How to query all data under a certain field in php

In the process of website development, we often need to query all the data under a certain field in the database. Here we take the PHP programming language as an example to briefly introduce how to use PHP to implement this function.

1. Connect to the database

Before querying the database, we need to connect to the database first. This can be achieved through the mysqli or PDO extension provided by PHP. Here we take the mysqli extension as an example to demonstrate:

$servername = "localhost"; //数据库服务器名称
$username = "username"; //数据库用户名
$password = "password"; //数据库密码
$dbname = "myDB"; //数据库名

// 创建连接
$conn = mysqli_connect($servername, $username, $password, $dbname);
// 检测连接
if (!$conn) {
    die("连接数据库失败: " . mysqli_connect_error());
}

In the above code, we first declare the parameters required for database connection: server name, user name, password, database name. Then use the mysqli_connect() function to create a database connection. If the connection is successful, the $conn variable will store the database connection object, otherwise an error message will be returned and the script will be terminated.

2. Query data

Next, you can use SQL statements to query data. Suppose we want to query all the data under the "name" field in the table, we can use the following code:

$sql = "SELECT * FROM myTable WHERE name='John'"; //查询语句
$result = mysqli_query($conn, $sql); //执行查询

if (mysqli_num_rows($result) > 0) {
    // 输出每行数据
    while($row = mysqli_fetch_assoc($result)) {
        echo "id: " . $row["id"]. " - Name: " . $row["name"]. " - Age: " . $row["age"]. "<br>";
    }
} else {
    echo "0 结果";
}

mysqli_close($conn); // 关闭连接

In the above code, we first define a SQL query statement, where "myTable" is the table to be queried Name, "name" is the field name to be queried, and "John" is the field value to be queried. Then use the mysqli_query() function to execute the SQL query. If the query is successful, the $result variable will store the query results, otherwise an error message will be returned and the script will terminate.

Next, use the mysqli_num_rows() function to obtain the number of rows in the query result set. If the number of rows is greater than 0, use the mysqli_fetch_assoc() function to obtain the data of each row in turn, and output the row data to the web page.

Finally use the mysqli_close() function to close the database connection.

3. Complete code example

The following is a complete PHP example code for querying all data under a certain field:

$servername = "localhost"; //数据库服务器名称
$username = "username"; //数据库用户名
$password = "password"; //数据库密码
$dbname = "myDB"; //数据库名

// 创建连接
$conn = mysqli_connect($servername, $username, $password, $dbname);
// 检测连接
if (!$conn) {
    die("连接数据库失败: " . mysqli_connect_error());
}

// 查询数据
$sql = "SELECT * FROM myTable WHERE name='John'";
$result = mysqli_query($conn, $sql);

if (mysqli_num_rows($result) > 0) {
    // 输出每行数据
    while($row = mysqli_fetch_assoc($result)) {
        echo "id: " . $row["id"]. " - Name: " . $row["name"]. " - Age: " . $row["age"]. "
";
    }
} else {
    echo "0 结果";
}

mysqli_close($conn); // 关闭连接

The above code is implemented Query all data with the value "John" under the "name" field in the "myTable" table, and print each piece of data to the web page. If the query result is empty, "0 results" are output.

Summary:

Through the above simple sample code, we can use PHP to query all data under a certain field, thereby meeting the needs of the development process. Of course, in actual development, we also need to pay attention to issues such as data security and performance optimization to ensure the reliability and efficiency of the program.

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