Concatenating strings from multiple rows into a single string in Google Bigquery – STRING_AGG()

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STRING_AGG() is instrumental in concatenating strings from multiple rows into a single string, significantly simplifying text data analysis and visualization. This article delves deep into the nuances of STRING_AGG(), complemented by hands-on examples that you can run in Google BigQuery.

Understanding STRING_AGG():

The STRING_AGG() function in Google BigQuery is designed to concatenate the values of string expressions from multiple rows into a single string row. It’s particularly useful in scenarios where you need to aggregate textual data from multiple records, such as combining comments, names, or even code snippets, and present them in a unified manner.

The typical syntax for STRING_AGG() is as follows:

STRING_AGG(expression, [delimiter] [ORDER BY sorting_expression] [LIMIT limit])

expression: The string column or expression that you’re aggregating.

delimiter (optional): The separator you wish to insert between each value. If not specified, BigQuery uses a comma (,).

ORDER BY sorting_expression (optional): An expression that dictates how the concatenated strings should be ordered.

LIMIT limit (optional): The maximum number of strings to include.

Sample data:

For demonstration purposes, let’s set up some sample data in BigQuery. Run the following commands in the BigQuery console:

-- Create a dataset

-- Create a table
CREATE TABLE IF NOT EXISTS company_data.EmployeeProjects (
  EmployeeID INT64,
  ProjectName STRING

-- Insert sample data
INSERT INTO company_data.EmployeeProjects (EmployeeID, ProjectName) VALUES
  (1, 'Alpha'),
  (1, 'Beta'),
  (2, 'Gamma'),
  (2, 'Delta'),
  (2, 'Epsilon');


Imagine we want to generate a report showing each employee and the projects they’re involved in, aggregated into one field. Here’s how we’d use STRING_AGG():

  STRING_AGG(ProjectName, ', ' ORDER BY ProjectName) as Projects

STRING_AGG() aggregates the ‘ProjectName’ for each ‘EmployeeID’ into a single string, separated by a comma and a space, and orders the project names alphabetically.


EmployeeID Projects
1 Alpha, Beta
2 Delta, Epsilon, Gamma

Delimiters and Sorting:

The delimiter is a powerful feature within STRING_AGG(), allowing you to define how individual strings will be separated in the final output. Delimiters can be more than just commas; they can be any string, including spaces, new lines, or special symbols.

The optional ORDER BY clause within STRING_AGG() lets you dictate the sequence in which the strings are concatenated. This is particularly useful when the order of information is crucial, such as chronological events, priority items, or alphabetized data.

STRING_AGG() function is an exceptional tool for data professionals who need to aggregate and streamline text data from multiple rows into a cohesive, single-string format. Its applications range from simplifying data visualization to preparing data for reports or further processing. By understanding and utilizing STRING_AGG(), you’re not just coding; you’re orchestrating a symphony of strings, bringing harmony to the potential cacophony of big data.

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Author: user