Mysql从多个表中选择行

Question

我正在一个用户可以浏览类别的目录网站上工作.类别可以包含其他类别和产品,产品可以属于多个类别.相关的数据库模式如下所示:

CREATE TABLE products (
    product_id INT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
    product_title VARCHAR(100) NOT NULL,
    product_status TINYINT UNSIGNED NOT NULL
);

CREATE TABLE product_categories (
    category_id INT UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
    parent_category_id INT UNSIGNED NOT NULL,
    category_title VARCHAR(100) NOT NULL,
    category_status TINYINT UNSIGNED NOT NULL,
    category_order INT UNSIGNED NOT NULL
);

CREATE TABLE products_categories (
    product_id INT UNSIGNED NOT NULL,
    category_id INT UNSIGNED NOT NULL,
    product_order INT UNSIGNED NOT NULL,
    PRIMARY KEY(product_id, category_id)
);

我遇到的问题是我需要使用LIMIT n, n以下命令对结果进行分页:

$perpage = 20;
$start = (isset($_GET['page'])) ? (int)$_GET['page'] * $perpage : 1;
$limitsql = "LIMIT $start, $perpage";

但我无法弄清楚如何在不加入和合并结果的情况下选择不同的类别和产品.理想情况下,我希望得到如下结果:

product_id      |       product_title   |       category_id     |       category_title
NULL            |       NULL            |       32              |       category foo
NULL            |       NULL            |       239             |       category bar
9391            |       product foo     |       NULL            |       NULL
325             |       product bar     |       NULL            |       NULL

我能做的最好的事情是得到这样的东西,但这并没有多大帮助:

product_id      |       product_title   |       category_id     |       category_title
9391            |       product foo     |       32              |       category foo
325             |       product bar     |       239             |       category bar
239             |       product foo2    |       32              |       category foo
115             |       product bar2    |       239             |       category bar

我能想到的唯一其他解决方案是查询类别中的所有子类别和产品,将它们粘贴到php数组中并使用提取当前页面array_slice.考虑到产品的数量(数千),这不是一个非常吸引人的选择.

否则,我可以查询类别的数量,并抵消$start了在LIMIT按类别的数量条款.如果有超过一整页的类别,这会变得混乱.

这是我当前的工作查询,它​​给出了上面的结果:

SELECT
    p.product_id, p.product_title,
    c.category_id, c.category_title
FROM products AS p
JOIN product_categories AS c
    ON c.parent_category_id='20'
INNER JOIN products_categories AS pc
    ON p.product_id=pc.product_id
WHERE p.product_status='1' AND pc.category_id='20'
ORDER BY pc.product_order ASC

编辑

我想我已经合作了UNION,我完全忘记了

SELECT
    c.category_id AS row_id, c.category_title AS row_title, 1 AS is_category
FROM product_categories AS c
WHERE c.parent_category_id='20'

UNION 

SELECT
    p.product_id AS row_id, p.product_title AS row_title, 0 AS is_category
FROM products AS p
INNER JOIN products_categories AS pc
    ON p.product_id=pc.product_id

编辑2

我猜联盟不会像我想的那样工作.由于两者都被视为单独的查询,因此我无法应用于LIMIT整个结果,只能应用于每个人SELECT.此外,从每个语句中选择的列似乎必须与另一个语句中相应类型的列相同.

Answer 1

使用:

SELECT *
  FROM (SELECT c.category_id AS row_id, c.category_title AS row_title, 1 AS is_category
          FROM product_categories AS c
         WHERE c.parent_category_id='20'
        UNION 
        SELECT p.product_id AS row_id, p.product_title AS row_title, 0 AS is_category
          FROM products AS p
          JOIN products_categories AS pc ON p.product_id=pc.product_id) x
LIMIT x, y