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> ;