Using the SELECT operator 'AND' only if a variable is set - sql

Question

What is the correct/efficient way to display results based on a variable if the variable is set? If variable is not set, the AND operator should not be used.
I apologize if this is a repeat, I clicked the suggested links and they did not make sense to me.
Near the end of code is my note with ^^^^^ marked.
For example:
$whatever = 123;
SELECT
DISTINCT terms.name as product_type_name,
tax.term_id as termidouter
FROM $wpdb->posts AS p
INNER JOIN wp_term_relationships r
ON p.ID = r.object_id
INNER JOIN wp_term_taxonomy tax
ON r.term_taxonomy_id = tax.term_taxonomy_id
INNER JOIN wp_terms terms
ON tax.term_id = terms.term_id
WHERE
tax.taxonomy = 'product_type'
AND p.post_status = 'publish'
AND p.post_type = 'product'
AND '$whatever ' = terms.term_id
^^^^ If $whatever is empty, I want to return results as if this line did not exist.
ORDER BY product_type_name
");
I was going to do an IF/ELSE but I figured that was the lazy way.
$whatever = 123;
if (empty($whatever)) {
// SELECT without the AND
} else {
// SELECT with the AND
}

Answer 1

You could do something like this:
AND CASE WHEN '$whatever ' IS NOT NULL THEN '$whatever ' ELSE terms.term_id END = terms.term_id

Answer 2

So... typically you'd want to use prepared statements, but if we're going this route, I would collect all my optional search criteria in an array as strings:
$myTerms=array();
if(!empty($whatever)) {
$myTerms[]="terms.term_id='" . $whatever . "'";
}
...
Then you can build your query easily like so:
$mySql = "SELECT * FROM whatever WHERE somefield='somevalue' ";
if(count($myTerms)>0) {
$mySql.=" AND " . implode(" AND ", $myTerms);
}
Please note that this is a basic example; you should also be checking any value you pass into your query for attacks etc.