How to change a UNION to a IN clause? - sql

Question

I need to get a maximum of 3 distinct records from the same table, so currently I'm doing:
SELECT 1, mycolumn FROM mytable WHERE id = #firstId
UNION ALL
SELECT 2, mycolumn FROM mytable WHERE id = #secondId
UNION ALL
SELECT 3, mycolumn FROM mytable WHERE id = #thirdId
The actual SELECT part contains over 20 columns and the FROM part contains a number of JOINs. The first column is a constant and is always fixed depending on the record. I don't know how many records might return. It could be anything from 0 to 3 records.
Is it possible to change the above query so that it uses IN like this:
SELECT ???, mycolumn FROM mytable WHERE id IN (#firstId, #secondId, #thirdId)
But how do I explicitly map each record to the fixed constant if I use IN?

Answer 1

You may use a CASE expression here with a single query:
SELECT
CASE id WHEN #firstId THEN 1
WHEN #secondId THEN 2
WHEN #thirdId THEN 3 END AS val,
mycolumn
FROM mytable
WHERE
id IN (#firstId, #secondId, #thirdId);
If you wish to also order by the computed column, then add ORDER BY val to the end of the above query.

Answer 2

You can use CASE like following.
SELECT
CASE
WHEN id= #firstId THEN 1
WHEN id=#secondId THEN 2
ELSE 3
END AS rn,
mycolumn
FROM mytable
WHERE id IN (#firstId,
#secondId,
#thirdId)
Another approach can be using DENSE_RANK if you have one record for each provided id and #firstId, #secondId & #thirdId are in ascending order.
SELECT DENSE_RANK()
OVER(
ORDER BY id) rn,
mycolumn
FROM mytable
WHERE id IN ( #firstId, #secondId, #thirdId )

Answer 3

I would recommend a table-valued constructor for this purpose:
select v.outputnum, my_column
from mytable t join
(values (#firstid, 1),
(#secondid, 2),
(#thirdid, 3)
) v(id, outputnum)
on t.id = v.id
order by v.outputnum;
I think this is simpler than other versions, because the list of ids is only present once in the query -- so no danger of different parts of the query getting out of sync.