How to select distinct for one column and any in another column?

  • I need to query an SQL database to find all distinct values of one column and I need an arbitrary value from another column. For example, consider the following table with two columns, key and value:

    key     value
    ===     =====
    one     test
    one     another
    one     value
    two     goes
    two     here
    two     also
    three   example
    

    I wish to get back one sample row, chosen arbitrarily, from each distinct key, perhaps getting these three rows:

    key     value
    ===     =====
    one     test
    two     goes
    three   example
    

    How can I formulate such a query in SQL?

    Which DBMS (Oracle, SQL-Server, DB2, MySQL, Postgres)?

    It is a proprietary system.

  • The easiest query to write is for MySQL (with not strict ANSI settings). It uses the non-standard construction:

    SELECT key, value
    FROM tableX
    GROUP BY key ;
    

    In recent version (5.7 and 8.0+) where the strict settings and ONLY_FULL_GROUP_BY are the default, you can use the ANY_VALUE() function, added in 5.7:

    SELECT key, ANY_VALUE(value) AS value
    FROM tableX
    GROUP BY key ;
    

    For other DBMSs, that have window functions (like Postgres, SQL-Server, Oracle, DB2), you can use them like this. The advantage is that you can select other columns in the result as well (besides the key and value) :

    SELECT key, value
    FROM tableX
        ( SELECT key, value,
                 ROW_NUMBER() OVER (PARTITION BY key 
                                    ORDER BY whatever)     --- ORDER BY NULL
                   AS rn                                   --- for example
          FROM tableX
        ) tmp 
    WHERE rn = 1 ;
    

    For older versions of the above and for any other DBMS, a general way that works almost everywhere. One disadvantage is that you cannot select other columns with this approach. Another is that aggregate functions like MIN() and MAX() do not work with some datatypes in some DBMSs (like bit, text, blobs):

    SELECT key, MIN(value) AS value
    FROM tableX
    GROUP BY key ;
    

    PostgreSQL has a special non-standard DISTINCT ON operator that can also be used. The optional ORDER BY is for selecting which row from every group should be selected:

    SELECT DISTINCT ON (key) key, value
    FROM tableX
    -- ORDER BY key, <some_other_expressions> ;
    

    @WilliamKF If by "chosen arbitrarily" you mean "chosen randomly" then simply replace the `ORDER BY whatever` in ypercube's query with a call to a function to randomize the results.

    @LeighRiffel It need not be random, any choice, as simple as the first one encountered works fine.

  • For MS-SQl Server:

    ;with FinalDataset as
    (
        select *,
            row_number() over(partition by key order by value) as rownum
        from YourOriginalTable
    )
    select
       key,
       value
    from FinalDataset 
    where rownum = 1
    

    Likewise, you could have rownum = 2 for your second result set

  • Similar to accepted answer, but instead of min() or max() you can use array_agg()

    SELECT key, (array_agg(value))[1] AS value
    FROM tableX
    GROUP BY key ;
    

    You can optionally order values inside array to select biggest or smallest of them:

    SELECT key, (array_agg(value) ORDER BY value DESC)[1] AS value
    FROM tableX
    GROUP BY key ;
    

    (checked on PostgreSQL)

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Content dated before 6/26/2020 9:53 AM