SQL How to perform multiple look-ups from a list, in one query - sql

Question

We have a weird database table (wt) for which I can construct a query that can return a single row with these fields:
wt.thing_a_id = 5, wt.thing_b_id = 12, wt.thing_c_id = 9
Then, there's another lookup table (dt) that holds descriptions for these numbers, you could imagine it like this:
id desc
5 "flour"
12 "cups"
9 "barley"
what I need to end up with is numbers from wt, along with its description from dt.
I can do 3 simple queries, one to look up each of my three thing_ values (select desc from dt where id = ) but I was hoping to do it all in one query.
Is there a way to do this?
Even better, is there way to do my query to get my single row of thing id's and combine them with their descriptions? I think the fundamental problem/challenge is that my thing id's are not one per row, but that they come back as fields in just one row. This makes it really hard to join against them, for example.
Michael

Answer 1

You seem to want conditional aggregation:
select
max(case when id = 3 then descr end) descr_3,
max(case when id = 12 then descr end) descr_12,
max(case when id = 9 then descr end) descr_9
from dt
where id in (3, 12, 9)
Note that desc is a SQL keyword, hence a poor choice for a column name. I renamed it descr in the query.

Answer 2

You will need multiple joins to the dt table to get the description of each of the "things" you want in a single row:
SELECT thing_a_id, dta.desc AS thing_a_desc,
thing_b_id, dtb.desc AS thing_b_desc,
thing_c_id, dtc.desc AS thing_c_desc
FROM wt
JOIN dt dta ON dta.id = wt.thing_a_id
JOIN dt dtb ON dtb.id = wt.thing_b_id
JOIN dt dtc ON dtc.id = wt.thing_c_id

Answer 3

I love to play with common table expressions (CTE), this is an ideal candidate for one.
In the example below, decriptions and dataset are substitutes for the actual tables you use. I am just building them in memory rather than an actual table.
In the "breakdown" CTE I am splitting up the CSV value from dataset into multiple rows.
In the last part of the select I am converting everything after the = sign to a number, and then matching that on id from the descriptions CTE. The resulting dataset is I believe what you requested.
WITH
descriptions AS
(SELECT 5 AS id, 'flour' AS description FROM DUAL
UNION ALL
SELECT 12 AS id, 'cups' AS description FROM DUAL
UNION ALL
SELECT 9 AS id, 'barley' AS description FROM DUAL),
dataset AS
(SELECT 'wt.thing_a_id = 5, wt.thing_b_id = 12, wt.thing_c_id = 9' AS result FROM DUAL),
breakdown ( result, REMAINDER ) AS
(SELECT TRIM( SUBSTR( result
, 1
, INSTR( result || ',', ',' ) - 1 ) ) AS result
, TRIM( SUBSTR( result, INSTR( result || ',', ',' ) + 1 ) || ',' ) AS REMAINDER
FROM dataset
UNION ALL
SELECT TRIM( SUBSTR( REMAINDER
, 1
, INSTR( REMAINDER, ',' ) - 1 ) )
, SUBSTR( REMAINDER, INSTR( REMAINDER || ',', ',' ) + 1 ) AS REMAINDER
FROM breakdown
WHERE REMAINDER IS NOT NULL)
SELECT result, TO_NUMBER( TRIM( SUBSTR( result, INSTR( result, '=' ) + 1 ) ) ) AS id, description
FROM breakdown
LEFT OUTER JOIN descriptions
ON TO_NUMBER( TRIM( SUBSTR( breakdown.result, INSTR( breakdown.result, '=' ) + 1 ) ) ) =
descriptions.id
Results:
Result ID DESCRIPTION
wt.thing_a_id = 5 5 flour
wt.thing_b_id = 12 12 cups
wt.thing_c_id = 9 9 barley