Add “Category” to Mysql Query

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If I’m not missing something, the below should work for you. It is pretty much straight from the Codex: WP_Query – Custom Field Parameters. Do it like this:

$args = array(
    'post_type' => 'post',
    'meta_query' => array(
        'relation' => 'AND',
        array(
            'key' => 'color',
            'value' => 'red',
            'compare' => '='
        ),
        array(
            'key' => 'size',
            'compare' => 'EXISTS'
        )
    )
);
$query = new WP_Query( $args );

Additional resources on here: