php - 查询从多个表中选择

Question

我知道有几篇关于此的帖子，但我无法让它发挥作用。我没有任何MySQL“join”或“left”的经验，只有简单的查询。

我有 3 个表:类别、公司和 catcomp

类别

id | name | ... | ... |

1 | Foo

2 | Bar

公司

id | name | ... | ...

1 | Company1

2 | Company2

Catcomp(为一家公司存储多个类别)

company_id | category_id

1 | 1

1 | 2

2 | 2

我只有这个:

$result = mysql_query("SELECT * FROM categories");
while($row = mysql_fetch_array($result))
{
    echo '<input type="checkbox" id="'.$row['id'].'" name="cat[]" value="'.$row['id'].'">'.$row['name'].'<br>
}

这会打印出带有复选框的所有类别。我希望为当前公司选中这些框。

有什么想法吗？

Answer 1

可能有更优雅的解决方案，但无论如何......

DROP TABLE IF EXISTS categories;
CREATE TABLE categories 
(id INT NOT NULL AUTO_INCREMENT PRIMARY KEY
,name VARCHAR(12) NOT NULL UNIQUE
);

INSERT INTO categories VALUES
(1 ,'Foo'),(2,'Bar'),(3,'Boo');

DROP TABLE IF EXISTS companies;
CREATE TABLE companies 
(id INT NOT NULL AUTO_INCREMENT PRIMARY KEY
,name VARCHAR(12) NOT NULL UNIQUE
);

INSERT INTO companies VALUES
(1,'Company1'),(2,'Company2'),(3,'Company3');;


DROP TABLE IF EXISTS company_category;
CREATE TABLE company_category
(company_id INT NOT NULL,category_id INT NOT NULL,PRIMARY KEY(company_id,category_id));

INSERT INTO company_category VALUES (1 ,1),(1 ,2),(2 ,2);

SELECT o.id company_id
     , o.name company_name
     , a.id category_id
     , a.name cateory_name
     , CASE WHEN oa.company_id IS NOT NULL THEN ' checked' ELSE '' END checked
  FROM companies o
  JOIN categories a
  LEFT 
  JOIN company_category oa
    ON oa.company_id = o.id
   AND oa.category_id = a.id;

 +------------+--------------+-------------+--------------+----------+
 | company_id | company_name | category_id | cateory_name | checked  |
 +------------+--------------+-------------+--------------+----------+
 |          1 | Company1     |           1 | Foo          |  checked |
 |          2 | Company2     |           1 | Foo          |          |
 |          3 | Company3     |           1 | Foo          |          |
 |          1 | Company1     |           2 | Bar          |  checked |
 |          2 | Company2     |           2 | Bar          |  checked |
 |          3 | Company3     |           2 | Bar          |          |
 |          1 | Company1     |           3 | Boo          |          |
 |          2 | Company2     |           3 | Boo          |          |
 |          3 | Company3     |           3 | Boo          |          |
 +------------+--------------+-------------+--------------+----------+

http://www.sqlfiddle.com/#!2/11e6e/1