Concatenating clob cloumn values in sql query - sql

Question

I am using this statement in my sql query to concate large clob column values but the output contains extra ","(commas) not able to figure out what is going wrong.?
SELECT RTRIM(
XMLAGG(
XMLELEMENT(
E,
CASE WHEN UNIQ_ID IN ( SELECT VAL
FROM SOME_TABLE
WHERE VAL_NM = 'SOME_TEXT' )
THEN TABLE1.COL_NAME
ELSE NULL
END,
', '
).EXTRACT('//text()')
ORDER BY TABLE1.UNIQ_ID
).GETCLOBVAL(),
','
) COMBINED_VAL

Answer 1

If you are asking about the trailing commas, then you are concatenating using comma then space so the trailing character is a space and not a comma.
If you are asking about adjacent separators with no value in between then when the WHEN UNIQ_ID IN ( ... ) part of your CASE statement is not matched you will have a NULL value; this is concatenated into the aggregated output and then you will find that you have two adjacent comma-space separators with no text in between.
For example:
WITH test_data ( id, value ) AS (
SELECT 1, 'a' FROM DUAL UNION ALL
SELECT 2, NULL FROM DUAL UNION ALL
SELECT 3, 'b' FROM DUAL
)
SELECT RTRIM(
XMLAGG(
XMLELEMENT(
E,
value,
', '
).EXTRACT('//text()')
ORDER BY id
).GETCLOBVAL(),
','
) AS COMBINED_VAL
FROM test_data;
Outputs:
| COMBINED_VAL |
| :----------- |
| a, , b, |
The trailing comma-space isn't trimmed as the last character is a space and the values are a then NULL then b and the NULL is represented as a zero-width substring.
db<>fiddle here

Answer 2

That's pretty easy:
do not aggregate rows which you don't want to get. To do that you just need to generate xmlelement only for required rows, and just return null for others.
Just put all characters you want to trim from your result into second parameter of rtrim:
SELECT RTRIM(
XMLAGG(
CASE WHEN UNIQ_ID IN ( SELECT VAL
FROM SOME_TABLE
WHERE VAL_NM = 'SOME_TEXT' )
and COL_NAME is not null
THEN XMLELEMENT(
E,
TABLE1.COL_NAME||', '
)
END
ORDER BY TABLE1.UNIQ_ID
).extract('//text()').GETCLOBVAL(),
', '
) COMBINED_VAL
from table1;
Full test case with sample data and results: https://dbfiddle.uk/?rdbms=oracle_11.2&fiddle=452c715247e8edda8735014ff2fb34f4
with
SOME_TABLE(VAL, VAL_NM) as (
select level*2, 'SOME_TEXT' from dual connect by level<=10
)
,TABLE1(UNIQ_ID, COL_NAME) as (
select level UNIQ_ID
, to_clob(level) COL_NAME
from dual
connect by level<=20
)
SELECT RTRIM(
XMLAGG(
CASE WHEN UNIQ_ID IN ( SELECT VAL
FROM SOME_TABLE
WHERE VAL_NM = 'SOME_TEXT' )
and COL_NAME is not null
THEN XMLELEMENT(
E,
TABLE1.COL_NAME||', '
)
END
ORDER BY TABLE1.UNIQ_ID
).extract('//text()').GETCLOBVAL(),
', '
) COMBINED_VAL
from TABLE1;
Results:
COMBINED_VAL
----------------------------------------
2, 4, 6, 8, 10, 12, 14, 16, 18, 20