LISTAGG FAQS

1. What is LISTAGG in Oracle?

LISTAGG is an aggregate function in Oracle that concatenates multiple rows of data into a single string with a specified delimiter between the values. It is useful for generating lists of items from multiple rows.

 

2. How do I use LISTAGG?

The syntax for LISTAGG is:

SELECT LISTAGG(column_name, 'delimiter')

       WITHIN GROUP (ORDER BY column_name)

FROM table_name;

• column_name: The column whose values you want to concatenate.
• delimiter: The string used to separate the concatenated values (e.g., comma ,, space, semicolon ;).
• WITHIN GROUP (ORDER BY column_name): Specifies the order in which the values are concatenated.

 

3. How does LISTAGG handle NULL values?

LISTAGG ignores NULL values by default. Only non-null values will be concatenated in the result.

 

4. Can I use LISTAGG with GROUP BY?

Yes, you can use LISTAGG with the GROUP BY clause to concatenate values within groups.

Example:

SELECT Department,

       LISTAGG(Employee, ', ') WITHIN GROUP (ORDER BY Employee) AS Employees_List

FROM Employees

GROUP BY Department;

 

5. Can I specify the order of the concatenated values?

Yes, you can specify the order of the values using the ORDER BY clause inside the WITHIN GROUP. This determines the order in which values are concatenated.

Example:

SELECT Department,

       LISTAGG(Employee, ', ') WITHIN GROUP (ORDER BY Salary DESC) AS Employees_List

FROM Employees

GROUP BY Department;

 

6. What happens if the result exceeds the maximum string length (4000 characters)?

If the concatenated result exceeds the 4000-character limit, Oracle will raise the error ORA-01489: result of string concatenation is too long. You can handle this by using the ON OVERFLOW clause or by truncating the result.

Example:

SELECT LISTAGG(Employee, ', ') WITHIN GROUP (ORDER BY Employee)

       ON OVERFLOW TRUNCATE '...' AS Employees_List

FROM Employees;

 

7. How can I concatenate multiple columns with LISTAGG?

You can concatenate values from multiple columns by combining them in the LISTAGG function.

Example:

SELECT Department,

       LISTAGG(Employee || ' (' || Salary || ')', ', ')

       WITHIN GROUP (ORDER BY Employee) AS Employee_Salary_List

FROM Employees

GROUP BY Department;

 

8. Is LISTAGG case-sensitive?

Yes, LISTAGG is case-sensitive when concatenating string values. To ensure case-insensitive concatenation, you may use functions like UPPER() or LOWER() in the ORDER BY clause.

 

9. Can I use LISTAGG to generate a comma-separated list?

Yes, LISTAGG is commonly used to create a comma-separated list or custom delimited string from a set of values.

Example:

SELECT LISTAGG(Product, ', ') WITHIN GROUP (ORDER BY Product) AS Product_List

FROM Products;

 

10. What is the performance impact of using LISTAGG?

The performance impact of LISTAGG can vary depending on the size of the dataset and the complexity of the query. Sorting the data before concatenation may cause performance issues with large datasets, but indexing the relevant columns can help improve performance.

 

11. How can I avoid exceeding the character limit in LISTAGG?

You can avoid exceeding the character limit by:

• Using the ON OVERFLOW clause to truncate the result.
• Using a WHERE clause to limit the dataset size.
• Splitting the result into smaller parts manually.

 

12. Can I use LISTAGG with DISTINCT?

Yes, you can use DISTINCT in conjunction with LISTAGG to concatenate only unique values.

Example:

SELECT LISTAGG(DISTINCT Employee, ', ') WITHIN GROUP (ORDER BY Employee) AS Employees_List

FROM Employees;

 

13. Can I concatenate large text fields using LISTAGG?

Yes, you can concatenate large text fields, but remember that the maximum size of a concatenated string is 4000 characters. If the result exceeds this limit, you'll encounter an error. You can use ON OVERFLOW to manage this issue.

 

14. Can I apply LISTAGG to multiple rows of a join?

Yes, you can use LISTAGG on the result of a join to concatenate values from multiple rows across different tables.

 

15. Can I use LISTAGG for data transformation?

Yes, LISTAGG can be used to transform data into a more readable or required format, such as generating CSV-style output or creating custom delimited lists.

 

16. What happens if the column values are long strings?

If the values being concatenated are long strings, you still face the 4000-character limit. To manage this, consider using ON OVERFLOW TRUNCATE or handle large concatenations in smaller chunks.