I have 2 tables:

Table bs_steinweg
id | laptopid | anzahl | datum | ustd

Table bs_laptops
id | laptop | anzahl

If users want to reserve a laptop, I want to query whether there are still laptops available.

$readanzahl = $fromform->laptop;

comes from the form field just like

$ustd = $fromform->time;

and

$newdate = date( 'Y-m-d', $fromform->date );

Now I want to connect the two tables. In addition, there is the WHERE clause with 3 conditions.

LBTB.laptopid must be the same as the one from the form field.
LBTB.datum must also be the same value as the one from the form field.
LBTB.ustd also comes from the form field.

If there is already a reservation, then I want to calculate how many laptops are still left.
In the MySQL query, I try to subtract the value LAPTOP.anzahl minus the number of laptops that have already been reserved.

If the number is less than = 0, I want to output a message that no laptops are available.

Unfortunately, I don’t get an error message, so I can’t go any further and I’m hoping for your help.

Thank you very much!

$newdate = date( 'Y-m-d', $fromform->date );
        $readanzahl = $fromform->laptop;
        $ustd = $fromform->time;
        $laptopsanzahl = $DB->get_records_sql( 'SELECT  LBTB.datum, LBTB.laptopid, LBTB.ustd, LAPTOP.id, LAPTOP.laptop, LAPTOP.anzahl - LBTB.anzahl AS differenz FROM {bs_steinweg} LBTB
        LEFT JOIN {bs_laptops} LAPTOP
        ON LBTB.laptopid =  LAPTOP.id
        WHERE LBTB.laptopid = @$readanzahl
        AND LBTB.datum = @newdate
        AND LBTB.ustd = @ustd
        ' );

    if ( $laptopsanzahl <= 0 ) {
        var_dump($laptopsanzahl);
    }