如何从第一个表中返回所有列，而从第二个表中只返回一列

Question

I have a first table called "purchases"

news_series, transaction_id, owner_id, amount

I have another table called "Events"

news_name, news_id, series_id, news_description

The issue that I'm running into is that if I do

purchases.news_series joins to events.series_id

The issues is that there can be multiple events with the series id....

I need to join just one to get the news_name from the joined table so the base select is

Select * from purchases where owner_id=29

140, asldkfj_sdfx34, 29, 40

then I add the joined table

Select * 
from purchases 
LEFT JOIN events on purchases.news_series=events.series_id 
where owner_id=29

140, asldkfj_sdfx34, 29, 40,"THIS EVENT", 606, 140, "MY FIRST EVENT"
140, asldkfj_sdfx34, 29, 40,"THIS EVENT", 607, 140, "MY FIRST EVENT"

and I end up with a few rows returned...I just need one to capture the new_name from the events table.

Answer 1

I just need one to capture the news_name from the events table.

This is what I would do:

PURCHASES TABLE:

+-------------+----------------+----------+--------+
| news_series | transaction_id | owner_id | amount |
+-------------+----------------+----------+--------+
| 140         | asldkfj_sdfx34 | 29       | 40     |
+-------------+----------------+----------+--------+

EVENTS TABLE:

+------------+---------+-----------+------------------+
| news_name  | news_id | series_id | news_description |
+------------+---------+-----------+------------------+
| THIS EVENT | 606     | 140       | MY FIRST EVENT   |
+------------+---------+-----------+------------------+
| THIS EVENT | 607     | 140       | MY FIRST EVENT   |
+------------+---------+-----------+------------------+

---

SELECT DISTINCT just the one column you want from the joined table:

SELECT DISTINCT p.*, e.news_name
FROM Purchases p
LEFT JOIN Events e ON p.news_series = e.series_id
WHERE p.owner_id = 29

---

If you do not SELECT DISTINCT , this is why you are getting two rows.

---

Test:

;WITH Purchases (news_series, transaction_id, owner_id, amount) AS (
    SELECT '140','asldkfj_sdfx34','29','40'
), Events (news_name,news_id,series_id,news_description) AS (
    SELECT 'THIS EVENT','606','140','MY FIRST EVENT' UNION ALL
    SELECT 'THIS EVENT','607','140','MY FIRST EVENT' )

SELECT DISTINCT p.*, e.news_name
FROM Purchases p
LEFT JOIN Events e ON p.news_series = e.series_id
WHERE p.owner_id = 29

Answer 2

Might I suggest:

Select p.*,
       (select e.news_name
        from events e
        where p.news_series = e.series_id 
        limit 1
       )
from purchases  p
where owner_id = 29;

This is guaranteed to return one row per purchase.