WHERE clause only if variable is set - mysql

Question

An input and script in index.php
<input type="text" class="filter" id="frClientName" name="cl_name">
<script>
$(".filter").on('change keydown keyup', function(){
var clname;
clname = document.getElementById("frClientName").value;
$("#spravaContent").load("php/search_results/sprava.php?cl_name=" + clname;
});
</script>
php/search_results/sprava.php
$clname = '';
if ( isset ( $_GET['cl_name'] ) ) {
$clname = $_GET['cl_name'];
}
$sql = ("
SELECT * FROM `db`.`table`
WHERE cl_full_name LIKE '%".$_GET['cl_name']."%'
");
How can I use WHERE clause right and only if variable isn't empty?
Thank you for some direction.
EDIT:
$sql = ("
SELECT * FROM `db`.`table`
ORDER BY {$oby} {$ohow}
WHERE cl_full_name LIKE '%".$_GET['cl_name']."%'
OR '' = '".$_GET['cl_name']."'
LIMIT $start_sprava,$per_page_sprava
");

Answer 1

It's a bit hacky, but I've handled situations like this with a CASE statement. You evaluate your argument and if it doesn't meet the condition you require, you use an obviously true statement like 1=1 which has the net effect of keeping that part of the WHERE clause from participating in filtering the result set.
SELECT *
FROM `db`.`table`
WHERE CASE WHEN TRIM('".$_GET['cl_name']."') IS NOT NULL THEN cl_full_name LIKE '%".$_GET['cl_name']."%'
ELSE 1=1
END
ORDER BY {$oby} {$ohow}
LIMIT {$start_sprava}, {$per_page_sprava}
;
Don't forget to police your inputs so you don't wind up with a Little Bobby Tables problem. Also, the ORDER BY and LIMIT clauses generally come after the WHERE clause.

Answer 2

Make the where clause work whether set or not.
Assuming your app language returns the text "null" if variable is not set:
"SELECT * FROM `db`.`table`
WHERE cl_full_name LIKE '%".$_GET['cl_name']."%'
OR 'null' = '".$_GET['cl_name']."'"