如何通过带有where子句的共享列ID联接同一表中的行？

Question

I'm trying to write a SQL query that looks at a single MySQL DB table ( exp_playa_relationships ) which stores relationship data of CMS' posts and which data structure looks like this:

rel_id  |  parent_entry_id  |  parent_field_id  |  child_entry_id
-----------------------------------------------------------------
55      |  3                |  2                |  1
56      |  3                |  2                |  4
58      |  1                |  2                |  4
59      |  8                |  4                |  2
60      |  8                |  5                |  1
63      |  4                |  2                |  3
64      |  9                |  4                |  6
65      |  9                |  5                |  3

rel_id is unique, other columns are not.

I would like to generate the following out of the data above:

event_data_id  |  user_id  |  event_id
--------------------------------------
8              |  1        |  2
9              |  3        |  6

The parent_field_id value itself is discarded in the final output but is needed to figure out if the row's child_entry_id signifies a user_id or event_id .

parent_entry_id is the event_data_id .

So in plain english I would like to:

1. Filter rows that have a parent_field_id value of either 4 or 5
2. Out of those rows, I want to join all those that share the same parent_entry_id .
3. Return the parent_entry_id as event_data_id .
4. Return the child_entry_id as a user_id if parent_field_id of the same row is 5 .
5. Return the child_entry_id as a event_id if parent_field_id of the same row is 4 .

My current SQL query (not working) is this:

SELECT
    t1.`parent_entry_id` AS event_data_id,
    t1.`child_entry_id` AS user_id,
    t1.`child_entry_id` AS event_id

FROM `exp_playa_relationships` AS t1
INNER JOIN `exp_playa_relationships` AS t2
ON t1.`parent_entry_id` = t2.`parent_entry_id`
WHERE t1.`parent_field_id` = 4 OR t1.`parent_field_id` = 5

What I cannot figure out specifically is how to avoid creating duplicates on the parent_entry_id (SQL creates 2 new rows per row) and how to return child_entry_id as either user_id or event_id based on the parent_field_id value.

Any ideas would be much appreciated.

Answer 1

You're sooo close:

SELECT t1.`parent_entry_id` AS event_data_id,
       t1.`child_entry_id` AS user_id,
       t2.`child_entry_id` AS event_id
FROM `exp_playa_relationships` AS t1
INNER JOIN `exp_playa_relationships` AS t2
        ON t2.`parent_entry_id` = t1.`parent_entry_id`
           AND t2.`parent_field_id` = 4
WHERE  t1.`parent_field_id` = 5

Specifically, you're having to tell it which row-set to pull the relevant data from.

By the way, your current database design will cause you more of these types of headaches... I'd recommend pulling the information out into 'result' tables (unless that's what this is for?).