How does while loop work with mysqli_fetch_assoc()?

问题

This is probably a silly beginner question and i don't think it's limited to mysqli_fetch_assoc() so it's probably a general programming question.

Anyways, i have this PHP code for getting data from a database using mysqli

$sql = "SELECT name FROM table1";
$result = mysqli_query($conn, $sql);

if($result){
    if(mysqli_num_rows($result) > 0){
        while($row = mysqli_fetch_assoc($result)){
            echo "Name: " . $row["name"]. "<br>";
        }
    }
}

What i don't understand is how the while loop there works. How does it iterates to the next element and know when to end? mysqli_fetch_assoc() returns a associative array which is stored in row variable which means it is not null therefore true and let's the while loop run. What i don't understand is how it iterates through the rows and ends when there are no more rows left. I'm not specifically doing anything to change the row to the next one so how does it do it on it's own?

Also mysqli_fetch_assoc() returns a associative array so shouldn't "name" key contain 1 element? Or is it a array of all the rows in that column?

(I hope you can understand what i'm trying to say, i'm not the best at explaining .-.)

Edit: What i don't understand is how this code iterates through all the rows. Is it part of the "inbuilt code" in this function? (I couldn't find it anywhere to confirm)

回答1:

Each time when mysqli_fetch_assoc($result) is accessed, the pointer moves to the next record. At last when no records are found, it returns null which breaks the while condition.

回答2:

Eventually, when mysqli_fetch_assoc($result) runs out of rows it will return NULL, which evaluates to false, which breaks out of the loop.

$row results in a single row in the database, so $row['name'] would be the value of 'name' for a particular row.

Let's break it down, with perhaps a dumbed down example of how it works internally (Please note, this works a lot more efficiently, and wont actually run multiple queries):

$result = ['currentRow' => 0, 'query' => 'SELECT name FROM table1'];

while ($row = mysqli_fetch_result($result)) {
}

//First iteration
mysqli_fetch_result queries 'SELECT name FROM table1 LIMIT 0, 1'
it increments internally $result['currentRow'] to 1
it returns the row that was found

//Second iteration
mysqli_fetch_result queries 'SELECT name FROM table1 LIMIT 1, 1'
it increments internally $result['currentRow'] to 2
it returns the row that was found

//Third iteration
mysqli_fetch_result queries 'SELECT name FROM table1 LIMIT 2, 1'
no rows returned! Just simply return null (This will cause your while loop to break out)

回答3:

You always pass $result as an argument to mysqli_fetch_assoc. This argument holds the iterator information, which will be used by mysqli_fetch_assoc to advance each time you call this function to the next row.

During each iteration, $row['name'] will be a single string.

来源：https://stackoverflow.com/questions/51573338/how-does-while-loop-work-with-mysqli-fetch-assoc

标签

php

mysqli

while-loop