Usage of mysqli_select_db and mysqli_query functions in PHP

In the previous article, I brought you "How to connect to the MySQL database in PHP? ", which introduces in detail how to connect to the MySQL database in PHP. In this article, we take a look at the relevant knowledge of PHP using the mysqli_select_db() function to select a database. I hope everyone has to help!

As mentioned in the previous article, PHP can connect to the MySQL database through the mysqli_connect() function. One parameter of this function is the name of the corresponding database. , this parameter is an optional parameter and can be omitted. If you omit this parameter, you need to specify a default database later. In PHP, you can specify a default database through the mysqli_select_db() function. Then let’s take a look at the usage of this function.

<strong><span style="font-size: 20px;">mysqli_select_db()</span></strong>Function

The syntax of this function The format is also divided into two situations. One is the object-oriented writing method, and its syntax format is as follows:

mysqli::select_db(string $dbname)

where $dbname represents the specified database name, and the other The first is a process-oriented writing method, and its syntax format is as follows:

mysqli_select_db(mysqli $link, string $dbname)

It should be noted that:

• $dbname is still expressed as specified Database name,

• $link represents the database connection returned by the mysqli_connect() function.

If the function is successfully executed, the return result is true. If the function fails, the return result is false.

Next, let’s take an example to see how to select a database through the mysqli_select_db() function. The example is as follows:

<?php
    $host     = &#39;localhost&#39;;
    $username = &#39;root&#39;;
    $password = &#39;root&#39;;
    $dbname   = &#39;test&#39;;
    $mysql    = new Mysqli($host, $username, $password);
    if($mysql -> connect_errno){
        die(&#39;数据库连接失败：&#39;.$mysql->connect_errno);
    }else{
        $mysql -> select_db($dbname);                  // 选择名为 test 的数据库
        $sql    = &#39;select name,sex,age from user&#39;;     // SQL 语句
        $result = $mysql -> query($sql);               // 执行上面的 SQL 语句
        $data   = $result -> fetch_all();
        $mysql -> close();
    }
    echo &#39;<pre class="brush:php;toolbar:false">&#39;;
    print_r($data);
?>

The above example selects a database named “test” through the mysqli_select_db() function. ” database, the output results are as follows:

The above example uses object-oriented writing. Next, let’s take a look at what process-oriented writing looks like. Example As follows:

<?php
    $host     = &#39;localhost&#39;;
    $username = &#39;root&#39;;
    $password = &#39;root&#39;;
    $dbname   = &#39;test&#39;;
    $link     = @mysqli_connect($host, $username, $password);
    if($link){
        mysqli_select_db($link, $dbname);           // 选择名为 test 的数据库
        $sql    = &#39;select name,sex,age from user&#39;;  // SQL 语句
        $result = mysqli_query($link, $sql);        // 执行 SQL 语句，并返回结果
        $data   = mysqli_fetch_all($result);        // 从结果集中获取所有数据
        mysqli_close($link);
    }else{
        echo &#39;数据库连接失败！&#39;;
    }
    echo &#39;<pre class="brush:php;toolbar:false">&#39;;
    print_r($data);
?>

In the above example, the difference between the two writing methods is not big, and the output results are the same. From this we specify a default database through the mysqli_select_db() function. Then let's take a look at the mysqli_query() function. What does it do and how is it used?

<strong><span style="font-size: 20px;">mysqli_query()</span></strong>## Function

mysqli_query() function. The syntax format of this function is as follows:

mysqli::query( string $query [, int $resultmode = MYSQLI_STORE_RESULT ] )

• $query represents the SQL statement to be executed;

• $resultmode is an optional parameter, used to modify the behavior of the function.

mysqli_query( mysqli $link , string $query [, int $resultmode = MYSQLI_STORE_RESULT ] )

• $link Represents the database connection returned by the mysqli_connect() function;

• $query Represents the SQL statement to be executed;

• $resultmode is an optional parameter used to modify the behavior of the function.

mysql> select * from user;

---- -------- ------ ------
| id | name | age | sex |
---- -------- ------ ------
| 1 | 一一 | 21 | Male |
| 2 | 二二 | 22 | Male |
| 3 | Sansan | 23 | Female |
| 4 | Four Four | 24 | Female |
---- --- ----- ------ ------
4 rows in set (0.00 sec)

<?php
    $host     = &#39;localhost&#39;;
    $username = &#39;root&#39;;
    $password = &#39;root&#39;;
    $dbname   = &#39;test&#39;;
    $mysql    = new Mysqli($host, $username, $password, $dbname);
    if($mysql -> connect_errno){
        die(&#39;数据库连接失败：&#39;.$mysql->connect_errno);
    }else{
        $mysql -> set_charset(&#39;UTF-8&#39;);             // 设置数据库字符集
        $sql = &#39;select name,sex,age from user&#39;;     // SQL 语句
        $result = $mysql -> query($sql);            // 执行上面的 SQL 语句
        $data = $result -> fetch_all();
        $mysql -> close();
    }
    echo &#39;<pre class="brush:php;toolbar:false">&#39;;
    print_r($data);
?>

<?php
    $host     = &#39;localhost&#39;;
    $username = &#39;root&#39;;
    $password = &#39;root&#39;;
    $dbname   = &#39;test&#39;;
    $link = @mysqli_connect($host, $username, $password, $dbname);
    if($link){
        $sql    = &#39;select name,sex,age from user&#39;;  // SQL 语句
        $result = mysqli_query($link, $sql);        // 执行 SQL 语句，并返回结果
        $data   = mysqli_fetch_all($result);        // 从结果集中获取所有数据
        mysqli_close($link);
    }else{
        echo &#39;数据库连接失败！&#39;;
    }
    echo &#39;<pre class="brush:php;toolbar:false">&#39;;
    print_r($data);
?>

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