WHERE clause only if variable is set

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']."'"