我正试图将一个产品分配给一个类别，我想到的唯一解决方案是将主键（类别类别中的类别Id）分配给外键（类别产品中的类别Id）。
我必须对SQL代码做点什么还是更改index.php中的代码？
代码是为了向读者展示我的所作所为！
类
public class Category {
public int Id{get;set;}
public string Name {get;set;}
}
public class Product{
public int Id{get;set;}
public int CategoryId{get;set;}
public String Brand {get;set;}
public String Name {get;set;}
public decimal Price{get;set;
}
mydatabase-sql-
CREATE TABLE IF NOT EXISTS `product` (
`id` INT NOT NULL AUTO_INCREMENT PRIMARY KEY ,
`category_id` INT NOT NULL ,
`brand` VARCHAR( 100 ) NOT NULL ,
`name` VARCHAR( 100 ) NOT NULL ,
`barcode` INT NOT NULL,
`price` DECIMAL NOT NULL
) ENGINE = INNODB;
INSERT INTO `product` (`id`, `category_id`, `brand`, `name`,`barcode`,`price`) VALUES
(1,1,'Coca Cola','Coke',123456789,0.99)
CREATE TABLE IF NOT EXISTS `category` (
`id` INT NOT NULL AUTO_INCREMENT PRIMARY KEY ,
`category_name` VARCHAR( 100 ) NOT NULL
) ENGINE = INNODB;
INSERT INTO `category` (`id`, `category_name`) VALUES
(1,'Drinks')
index.php
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<link href="css/bootstrap.min.css" rel="stylesheet">
<script src="js/bootstrap.min.js"></script>
</head>
<body>
<div class="container">
<div class="row">
<h3>PHP CRUD Grid</h3>
</div>
<div class="row">
<p>
<a href="create.php" class="btn btn-success">Create</a>
</p>
<table class="table table-striped table-bordered">
<thead>
<tr>
<th>Id</th>
<th>CategoryId</th>
<th>Brand</th>
<th>Name</th>
<th>Barcode</th>
<th>Price</th>
</tr>
</thead>
<tbody>
<?php
include_once 'database.php';
$pdo = Database::connect();
$sql = 'SELECT * FROM product ORDER BY id DESC';
foreach ($pdo->query($sql) as $row) {
echo '<tr>';
echo '<td>'. $row['id'] . '</td>';
echo '<td>'. $row['category_id'] . '</td>';
echo '<td>'. $row['brand'] . '</td>';
echo '<td>'. $row['name'] . '</td>';
echo '<td>'. $row['barcode'] . '</td>';
echo '<td>'. $row['price'] . '</td>';
echo '<td width=250>';
echo '<a class="btn" href="read.php?id='.$row['id'].'">Read</a>';
echo ' ';
echo '<a class="btn btn-success" href="update.php?id='.$row['id'].'">Update</a>';
echo ' ';
echo '<a class="btn btn-danger" href="delete.php?id='.$row['id'].'">Delete</a>';
echo '</td>';
echo '</tr>';
}
?>
</tbody>
</table>
<div class="container">
<div class="row">
<h3>PHP CRUD Grid</h3>
</div>
<div class="row">
<p>
<a href="createCategory.php" class="btn btn-success">Create</a>
</p>
<table class="table table-striped table-bordered">
<thead>
<tr>
<th>CategoryId</th>
<th>Category Name</th>
</tr>
</thead>
include_once 'database.php';
<?php
$sql2 = 'SELECT * FROM category ORDER BY id DESC';
foreach ($pdo->query($sql2) as $row) {
echo '<tr>';
echo '<td>'. $row['id'] . '</td>';
echo '<td>'. $row['category_name'] . '</td>';
echo '<td width=250>';
echo '<a class="btn" href="readCategory.php?id='.$row['id'].'">Read</a>';
echo ' ';
echo '<a class="btn btn-success" href="updateCategory.php?id='.$row['id'].'">Update</a>';
echo ' ';
echo '<a class="btn btn-danger" href="deleteCategory.php?id='.$row['id'].'">Delete</a>';
echo '</td>';
echo '</tr>';
}
Database::disconnect();
?>
</tbody>
</table>
</div>
</div>
</body>
</html>
<!-- <div class="container">
<div class="row">
<h3>PHP CRUD Grid</h3>
</div>
<div class="row">
<p>
<a href="create.php" class="btn btn-success">Create</a>
</p>
<table class="table table-striped table-bordered">
<thead>
<tr>
<th>CategoryId</th>
<th>Catengory Name</th>
</tr>
</thead>
$sql2 = 'SELECT * FROM category ORDER BY id DESC';
foreach ($pdo->query($sql2) as $row) {
echo '<tr>';
echo '<td>'. $row['id'] . '</td>';
echo '<td>'. $row['category_name'] . '</td>';
echo '<td width=250>';
echo '<a class="btn" href="readCategory.php?id='.$row['id'].'">Read</a>';
echo ' ';
echo '<a class="btn btn-success" href="updateCategory.php?id='.$row['id'].'">Update</a>';
echo ' ';
echo '<a class="btn btn-danger" href="deleteCategory.php?id='.$row['id'].'">Delete</a>';
echo '</td>';
echo '</tr>';
</div>
}
Database::disconnect();
?>
</tbody>
</table> -->
Basicly、主键和外键是数据库的概念。您必须使产品表的category_id成为索引，并为类别表