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I have relation A (reviewer_id, book_id, trivalFields) Suppose at only 1 or 2 reviews exist for the book_id. I would like a result set that review_id (first), review_id (second), book_id. Example:

  johnson, smith, 'horton hears a who' 
  johnson, smith, 'green eggs and ham'.

But not

  johnson, smith, 'horton hears a who' 
  smith, johnson,  'horton hears a who' 
  johnson, smith, 'green eggs and ham'.
  smith, johnson,  'green eggs and ham'.

I know how do something like this with brute strength code - but how to do it in pure SQL??

PostgreSQL - by the way.

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SELECT t1.reviewer_id AS first, 
       t2.reviewer_id AS second, 
       t1.book_id
FROM table AS t1 
LEFT JOIN table AS t2 ON t2.book_id = t1.book_id 
                     AND t2.reviewer_id > t1.reviewer_id

or

SELECT MIN(reviewer_id) AS first, 
       CASE WHEN COUNT(reviewer_id) > 1 
            THEN MAX(reviewer_id) 
            ELSE NULL
            END AS second, 
       book_id
FROM table
GROUP BY book_id
  • Brilliant !!!!! – terary Oct 15 at 8:07

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