Getting record from three wpdb tables

Question

I'm trying to deal with wpdb but unfortunetelly things that I'm creating are not even close to reaching what I need. I have data:

seller ID (post_author = user_id) - $seller_id

and

(post_parent = transaction_id) - $ordernum

I have 3 tables:

wp_dopbsp_calendars

id
user_id
post_id name
max_year
hours_enabled
hours_interval_enabled
price_min
price_max 
rating 
address address_en
address_alt
address_alt_en
coordinates

wp_dopbsp_reservations

id
calendar_id
language
currency
currency_code
check_in
check_out
start_hour
end_hour
no_items
price
price_total
refund
extras
extras_price
discount
discount_price
coupon
coupon_price
fees
fees_price
deposit
deposit_price
days_hours_history
form
address_billing
address_shipping
email
status
payment_method
payment_status
transaction_id
token
date_created

and wp_posts

ID
post_author
post_date
post_date_gmt
post_content
post_title
post_excerpt
post_status
comment_status
ping_status
post_password
post_name
to_ping
pinged
post_modified
post_modified_gmt
post_content_filtered
post_parent
guid
menu_order
post_type
post_mime_type
comment_count

I need to create table and get its only record:

by seller ID (post_author = user_id) get calendar ID ("id" table wp_dopbsp_calendars) and post_parent ("post_parent" from wp_posts)

And with these two look for rows in table wp_dopbsp_reservations, and get from there only the ones where transaction_id is post_parent (from wp_posts) and calendar_id is id (from wp_dopbsp_calendars)

The columns that I need from this table are transaction_id, calendar_id, check_in, check_out, start_hour, end_hour and price_total.

I couldn't go any further than this:

Select
  $wpdb->posts.post_author,
  $wpdb->dopbsp_calendars.user_id,
  $wpdb->dopbsp_reservations.price_total,
 $wpdb->dopbsp_reservations.check_in,
  $wpdb->dopbsp_reservations.check_out,
  $wpdb->dopbsp_reservations.start_hour,
  $wpdb->dopbsp_reservations.end_hour,
  $wpdb->dopbsp_calendars.id,
  $wpdb->posts.post_title,
  $wpdb->posts.ID As ID1,
  $wpdb->posts.post_parent,
  $wpdb->dopbsp_reservations.transaction_id
From
  wp_posts
  INNER JOIN $wpdb->posts.post_author ON $wpdb->dopbsp_calendars.user_id
  $wpdb->dopbsp_calendars
    On wp_posts.post_author = $wpdb->dopbsp_calendars.user_id,
  $wpdb->dopbsp_reservations

Answer 1

You don't want to ask wpdb for each field. This is the wrapper for SQL queries

Like so (not tested):

<?php
$my_results = $wpdb->get_results(
    SELECT
        posts.post_author, dopbsp_calendars.user_id, dopbsp_reservations.price_total, dopbsp_reservations.check_in, dopbsp_reservations.check_out, dopbsp_reservations.start_hour, dopbsp_reservations.end_hour, dopbsp_calendars.id, posts.post_title, posts.ID As ID1, posts.post_parent, dopbsp_reservations.transaction_id
    FROM wp_posts
    INNER JOIN
        posts.post_author
    ON
        dopbsp_calendars.user_id
        dopbsp_calendars
    ON
        wp_posts.post_author = dopbsp_calendars.user_id,
        dopbsp_reservations
);

Then see what you get in this object:

<?php
echo '<pre>';
print_r($my_results);
echo '</pre>';

Warning: Methods in the wpdb() class should not be called directly. Use the global $wpdb object instead! Class Reference/wpdb

So, it's better to make $wpdb global:

global $wpdb;
$my_results = $wpdb->get_results(..................);

Notice: I didn't checked the query itself.