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. 表category_details(cat_id，id，.)

我有如下所示的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);

}

我能这样用吗？

Answer 1

使用JOIN。

示例

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

上面的查询将返回所有category和相关的category detail。

OP注释响应

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);

完全代码

<?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');

积表

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);

上面的将输出

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

你想把COUNT和GROUP BY结合使用吗？

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

这将返回与其关联的category_details计数的每个类别。使用LEFT JOIN将返回所有类别--如果您只想要那些有详细信息的类别，请用INNER JOIN替换。