与mysql两表数据交叉匹配，表中数值显示使用PHP mySql

Question

I am having some trouble with php and mysql, I am even not sure how to properly ask the question, it seems very complex. Still if anyone can help me, i will be very thankful.

i have two tables (allunit.sql)

id - unit_name
12       -   MIS
14       -   MIT
15       - ENG

when someone click enroll button from browser (unit_id) will store in enrollment table. if some one enroll into the unit, button will show (Already Enrolled), not not it will show "Enroll" enrollment.sql

enroll_id - unit_id
1         - 12
2         - 14

I am using this query

$unit = SELECT * FROM allunit;

$enroll = SELECT * FROM enrollment;

$row_enroll = mysqli_fetch_assoc($enroll);

while($row = mysqli_fetch_assoc($unit)) {
    if($row['id']==$row_enroll['unit_id']){
        $button = 'Already enrolled';
    }else{
        $button = 'Enroll';
    }
?>

<tr>
     <td><?php echo $row['id']; ?></td>
            <td><?php echo $row['unit_name']; ?></td>

           <td><?php echo $button; ?></td>
</tr>
<?php } ?>

if i add one unit button changes to "already Enrolled" for that unit, but if i add more than one, still only one button changes. other stays same "enroll".

I know my question is reallty messy, hope you will understand. Badly need help. Thank you

Answer 1

There are two problems I see in your code:

1. mysqli_fetch_assoc() is called on a MySQL result, not a query. You need to call mysqli_query() first. You can see an example in the docs: https://www.php.net/manual/en/mysqli-result.fetch-assoc.php
2. When you get a result, such as $row_enroll , it's a collection of rows, so you can't use it with a column directly, ie $row_enroll['unit_id'] won't give you anything.

Finally, it doesn't appear that a comparison between two separate datasets like this is going to work well for you, at least with the current code. Consider using JOIN s to return just one dataset.

Answer 2

First, you have to tell the database to run your query, it is not enough to place a query in a text string. This is done, in this case using the query() method.

Second, as you want to process the Enrolments once for each of the Units, it would be useful to unload at least the Enrolment into an array so it is easily reusable

// assuming you have a connection and its in $con

$sql    = 'SELECT * FROM allunit';
$units   = $con->query($sql);

$sql    = 'SELECT unit_id FROM enrollment';
$res2   = $con->query($sql);
// make an array of just the enrolment id's as that all you need
// so we can use in_array() later to do the test for are you already enrolled
$enrols = [];
while ($row = $res2->fetch_assoc()){
    $enrols[] = $row['unit_id'];
}

while ($unit = $units->fetch_assoc() ) {
    if ( in_array($unit['id'], $enrols) ) {
        $button = 'Already enrolled';
    }else{
        $button = 'Enroll';
    }

?>
<tr>
    <td><?php echo $unit['id']; ?></td>
    <td><?php echo $unit['unit_name']; ?></td>
    <td><?php echo $button; ?></td>
</tr>
<?php 
} // endwhile
?>