How to perform multiple AND/OR meta_query using using $wpdb - sql

Question

SCENARIO: I have a custom post type with start and end dates saved as post meta. This is to allow the admin to schedule dates to show AND hide a post. There is also the option to leave these dates blank, in which case the post just shows by default. As meta_query doesn't appear to allow you to perform an AND/OR comparison, I've dabbled with creating a custom SQL query using $wpdb. SQL is new territory for me. Currently, this works for the start date:
$today = date('Y-m-d');
$results = $wpdb->get_results($wpdb->prepare("
SELECT
$wpdb->posts.post_title,
$wpdb->posts.ID,
$wpdb->posts.post_content
FROM
$wpdb->posts,
$wpdb->postmeta
WHERE
$wpdb->posts.ID = $wpdb->postmeta.post_id
AND $wpdb->posts.post_type = 'announcements'
AND ($wpdb->postmeta.meta_key = 'sap_start_date'
AND ($wpdb->postmeta.meta_value <= '$today' OR $wpdb->postmeta.meta_value = ''))
"));
However, when I try and amend the above to query the end date, I get nothing back. I tried changing the WHERE section to:
WHERE
$wpdb->posts.ID = $wpdb->postmeta.post_id
AND $wpdb->posts.post_type = 'announcements'
AND ($wpdb->postmeta.meta_key = 'sap_start_date'
AND ($wpdb->postmeta.meta_value <= '$today' OR $wpdb->postmeta.meta_value = ''))
AND ($wpdb->postmeta.meta_key = 'sap_end_date'
AND ($wpdb->postmeta.meta_value >= '$today' OR $wpdb->postmeta.meta_value = ''))
This returns an empty array.
QUESTION: How would I write this query to check both the sap_start_date and sap_end_date post meta values?

Answer 1

I figured out my problem was that I was using a logical AND against the same meta key object twice; how can postmeta.meta_key be equal to start_date and end_date? Using an answer from this SO question I managed to re-write the query to use aliases so that I could get the postmeta twice and query the two different keys:
SELECT
$wpdb->posts.post_title,
$wpdb->posts.ID,
$wpdb->posts.post_content
FROM
$wpdb->posts,
$wpdb->postmeta AS postmeta1,
$wpdb->postmeta AS postmeta2
WHERE
$wpdb->posts.ID = postmeta1.post_id
AND $wpdb->posts.ID = postmeta2.post_id
AND $wpdb->posts.post_type = 'announcements'
AND (postmeta1.meta_key = 'sap_start_date'
AND (postmeta1.meta_value <= '$today' OR postmeta1.meta_value = ''))
AND (postmeta2.meta_key = 'sap_end_date'
AND (postmeta2.meta_value >= '$today' OR postmeta2.meta_value = ''))
This works, although I'm still a little fuzzy on just how it works. If anyone could shed some further light on this purely for educational purposes it would be much appreciated.