Collapse Multiple Records Into a Single Record With Multiple Columns - sql

Question

In a program I'm maintaining we were given a massive (~500 lines) SQL statement by the customer. It is used for generating flat files with fixed length records for transmitting data to another big business. Since its a massive flat file its not relational and the standard normal forms of data are collapsed. So, if you have a record that can have multiple codes associated, in this case upto 19, they all have be written into single line, but seperate fields, in the flat file.
Note: this example is simplified.
The data might look like this, with three tables:
RECORDS
record_id firstname lastname
--------------------------------
123 Bob Schmidt
324 George Washington
325 Ronald Reagan
290 George Clooney
CODE_TABLE
code_id code_cd code_txt
--------------------------------
5 3 President
2 4 Actor
3 7 Plumber
CODES_FOR_RECORDS
record_id code_cd
-------------------
123 7
325 3
290 4
324 3
325 4
123 4
This needs to produce records like:
firstname lastname code1 code2 code3
Bob Schmidt Actor Plumber NULL
George Washington President NULL NULL
Ronald Reagon Actor President NULL
George Clooney Actor NULL NULL
The portion of the current query we were given looks like this, but with 19 code columns instead of the 5:
select
x.record_id,
max(case when x.rankk = 1 then code_txt end) as CodeColumn1,
max(case when x.rankk = 2 then code_txt end) as CodeColumn2,
max(case when x.rankk = 3 then code_txt end) as CodeColumn3,
max(case when x.rankk = 4 then code_txt end) as CodeColumn4,
max(case when x.rankk = 5 then code_txt end) as CodeColumn5,
from
(
select
r.record_id,
ct.code_txt as ctag ,
dense_rank() over (partition by r.record_id order by cfr.code_id) as rankk
from
records as r
codes_for_records as cfr,
code_table as ct
where
r.record_id = cfr.record_id
and ct.code_cd = cfr.code_cd
and cfr.code_cd is not null
and ct.code_txt not like '%V%'
) as x
where
x.record_id is not null
group by
x.record_id
I trimmed down things for simplicties sake, but the actual statment includes an inner query and a join and more where conditions, but that should get the idea across. My brain is telling me there has to be a better way, but I'm not an SQL expert. We are using DB2 v8 if that helps. And the codes have to be in seperate columns, so no coalescing things into a single string. Is there a cleaner solution than this?
Update:
I ended up just refacorting the original query, it sill uses the ugly MAX() business, but overall the query is much more readable due to reworking other parts.

Answer 1

It sounds like what you are looking for is pivoting.
WITH joined_table(firstname, lastname, code_txt, rankk) AS
(
SELECT
r.firstname,
r.lastname,
ct.code_txt,
dense_rank() over (partition by r.record_id order by cfr.code_id) as rankk
FROM
records r
INNER JOIN
codes_for_records cfr
ON r.record_id = cfr.record_id
INNER JOIN
codes_table ct
ON ct.code_cd = cfr.code_cd
),
decoded_table(firstname, lastname,
CodeColumn1, CodeColumn2, CodeColumn3, CodeColumn4, CodeColumn5) AS
(
SELECT
firstname,
lastname,
DECODE(rankk, 1, code_txt),
DECODE(rankk, 2, code_txt),
DECODE(rankk, 3, code_txt),
DECODE(rankk, 4, code_txt),
DECODE(rankk, 5, code_txt)
FROM
joined_table jt
)
SELECT
firstname,
lastname,
MAX(CodeColumn1),
MAX(CodeColumn2),
MAX(CodeColumn3),
MAX(CodeColumn4),
MAX(CodeColumn5)
FROM
decoded_table dt
GROUP BY
firstname,
lastname;
Note that I've never actually done this myself before. I'm relying on the linked document as a reference.
You might need to include the record_id to account for duplicate names.
Edit: Added the GROUP BY.

Answer 2

One of the possible solutions is using of recursive query:
with recursive_view (record_id, rankk, final) as
(
select
record_id,
rankk,
cast (ctag as varchar (100))
from inner_query t1
union all
select
t1.record_id,
t1.rankk,
/* all formatting here */
cast (t2.final || ',' || t1.ctag as varchar (100))
from
inner_query t1,
recursive_view t2
where
t2.rankk < t1.rankk
and t1.record_id = t2.record_id
and locate(t1.ctag, t2.final) = 0
)
select record_id, final from recursive_view;
Can't guarantee that it works, but hope it will be helpful. Another way is using of custom aggregate function.