How can I transform my N little queries into one query? - sql

Question

I have a query that gives me the first available value for a given date and pair.
SELECT
TOP 1 value
FROM
my_table
WHERE
date >= 'myinputdate'
AND key = 'myinpukey'
ORDER BY date
I have N pairs of key and dates, and I try to find out how not to query each pair one by one. The table is rather big, and N as well, so it's currently heavy and slow.
How can I query all the pairs in one query ?

Answer 1

A solution is to use APPLY like a "function" created on the fly with one or many columns from another set:
DECLARE #inputs TABLE (
myinputdate DATE,
myinputkey INT)
INSERT INTO #inputs(
myinputdate,
myinputkey)
VALUES
('2019-06-05', 1),
('2019-06-01', 2)
SELECT
I.myinputdate,
I.myinputkey,
R.value
FROM
#inputs AS I
CROSS APPLY (
SELECT TOP 1
T.value
FROM
my_table AS T
WHERE
T.date >= I.myinputdate AND
T.key = I.myinputkey
ORDER BY
T.date ) AS R
You can use OUTER APPLY if you want NULL result values to be shown also. This supports fetching multiple columns and using ORDER BY with TOP to control amount of rows.

Answer 2

This solution is without variables. You control your N by setting the right value to the row_num predicate.
There are plenty of ways how to do you what you want and it all depends on your specific needs. As it answered already, that you can use temp/variable table to store these conditions and then join it on the same conditions you use predicates. You can also create user defined data type and use it as param to the function/procedure. You might use CROSS APPLY + VALUES clause to get that list and then join it.
DROP TABLE IF EXISTS #temp;
CREATE TABLE #temp ( d DATE, k VARCHAR(100) );
GO
INSERT INTO #temp
VALUES ( '20180101', 'a' ),
( '20180102', 'b' ),
( '20180103', 'c' ),
( '20180104', 'd' ),
( '20190101', 'a' ),
( '20190102', 'b' ),
( '20180402', 'c' ),
( '20190103', 'c' ),
( '20190104', 'd' );
SELECT a.d ,
a.k
FROM ( SELECT d ,
k ,
ROW_NUMBER() OVER ( PARTITION BY k ORDER BY d DESC ) row_num
FROM #temp
WHERE (d >= '20180401'
AND k = 'a')
OR (d > '20180401'
AND k = 'b')
OR (d > '20180401'
AND k = 'c')
) a
WHERE a.row_num <= 1;
-- VALUES way
SELECT a.d ,
a.k
FROM ( SELECT t.d ,
t.k ,
ROW_NUMBER() OVER ( PARTITION BY t.k ORDER BY t.d DESC ) row_num
FROM #temp t
CROSS APPLY (VALUES('20180401','a'), ('20180401', 'b'), ('20180401', 'c')) f(d,k)
WHERE t.d >= f.d AND f.k = t.k
) a
WHERE a.row_num <= 1;

Answer 3

If all the keys are using the same date, then use window functions:
SELECT key, value
FROM (SELECT t.*, ROW_NUMBER() OVER (PARTITION BY key ORDER BY date) as seqnum
FROM my_table t
WHERE date >= #input_date AND
key IN ( . . . )
) t
WHERE seqnum = 1;

Answer 4

SELECT key, date,value
FROM (SELECT ROW_NUMBER() OVER (PARTITION BY key,date ORDER BY date) as rownum,key,date,value
FROM my_table
WHERE
date >= 'myinputdate'
) as d
WHERE d.rownum = 1;