Multiple Dropdowns Dynamic Query AJAX PHP MySQL - php

Question

I am currently creating a multiple dropdown query where the user can query by (3) factors for returning results, my issue is how can I do this in an efficient manner so I am not obscurely writing multiple possible MySQL Queries.
<select name="class">
<OPTION VALUE='any'>Choose Class
<option value="a">Block A</option>
<option value="b">Block B</option>
<option value="c">Block C</option>
</select>
<select name="category">
<OPTION VALUE='any'>Choose Type
<option value="math">Math</option>
<option value="science">Science</option>
<option value="history">History</option>
</select>
How can I successfully create a MySQL query that will correctly select the right parameters in the case that a variable is missing. In example, if they choose to do the first dropdown and only search for the "class" and not choose the second dropdown. I want to be able to do the first query, the second query or both of them. I have the PHP, Ajax written, I'm just stumped as to the correct structure of the MySQL query.

Answer 1

Put your "WHERE" clause conditions in one array. I would do like this:
// filter out invalid values is important
$valid_class = array('a', 'b', 'c');
$valid_category = array('math', 'science', 'history');
// initialize array for WHERE clause conditions
$where = array('TRUE');
if (in_array($_POST['class'], $valid_class))
{
$where[] = 'class = "' . $_POST['class'] . '"';
}
if (in_array($_POST['category'], $valid_category))
{
$where[] = 'category = "' . $_POST['category'] . '"';
}
// use the array with the "implode" function to join its parts
$sql = 'SELECT * FROM table WHERE ' . implode(' AND ', $where);
You may want to initialize the $where array with something more interesting than "TRUE" (which is there in case the user does not filter by class neither category, because we don't want an empty $where array reaching the last line). For example:
$where = array();
$where[] = 'name = "' . mysql_real_escape_string($_POST['name']) . '"';

Answer 2

<select name="class">
<OPTION VALUE="">Choose Class
<option value="a">Block A</option>
<option value="b">Block B</option>
<option value="c">Block C</option>
</select>
<select name="category">
<OPTION VALUE="">Choose Type
<option value="math">Math</option>
<option value="science">Science</option>
<option value="history">History</option>
</select>
set the column of the table on ur MySQL to allow null
so when user select " <OPTION VALUE="">Choose Type" or "<OPTION VALUE="">Choose Class", pass Null into the column

Answer 3

Break the query in pieces and use it conditionally
suppose: $class is for class select box and $category is for category select box
$selectquery = "select * from tablename ";
if($class != "" && $category == ""){
$selectquery .= "WHERE class='".$class."'";
}elseif($class == "" && $category != ""){
$selectquery .= "WHERE category ='".$category."'";
}elseif($class != "" && $category != ""){
$selectquery .= "WHERE category ='".$category."' AND class='".$class."'";
}