php query database according to conditions

PHP is a very popular web development language and one of the favorite languages ​​​​of many developers. Applications written in the PHP language often need to interact with a database in order to obtain necessary information. In this article, we will demonstrate how to retrieve data from a database using PHP.

Database query is a very critical task because it determines how the developer obtains the required information and displays it to the user. During the development process, sometimes you need to query the database based on conditions to obtain data that meets specific needs. Here are some steps and sample code that shows how to conditionally retrieve data from a database using PHP.

Step 1: Create a database connection

To use PHP to retrieve data from the database, you must first establish a connection to the database. Database connections can be established using PHP's built-in mysqli or PDO extensions.

The following is a sample code that uses the mysqli extension to establish a connection to the database:

$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database_name";

// 创建连接
$conn = new mysqli($servername, $username, $password, $dbname);

// 检查连接是否成功建立
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

Step 2: Execute the query

After establishing the connection to the database, next A query needs to be executed. Queries can be executed using mysqli or PDO extensions.

The following is a sample code that uses the mysqli extension to execute a query:

$sql = "SELECT * FROM table_name WHERE column_name = 'value'";

$result = $conn->query($sql);

if ($result->num_rows > 0) {
    // 处理结果
} else {
    echo "0 结果";
}

Step 3: Process the results

After executing the query, you will get a result set. Before processing the result set, you must check whether there is data to return.

The following is a sample code that uses the mysqli extension to process the results:

if ($result->num_rows > 0) {
    // 循环输出每一行数据
    while($row = $result->fetch_assoc()) {
        echo "id: " . $row["id"]. " - Name: " . $row["name"]. " " . $row["email"]. "<br>";
    }
} else {
    echo "0 结果";
}

In the above sample code, the fetch_assoc() function is used to obtain each row of data. Different functions can be used to obtain different results.

In the examples in this article, we enclose the conditional value in single quotes. If the condition value is a variable, you will need to use double quotes instead to insert the variable's value into the query string.

Using mysqli or PDO extension to query the database conditionally is a very basic task, but it is also a very critical task. This article provides some sample code and steps so that developers can better understand the technology and practical applications in this area.

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