如何检查字符串是否已在SQL db中

Question

I am creating a small plugin which gives a serial number to each post, each serial number has to be unique, I don't know how to check if the generated serial number is already in the SQL db.

This is the key generator code:

function generateRandomString($length = 5) {
    $characters = '123456789';
    $charactersLength = strlen($characters);
    $randomString = '';
    for ($i = 0; $i < $length; $i++) {
        $randomString .= $characters[rand(0, $charactersLength - 1)];
    }
    return '#'.$randomString;   
$new_SN = generateRandomString(); }

How do I check if the value of $new_SN is already stored in a relevant SQL table cell - in my case the table is wp_postmeta, meta_key is serial and the meta_value is, well, the value populated by the function

Answer 1

Why are you generating a unique number? There are lots of tools specifically designed for this purpose - since you've mentioned MySQL, it supports auto-increment integers, PHP supports uuid (uniqid()) and several hashing functions (if you don't want to use sequential numbers). And as developerwjk says - if your generation function is at all effective then you shouldn't bother with a checking before saving the data - just add a unique index and trap duplicates when they fail to insert.