PHP 当 mysqli_fetch_assoc 表达式仅用一个变量替换时提供无限 While 循环

Question

I'm a complete newbie in PHP and I just don't understand how while loop works with mysqli_fetch_assoc . The former execute statements based upon expressions, the latter retrieves a row from the database. So if you don't end the loop, it will continue to iterate the field in the first row infinitely;

<?php
$connect_db=mysqli_connect('localhost','root','root','db');
$fetch_data="SELECT * FROM tabel";
$query_db=mysqli_query($connect_db,$fetch_data);
$fetch_row=mysqli_fetch_assoc($query_db);
while($fetch_row){
    $column_2_array=$fetch_row['column_2'];
    echo($column_2_array);
    break;
}
?>

What I'm scratching my head is if the whole expression is put inside the parentheses instead of just the variable, then the loop will iterates the whole content of the column.

<?php
$connect_db=mysqli_connect('localhost','root','root','db');
$fetch_data="SELECT * FROM tabel";
$query_db=mysqli_query($connect_db,$fetch_data);
while($fetch_row=mysqli_fetch_assoc($query_db)){
    $column_2_array=$fetch_row['column_2'];
    echo($column_2_array);
}
?>

Why is it that on the second example above, after the first loop, while is iterating the subsequent rows on the selected column?

Clarifying my question further, if I take away break from the first example, then what is the difference between example 1 and 2? This is what confuses me since I thought they're identical.

Answer 1

To understand why this approach of looping mysqli_result works with a while loop you need to understand two things:

• PHP uses type juggling . Each row returned by mysqli_fetch_assoc() will be an array. It is safe to assume that this array will never be empty, so the value will always be cast to true when used in boolean context.
• mysqli_fetch_assoc() returns a single row from the result and moves an internal pointer to the next row. When pointer reaches the last row, each subsequent call will return NULL .

This while loop is equivalent to:

$fetch_row = mysqli_fetch_assoc($query_db); // save the first row in the variable. It could also be false
while($fetch_row) { // as long as $fetch_row is not false-ish
    $column_2_array = $fetch_row['column_2'];
    echo $column_2_array;

    // fetch false or the next row
    $fetch_row = mysqli_fetch_assoc($query_db)
}

Worth noting that such an approach to looping is not recommended. It is much easier to use a foreach loop. This will loop on the whole resultset one by one from the first row until the last. You can loop the same mysqli_result object multiple times.

foreach($query_db as $fetch_row) {
    $column_2_array = $fetch_row['column_2'];
    echo $column_2_array;
}

An even better alternative is to use fetch_all(MYSQLI_ASSOC) and store all rows in a multi-dimensional array. And as always, use PDO instead of mysqli whenever possible.

Answer 2

The simplest explanation is,

When you run mysqli_query() For successful SELECT, SHOW, DESCRIBE or EXPLAIN queries mysqli_query() will return a mysqli_result object which contains data from the database.

---

• mysqli_fetch_assoc() - Fetch a result row(one row from the mysqli_result object) as an associative array at each time it called and moves the internal pointer to the next. $fetch_row is used to store that associative array containing data of single row getting from the mysqli_result object '

you can retrieve data belongs to that raw by using $fetch_row['column_name']

• The while loop does the calling of mysqli_fetch_assoc() over and over. After one row is fetched, it goes to the next one and so on till it reaches the end of the object and returns NULL , which means false and breaks the while loop.

---

So in your first code sample,

$fetch_row=mysqli_fetch_assoc($query_db); will return and assign only one data row to $fetch_row (data type- assoc. array). Which makes the condition of the while loop true .