How Can I Efficiently Concatenate Multiple Rows into a Single Row in Oracle SQL?

Data Manipulation in Oracle: Concatenating Multiple Rows into One

In Oracle, efficiently concatenating multiple rows into a single row is often a desired operation. This question delves into a specific scenario where a table contains data in the following format:

question_id	element_id
1	7
1	8
2	9
3	10
3	11
3	12

The objective is to transform this data into the following desired result:

question_id	element_id
1	7,8
2	9
3	10,11,12

To accomplish this task effectively without resorting to stored procedures, Oracle 11gR2 introduced the LISTAGG clause. This remarkable clause allows for such concatenation operations to be performed directly within a single SQL statement.

Here's how to implement it in your Oracle query:

SELECT question_id,
       LISTAGG(element_id, ',') WITHIN GROUP (ORDER BY element_id)
FROM YOUR_TABLE
GROUP BY question_id;

The LISTAGG clause takes the following form:

LISTAGG(expression, delimiter) WITHIN GROUP (ORDER BY expression)

In this case, we specify the "element_id" column as the expression and "," as the delimiter to separate the concatenated values. The WITHIN GROUP clause ensures that the concatenation is performed within each group, which is defined by the "question_id" column.

It's worth noting that the resulting concatenated string may exceed the maximum length limit for a VARCHAR2 data type (4000 characters). To address this potential issue, Oracle 12cR2 introduced the ON OVERFLOW TRUNCATE/ERROR option. By incorporating this option, you can specify whether to truncate the string or raise an error if the length limit is exceeded.

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