This is an extended version of a related previous question. I have posted it a new question for Erwin Brandstetter suggested me to do so. (I realized that I actually wanted this, after people replied to my first question)
Having the following data (blank means NULL):
ID User ColA ColB ColC
1 1 15 20
2 1 11 4
3 1 3
4 2 5 5 10
5 2 6
6 2 8
7 1 1
How can I get the last not-NULL values of each column for all users, the simplest way? So the resulting for the given data would be:
User ColA ColB ColC
1 11 1 20
2 6 8 10
I have not found much, the function that seemed to do something similar to what I describe was COALESCE, but it does not work as expected in my case.
Note: Standard SQL if possible, PostgreSQL otherwise. The count of the involved columns might change, so a solution that is not tied to these three specific columns would be best.
"Standard" SQL
Similar to what I posted on the previous question, a recursive CTE is elegant and probably the fastest way to do it in standard SQL - especially for many rows per user.
WITH RECURSIVE t AS (
SELECT row_number() OVER (PARTITION BY usr ORDER BY id DESC) AS rn
,usr, cola, colb, colc
FROM tbl
)
, x AS (
SELECT rn, usr, cola, colb, colc
FROM t
WHERE rn = 1
UNION ALL
SELECT t.rn, t.usr
, COALESCE(x.cola, t.cola)
, COALESCE(x.colb, t.colb)
, COALESCE(x.colc, t.colc)
FROM x
JOIN t USING (usr)
WHERE t.rn = x.rn + 1
AND (x.cola IS NULL OR x.colb IS NULL OR x.colc IS NULL)
)
SELECT DISTINCT ON (usr)
usr, cola, colb, colc
FROM x
ORDER BY usr, rn DESC;
-> sqlfiddle for requested PostgreSQL.
The only non-standard element is DISTINCT ON, which is an extension to DISTINCT in the standard. Replace the final SELECT with this for a standard SQL:
SELECT usr
,max(cola) As cola
,max(colb) As colb
,max(colc) As colc
FROM x
GROUP BY usr
ORDER BY usr;
The request for "standard SQL" is of limited use. The standard only exists on paper. No RDBMS implements 100 % standard SQL - it would be rather pointless, too, since the standard includes nonsensical parts here and there. Arguably, PostgreSQL's implementation is among the closest to the standard.
PL/pgSQL function
This solution is specific to PostgreSQL, but should perform very well.
I am building on the same table as demonstrated in the fiddle above.
CREATE OR REPLACE FUNCTION f_last_nonull_per_user()
RETURNS SETOF tbl AS
$func$
DECLARE
_row tbl; -- table name can be used as row type
_new tbl;
BEGIN
FOR _new IN
SELECT * FROM tbl ORDER BY usr, id DESC
LOOP
IF _new.usr = _row.usr THEN
_row.id := _new.id; -- copy only id
IF _row.cola IS NULL AND _new.cola IS NOT NULL THEN
_row.cola := _new.cola; END IF; -- only if no value found yet
IF _row.colb IS NULL AND _new.colb IS NOT NULL THEN
_row.colb := _new.colb; END IF;
IF _row.colc IS NULL AND _new.colc IS NOT NULL THEN
_row.colc := _new.colc; END IF;
ELSE
IF _new.usr <> _row.usr THEN -- doesn't fire on first row
RETURN NEXT _row;
END IF;
_row := _new; -- remember row for next iteration
END IF;
END LOOP;
RETURN NEXT _row; -- return row for last usr
END
$func$ LANGUAGE plpgsql;
Call:
SELECT * FROM f_last_nonull_per_user();
Returns the whole row - including the min id we need to fill all columns.
This query is easy convert to MS SQL. If you need more something specific add comment. Mysql Query:
SQLFIDDLEExample
SELECT
t1.User,
(SELECT ColA
FROM Table1
WHERE ColA is not null
AND Table1.User = t1.User
ORDER BY ID DESC
LIMIT 1 ) as ColA,
(SELECT ColB
FROM Table1
WHERE ColB is not null
AND Table1.User = t1.User
ORDER BY ID DESC
LIMIT 1 ) as ColB,
(SELECT ColC
FROM Table1
WHERE ColC is not null
AND Table1.User = t1.User
ORDER BY ID DESC
LIMIT 1 ) as ColC
FROM Table1 t1
GROUP BY t1.User
Result:
| USER | COLA | COLB | COLC |
-----------------------------
| 1 | 11 | 1 | 20 |
| 2 | 6 | 8 | 10 |
