SELECT distinct values for multiple rows of same ID

Question

I have a table that looks like this:

ID | FIELD_NAME   | VALUE
23 |  sign_up     |  yes
23 |  first_name  |  Fred
23 |  street      |  Barber Lane
24 |  sign         


        

Answer 1

One approach is to use a correlated subquery to return each field value as a column,

SELECT t.id
     , (SELECT f1.value FROM mytable f1 
         WHERE f1.id = t.id AND f1.field_name = 'sign_up' 
         ORDER BY f1.value LIMIT 1
       ) AS SIGN_UP
     , (SELECT f2.value FROM mytable f2 
         WHERE f2.id = t.id AND f2.field_name = 'first_name' 
         ORDER BY f2.value LIMIT 1
       ) AS FIRST_NAME
     , (SELECT f3.value FROM mytable f3 
         WHERE f3.id = t.id AND f3.field_name = 'street'
         ORDER BY f3.value LIMIT 1
       ) AS STREET
  FROM (SELECT s.id
          FROM mytable s
         GROUP BY s.id
         ORDER BY s.id
       ) t

This isn't the only way, but it's a workable approach, especially if you are concerned that you will get exactly four columns returned, and that they will be returned in a specific sequence.

Note that this approach works when a particular field_name is "missing" for a particular ID (it will return a NULL in place of a value). It also works if there are multiple occurrences of the same field_name for a particular ID. (This query will return only one of them, and disregard the other.)

This same result set can also be obtained with a query written like this:

SELECT t.id          AS ID
     , f1.sign_up    AS SIGN_UP
     , f2.first_name AS FIRST_NAME
     , f3.street     AS STREET       
  FROM (SELECT s.id
          FROM mytable s
         GROUP BY s.id
         ORDER BY s.id
       ) t
   LEFT      
   JOIN (SELECT s1.id
              , MIN(s1.value) AS sign_up
           FROM mytable s1
          WHERE s1.field_name = 'sign_up'
            AND s1.value IS NOT NULL 
          GROUP BY s1.id
        ) f1
     ON f1.id = t.id   
   LEFT
   JOIN (SELECT s2.id
              , MIN(s2.value) AS first_name
           FROM mytable s2
          WHERE s2.field_name = 'first_name'
            AND s2.value IS NOT NULL
          GROUP BY s2.id
        ) f2
     ON f2.id = t.id
   LEFT
   JOIN (SELECT s3.id
              , MIN(s3.value) AS street
           FROM mytable s3
          WHERE s3.field_name = 'street'
            AND s3.value IS NOT NULL
          GROUP BY s3.id
        ) f3
     ON f3.id = t.id

With other queries, ensure you are getting the desired behavior when a field_name is "missing" for a given ID, or when there are duplicate field_name for a given ID, or when there are additional field_name values in the table which you are not concerned with.