如何在sql查询中使用sql查询

Question

我有两张桌子：

1. 表类别（cat_id，类别）
2. table category_details（cat_id，id，...）

我有像下面这样的sql代码：

$sql=mysql_query("select * from category");
while($rows=mysql_fetch_array($sql)){
$id_count=$rows['cat_id'];      

$sql1=mysql_query("select * from category_details where cat_id='$id_count'");
$count=mysql_num_rows($sql1);

}

我可以这样使用吗？

Answer 1

使用JOIN。

示例

SELECT * FROM category LEFT JOIN 
   category_details ON category_details.cat_id = category.cat_id;

以上查询将返回所有category和关联的category detail。

OP评论回复

SELECT    c.name,
          IFNULL(sub_c.total, 0) num
FROM      category c
LEFT JOIN ( SELECT   COUNT(*) total, cat_id
            FROM     category_details
            GROUP BY cat_id
          ) sub_c ON (sub_c.cat_id = c.cat_id);

完整代码

<?php
    $query = "SELECT    c.name,
                IFNULL(sub_c.total, 0) num
            FROM      products_category c
                LEFT JOIN ( SELECT   COUNT(*) total, cat_id
                    FROM     product
                    GROUP BY cat_id
                ) sub_c ON (sub_c.cat_id = c.id)";

    $result = mysql_query($query)or die(mysql_error());

    echo "<table><tr><td>NameCount</td></tr>";
    while($row = mysql_fetch_assoc($result))
    {
        echo "<tr><td>".$row['name']."(".$row['num'].")"."</td></tr>";
    }
    echo "</table>";
?>

MySQL表

ProductCategory表

CREATE TABLE IF NOT EXISTS `products_category` (
  `id` int(11) NOT NULL,
  `name` varchar(200) COLLATE utf8_unicode_ci NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

--
-- Dumping data for table `products_category`
--

INSERT INTO `products_category` (`id`, `name`) VALUES
(1, 'Erasmus'),
(2, 'Preston'),
(3, 'Ulric'),
(4, 'Gray'),
(5, 'Joseph'),
(6, 'Merrill'),
(7, 'Alan'),
(8, 'Jeremy'),
(9, 'Solomon'),
(10, 'Andrew'),
(11, 'Galvin'),
(12, 'Craig'),
(13, 'Cameron'),
(14, 'Omar'),
(15, 'Addison');

产品表

CREATE TABLE IF NOT EXISTS `product` (
  `id` int(11) NOT NULL,
  `name` varchar(200) COLLATE utf8_unicode_ci NOT NULL,
  `cat_id` int(11) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

--
-- Dumping data for table `product`
--

INSERT INTO `product` (`id`, `name`, `cat_id`) VALUES
(1, 'Wesley', 1),
(2, 'Graiden', 2),
(3, 'Cruz', 5),
(4, 'Hayden', 5),
(5, 'Kennedy', 6),
(6, 'Uriah', 8),
(7, 'Alan', 8),
(8, 'Cade', 1),
(9, 'Ryan', 5),
(10, 'Brody', 7);

以上将输出

Erasmus(2)
Preston(1)
Ulric(0)
Gray(0)
Joseph(3)
Merrill(1)
Alan(1)
Jeremy(2)
Solomon(0)
Andrew(0)
Galvin(0)
Craig(0)
Cameron(0)
Omar(0)
Addison(0)

Answer 2

您是否希望COUNT使用GROUP BY：

select c.cat_id, c.cat_name, count(cd.*)
from category c
   left join category_details cd 
       on c.cat_id = cd.cat_id
group by c.cat_id, c.cat_name

这将返回每个类别，其中包含与其关联的category_details计数。使用LEFT JOIN将返回所有类别 - 如果您只想要包含详细信息，请使用INNER JOIN替换。