“`html

When aiming to analyze more extensive datasets, the duration of processing will increase as the query must inspect all rows and columns within a database. To streamline processing time, you can amalgamate column values from various rows utilizing Oracle. This enhances the query’s efficiency and simplifies data analysis. Additionally, it aids in comprehending the data and facilitates data retrieval. In this article, you will delve into the various techniques to concatenate column values from multiple rows in Oracle.

Table of Contents:

• Techniques for Amalgamating Column Values from Multiple Rows in Oracle
  • Technique 1: Utilizing LISTAGG() in Oracle
  • Technique 2: Utilizing XMLAGG() in Oracle
  • Technique 3: Utilizing COLLECT() in Oracle
  • Technique 4: JSON_ARRAYAGG() in Oracle
• Alternative Approach Using CURSOR and STRING AGGREGATION in PL/SQL
• Performance Considerations for Each Approach
• Practical Applications
• Real-World Cases
• Conclusion
• How to Amalgamate Column Values from Multiple Rows in Oracle – FAQs

Techniques for Amalgamating Column Values from Multiple Rows in Oracle

There are several approaches to concatenate column values from multiple rows, such as LISTAGG(), XMLAGG(), and COLLECT() with CAST in Oracle Database.

Let’s establish a table to perform operations on the amalgamation of column values from various rows in Oracle.

-- Create the table
CREATE TABLE hotel_dishes (
    hotel_id NUMBER,
    hotel_name VARCHAR2(100),
    dish_name VARCHAR2(100)
);

-- Insert sample data
INSERT INTO hotel_dishes VALUES (1, 'Grand Palace', 'Butter Chicken');
INSERT INTO hotel_dishes VALUES (1, 'Grand Palace', 'Paneer Tikka');
INSERT INTO hotel_dishes VALUES (1, 'Grand Palace', 'Mutton Biryani');
INSERT INTO hotel_dishes VALUES (2, 'Ocean View', 'Grilled Salmon');
INSERT INTO hotel_dishes VALUES (2, 'Ocean View', 'Lobster Thermidor');
INSERT INTO hotel_dishes VALUES (3, 'Mountain Retreat', 'Stuffed Mushrooms');
INSERT INTO hotel_dishes VALUES (3, 'Mountain Retreat', 'Veg Lasagna');

-- Commit the transaction
COMMIT;
COLUMN hotel_id FORMAT 9999
COLUMN hotel_name FORMAT A30
COLUMN best_dishes FORMAT A100
SET LINESIZE 200
SET PAGESIZE 50
SELECT * FROM hotel_dishes;

Output:

After creating and inserting values, the table will appear as follows.

Technique 1: Utilizing LISTAGG() in Oracle

The LISTAGG() function in Oracle is highly effective when converting column values from numerous rows. The LISTAGG() performs optimally with substantial datasets.

Syntax:

SELECT column1,  
       LISTAGG(column2, ', ') WITHIN GROUP (ORDER BY column2) AS concatenated_values  
FROM table_name  
GROUP BY column1;

Example:

-- Execute the main query with appropriate formatting
SELECT hotel_id, 
       RPAD(hotel_name, 30) AS hotel_name, 
       LISTAGG(dish_name, ', ') WITHIN GROUP (ORDER BY dish_name) AS best_dishes
FROM hotel_dishes
GROUP BY hotel_id, hotel_name
ORDER BY hotel_id;

Output:

Explanation: In this case, the LISTAGG(dish_name, ‘, ‘) function converts the data into strings using a delimiter that separates the strings. Then, the GROUP BY clause categorizes the strings accordingly.

Technique 2: Utilizing XMLAGG() in Oracle

XMLAGG() function behaves similarly to LISTAGG() but is generally implemented in earlier versions of Oracle. It manages substantial tables effectively.

XMLAGG() Syntax in Oracle:

SELECT column1, 
       RTRIM(XMLAGG(XMLELEMENT(E, column2 || ', ')).EXTRACT('//text()'), ', ') AS concatenated_value
FROM table_name
GROUP BY column1;

Example:

SELECT hotel_name,
       RTRIM(XMLAGG(XMLELEMENT(E, dish_name || ', ')).EXTRACT('//text()'), ', ') AS best_dishes
FROM hotel_dishes
GROUP BY hotel_name;

Output:

Explanation: In this case, the XMLAGG() function separates the strings using designated delimiters and treats them as elements, subsequently allowing the GROUP BY clause to organize all elements by their respective category (hotel name).

Technique 3: Utilizing COLLECT() in Oracle

The COLLECT() function in Oracle is generally unsuitable if you desire output in an easily readable format. However, it can yield output with the DBMS_DEBUG_VC2COLL tag. This function is advantageous when requiring aggregated data in PL/SQL for further processing.

COLLECT() Syntax:

“““html
class=”wp-block-code”>SELECT column1,
CAST(COLLECT(column2) AS collection_type) AS combined_values
FROM table_name
GROUP BY column1;

Sample:

SELECT hotel_name, 
       CAST(COLLECT(dish_name) AS SYS.DBMS_DEBUG_VC2COLL) AS favorite_dishes
FROM hotel_dishes
GROUP BY hotel_name;

Result:

Clarification: In this context, the COLLECT() function yielded the output in a textual format with a tag of DBMS_DEBUG_VC2COLL. This is saved in a PL/SQL collection structure, which is beneficial for subsequent processing in PL/SQL procedures.

Technique 4: JSON_ARRAYAGG() in Oracle

The JSON_ARRAYAGG() function in Oracle serves as an aggregate function that outputs a JSON array composed of values from several rows. It amalgamates values from disparate rows and transforms them into a singular JSON array string.

Formula:

JSON_ARRAYAGG(expression [ RETURNING data_type ] [ FORMAT JSON ])

Sample:

SELECT JSON_ARRAYAGG(hotel_name) AS names_list
FROM hotel_dishes;

Result:

Clarification: Here, the JSON_ARRAYAGG() function compiles all values from various rows and presents them as a single JSON array string.

Alternative Technique Utilizing CURSOR and STRING AGGREGATION in PL/SQL

This method is notably efficient since it imposes no character constraints. It employs a cursor that manually concatenates rows. It is particularly effective within the PL/SQL environment.

Sample:

-- Create the table
CREATE TABLE hotel_dishes (
    hotel_id NUMBER,
    hotel_name VARCHAR2(100),
    dish_name VARCHAR2(100)
);

-- Insert sample data
INSERT INTO hotel_dishes VALUES (1, 'Grand Palace', 'Butter Chicken');
INSERT INTO hotel_dishes VALUES (1, 'Grand Palace', 'Paneer Tikka');
INSERT INTO hotel_dishes VALUES (1, 'Grand Palace', 'Mutton Biryani');
INSERT INTO hotel_dishes VALUES (2, 'Ocean View', 'Grilled Salmon');
INSERT INTO hotel_dishes VALUES (2, 'Ocean View', 'Lobster Thermidor');
INSERT INTO hotel_dishes VALUES (3, 'Mountain Retreat', 'Stuffed Mushrooms');
INSERT INTO hotel_dishes VALUES (3, 'Mountain Retreat', 'Veg Lasagna');

-- Commit the transaction
COMMIT;
COLUMN hotel_id FORMAT 9999
COLUMN hotel_name FORMAT A30
COLUMN best_dishes FORMAT A100
SET LINESIZE 200
SET PAGESIZE 50
DECLARE
    v_result VARCHAR2(4000);
BEGIN
    FOR rec IN (SELECT dish_name FROM hotel_dishes WHERE hotel_name = 'Grand Palace') LOOP
        v_result := v_result || rec.dish_name || ', ';
    END LOOP;
    
    -- Remove the last comma and space
    v_result := RTRIM(v_result, ', ');
    
    DBMS_OUTPUT.PUT_LINE(v_result);
END;
/

Result:

Clarification: Here, the WHERE clause retrieves only the records pertaining to the hotel identified as “Grand Palace” using the SELECT statement.

Performance Assessment of Each Technique

Practical Applications

1. It merges columns into multiple rows, rendering the data more interpretable and organized, thereby decreasing query redundancy. This can be accomplished by using LISTAGG() to structure the data and XMLAGG() for larger datasets.
2. It improves query efficiency, particularly for expansive tables, as it diminishes the number of rows by consolidating several rows. This is particularly advantageous when employing XMLAGG().
3. It facilitates efficient data transfer by transmitting multiple rows as a single comma-separated string, which minimizes memory usage during transfer. XML enables file transfer in XML format, simplifying the process.
4. It utilizes PL/SQL logic for effective data storage, consequently reducing memory consumption. COLLECT() in conjunction with CAST() is optimal due to its capability for bulk processing.

Real-Life Instances

Instance 1: Merging Employee Skills in an HR Database

CREATE TABLE employee_skills (
    employee_id NUMBER,
    employee_name VARCHAR2(100),
    skill VARCHAR2(100)
);

INSERT INTO employee_skills VALUES (101, 'Alice', 'Java');
INSERT INTO employee_skills VALUES (101, 'Alice', 'Python');
INSERT INTO employee_skills VALUES (101, 'Alice', 'SQL');
INSERT INTO employee_skills VALUES (102, 'Bob', 'C++');
INSERT INTO employee_skills VALUES (102, 'Bob', 'JavaScript');
INSERT INTO employee_skills VALUES (103, 'Charlie', 'Python');
INSERT INTO employee_skills VALUES (103, 'Charlie', 'R');
COMMIT;
COLUMN employee_id FORMAT 9999
COLUMN employee_name FORMAT A20
COLUMN skills FORMAT A50
SET LINESIZE 100
SET PAGESIZE 50

SELECT employee_id, 
       RPAD(employee_name, 20) AS employee_name, 
       LISTAGG(skill, ', ') WITHIN GROUP (ORDER BY skill) AS skills
FROM employee_skills
GROUP BY employee_id, employee_name
ORDER BY employee_id;

Result:

Clarification: In this example, the LISTAGG() function aggregates the skills into a string separated by commas, while GROUP BY categorizes them according to the employee specifics from the HR database.

Illustration 2: showcasing products categorized by type.

CREATE TABLE product_categories (
    product_id NUMBER,
    product_name VARCHAR2(100),
    category VARCHAR2(100)
);

INSERT INTO product_categories VALUES (1, 'Laptop', 'Electronics');
INSERT INTO product_categories VALUES (1, 'Laptop', 'Computers');
INSERT INTO product_categories VALUES (2, 'Smartphone', 'Electronics');
INSERT INTO product_categories VALUES (2, 'Smartphone', 'Mobiles');
INSERT INTO product_categories VALUES (3, 'Sofa', 'Furniture');
INSERT INTO product_categories VALUES (3, 'Sofa', 'Home Decor');
COMMIT;
COLUMN product_id FORMAT 9999
COLUMN product_name FORMAT A20
COLUMN categories FORMAT A50
SET LINESIZE 100
SET PAGESIZE 50

SELECT product_id, 
       RPAD(product_name, 20) AS product_name, 
       RTRIM(XMLAGG(XMLELEMENT(E, category || ', ')).EXTRACT('//text()'), ', ') AS categories
FROM product_categories
GROUP BY product_id, product_name
ORDER BY product_id;

Result:

Clarification: In this case, the XMLAGG() function retrieves the product categories and organizes them according to the product name.

Summary

You are able to concatenate column data from various rows in Oracle using functions such as LISTAGG(), XMLAGG(), and COLLECT(), as well as other methods for selective concatenation like CURSOR and STRING AGGREGATION. These techniques are highly efficient and enhance the integrity of data within a table, while also conserving memory space by transforming extensive datasets into comma-separated strings, XML formats, or PL/SQL objects. You may choose any of these approaches based on the specifics of your dataset.

Enhance your expertise by signing up for our SQL Training Course today and acquire practical experience. Additionally, get ready for job interviews with our SQL interview questions, compiled by industry professionals.

How to Combine Column Values from Multiple Rows in Oracle – FAQs

The article How to Concatenate Column Values From Multiple Rows in Oracle was first published on Intellipaat Blog.