How to concatenate rows delimited with comma using standard SQL? - sql

Question

Let's suppose we have a table T1 and a table T2. There is a relation of 1:n between T1 and T2. I would like to select all T1 along with all their T2, every row corresponding to T1 records with T2 values concatenated, using only SQL-standard operations.
Example:
T1 = Person
T2 = Popularity (by year)
for each year a person has a certain popularity
I would like to write a selection using SQL-standard operations, resulting something like this:
Person.Name Popularity.Value
John Smith 1.2,5,4.2
John Doe NULL
Jane Smith 8
where there are 3 records in the popularity table for John Smith, none for John Doe and one for Jane Smith, their values being the values represented above. Is this possible? How?
I'm using Oracle but would like to do this using only standard SQL.

Answer 1

Here's one technique, using recursive Common Table Expressions. Unfortunately, I'm not confident on its performance.
I'm sure that there are ways to improve this code, but it shows that there doesn't seem to be an easy way to do something like this using just the SQL standard.
As far as I can see, there really should be some kind of STRINGJOIN aggregate function that would be used with GROUP BY. That would make things like this much easier...
This query assumes that there is some kind of PersonID that joins the two relations, but the Name would work too.
WITH cte (id, Name, Value, ValueCount) AS (
SELECT id,
Name,
CAST(Value AS VARCHAR(MAX)) AS Value,
1 AS ValueCount
FROM (
SELECT ROW_NUMBER() OVER (PARTITION BY Name ORDER BY Name) AS id,
Name,
Value
FROM Person AS per
INNER JOIN Popularity AS pop
ON per.PersonID = pop.PersonID
) AS e
WHERE id = 1
UNION ALL
SELECT e.id,
e.Name,
cte.Value + ',' + CAST(e.Value AS VARCHAR(MAX)) AS Value,
cte.ValueCount + 1 AS ValueCount
FROM (
SELECT ROW_NUMBER() OVER (PARTITION BY Name ORDER BY Name) AS id,
Name,
Value
FROM Person AS per
INNER JOIN Popularity AS pop
ON per.PersonID = pop.PersonID
) AS e
INNER JOIN cte
ON e.id = cte.id + 1
AND e.Name = cte.Name
)
SELECT p.Name, agg.Value
FROM Person p
LEFT JOIN (
SELECT Name, Value
FROM (
SELECT Name,
Value,
ROW_NUMBER() OVER (PARTITION BY Name ORDER BY ValueCount DESC)AS id
FROM cte
) AS p
WHERE id = 1
) AS agg
ON p.Name = agg.Name
This is an example result:
--------------------------------
| Name | Value |
--------------------------------
| John Smith | 1.2,5,4.2 |
--------------------------------
| John Doe | NULL |
--------------------------------
| Jane Smith | 8 |
--------------------------------

Answer 2

As per in Oracle you can use listagg to achive this -
select t1.Person_Name, listagg(t2.Popularity_Value)
within group(order by t2.Popularity_Value)
from t1, t2
where t1.Person_Name = t2.Person_Name (+)
group by t1.Person_Name
I hope this will solve your problem.
But the comment you have given after #DavidJashi question .. well this is not sql standard and I think he is correct. I am also with David that you can not achieve this in pure sql statement.

Answer 3

I know that I'm SUPER late to the party, but for anyone else that might find this, I don't believe that this is possible using pure SQL92. As I discovered in the last few months fighting with NetSuite to try to figure out what Oracle methods I can and cannot use with their ODBC driver, I discovered that they only "support and guarantee" SQL92 standard.
I discovered this, because I had a need to perform a LISTAGG(). Once I found out I was restricted to SQL92, I did some digging through the historical records, and LISTAGG() and recursive queries (common table expressions) are NOT supported in SQL92, at all.
LISTAGG() was added in Oracle SQL version 11g Release 2 (2009 – 11 years ago: reference https://oracle-base.com/articles/misc/string-aggregation-techniques#listagg) , CTEs were added to Oracle SQL in version 9.2 (2007 – 13 years ago: reference https://www.databasestar.com/sql-cte-with/).
VERY frustrating that it's completely impossible to accomplish this kind of effect in pure SQL92, so I had to solve the problem in my C# code after I pulled a ton of extra unnecessary data. Very frustrating.