How can I insert variable into SQL query only if it exist? - mysql

Question

Let's say I have such html:
<form id="form">
Level:
<br/>
<select id="userLevel" name="userLevel">
<option value="">Choose level</option>
<option value="novice">novice</option>
<option value="intermediate">intermediate</option>
<option value="advanced">advanced</option>
</select>
Days per week:
<select id="days" name="days">
<option value="">Days per week:</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
</select>
<input type="button" id="btn" value="Submit"/>
</form>
<br />
<div id="txtHint"><b>Table goes here...</b></div>
Also I have a database with "user_level" and "days_per_week" columns. I want my code to show certain rows from that table, depends on wich parametrs user has chosen. But, if he didn't choose one of them, 'days' for example, program must display rows depending on only chosen parameters and dismising not chosen ones. How can I constuct SQL query properly? Something to check if that variable was set and if it wasn't - neglect it.
$userLevel = $_POST['userLevel'];
$days = $_POST['days'];
$sql="SELECT *
FROM table
WHERE ifset(user_level){
user_level = $userLevel
} else {
neglect this variable
}
AND ifset(days_per_week){
days_per_week= $days
} else {
neglect this variable
}"
EDIT: that table has about dozen of columns, and eventually that sql query will operate with several variables. I brought two of them just for example.

Answer 1

Here is your code :
$q = "";
if(isset($_POST['userLevel']) && $_POST['userLevel'] != "") {
$userLevel = $_POST['userLevel'];
$q .= " and user_level = '$userLevel'";
}
if(isset($_POST['days']) && $_POST['days'] != "") {
$days = $_POST['days'];
$q .= " and days_per_week= '$days'";
}
$sql="SELECT *
FROM table
WHERE id is not null $q"