AS num
FROM city_category_company ccc
INNER JOIN categories c ON ccc.category_id = cc.id
WHERE ccc.city_id={city_id}
GROUP BY c.id

Answer 2

SELECT COUNT(co.id) AS company_count, cc.category_id, ca.name AS category_name 
FROM companies co 
INNER JOIN city_category_company cc ON co.id=cc.company_id 
INNER JOIN categories ca ON ca.id=cc.category_id 
GROUP BY cc.category_id, ca.name

从四个表中选择计数

2 个答案: