How to Retrieve Only the First Row in a LEFT JOIN for a Specific Feed?

LEFT JOIN Only the First Row

In the world of database queries, the LEFT JOIN operation is a powerful tool for combining data from multiple tables. However, sometimes you may encounter the challenge of selectively retrieving only the first row from a LEFT JOIN, leading to confusion and seemingly unsuccessful results.

One such scenario involves fetching just the first artist associated with a feed. Consider the following simplified database structure:

• Feeds: id, title, content
• Artists: artist_id, artist_name
• feeds_artists: rel_id, artist_id, feed_id

To retrieve the designated feeds and their corresponding first artists, an intuitive approach might involve something like the following:

SELECT *
FROM feeds 
LEFT JOIN feeds_artists ON feeds.id = (
  SELECT
    feeds_artists.feed_id
  FROM feeds_artists
  WHERE
    feeds_artists.feed_id = feeds.id 
  LIMIT 1
)
WHERE
  feeds.id = '13815'

However, this query may not yield the desired outcome. Instead, try the following optimized query:

SELECT *
FROM feeds f
LEFT JOIN artists a ON a.artist_id = (
  SELECT
    artist_id
  FROM feeds_artists fa 
  WHERE
    fa.feed_id = f.id
  LIMIT 1
)
WHERE
  f.id = '13815'

In this query, the trick lies in using an inline query within the LEFT JOIN statement. This inline query selects the artist_id of the earliest artist associated with each feed, leveraging the assumption that the artist IDs increment over time. Consequently, the resulting query retrieves the desired row, showcasing the power of database querying.

The above is the detailed content of How to Retrieve Only the First Row in a LEFT JOIN for a Specific Feed?. For more information, please follow other related articles on the PHP Chinese website!