Using PostgreSQL STRING_AGG Function with Examples 📋

Learn how to use the PostgreSQL STRING_AGG function to concatenate strings with a specified delimiter. Improve query results and customize output strings ⚙️

How to Use STRING_AGG() Function in PostgreSQL

The STRING_AGG() function is an aggregate function in PostgreSQL used to concatenate values from multiple rows into a single string. This handy function allows easy string concatenation directly in your SQL queries. In this article, we’ll explore the basics of using STRING_AGG() and look at some examples of it in action.

What is STRING_AGG()?

STRING_AGG() is a built-in aggregate function introduced in PostgreSQL 9.0. It takes a column or expression as input and concatenates the values into a single string.

The basic syntax of STRING_AGG() is:

SELECT STRING_AGG(column, separator) FROM table GROUP BY group_column

Where:

• column is the column or expression you want to concatenate
• separator is the delimiter string added between values
• group_column is used with GROUP BY to specify the groups to concatenate within

Some key things to note about STRING_AGG():

• It requires the GROUP BY clause like other aggregate functions
• The separator is not added after the last value
• NULL values are skipped and not included in the result
• Values can be sorted using ORDER BY in the function

Why Use STRING_AGG()?

Concatenating values from multiple rows into a single string is a common need when generating reports or analyses from database tables.

Doing this concatenation in application code can be cumbersome. STRING_AGG() provides an easy way to handle it directly in SQL.

Some examples of when STRING_AGG() is useful:

• Generating a list of comma-separated names for a report
• Combining values from related rows into a single string
• Aggregating data while retaining a string representation
• Concatenating columns for easier analysis

By handling string concatenation in the database, STRING_AGG() can save application code from doing this work.

STRING_AGG() Examples

To demonstrate STRING_AGG() in action, let’s look at some example uses with a sample database table.

First we’ll create a simple users table with some data:

CREATE TABLE users ( id INT, first_name VARCHAR(50), last_name VARCHAR(50), country VARCHAR(50) ); INSERT INTO users VALUES (1, 'John', 'Doe', 'USA'), (2, 'Jane', 'Doe', 'Canada'), (3, 'Bob', 'Smith', 'USA'), (4, 'Mary', 'Jones', 'Mexico');

This gives us a table with users from different countries.

Basic STRING_AGG() Example

To get a list of user first names by country, we can use STRING_AGG():

SELECT country, STRING_AGG(first_name, ', ') AS names FROM users GROUP BY country;

This would return:

USA, John, Bob Canada, Jane Mexico, Mary

The first names are concatenated by country with commas between them.

Sorting Concatenated Values

To sort the concatenated values, add an ORDER BY clause:

SELECT STRING_AGG(first_name, ', ' ORDER BY first_name DESC) FROM users;

Now the names are concatenated in descending order:

John, Mary, Jane, Bob

Filtering Values

You can filter which values get concatenated using a CASE statement:

SELECT STRING_AGG(CASE WHEN country = 'USA' THEN first_name END, ', ') FROM users;

This concatenates only names where the country is ‘USA’:

John, Bob

As you can see, STRING_AGG() provides a flexible way to concatenate values directly in PostgreSQL without needing procedural code.

When to Avoid STRING_AGG()

While handy, STRING_AGG() isn’t ideal for every situation. Some cases where alternatives may be better:

• You need the raw concatenated string for additional string manipulation. Doing this in application code may be easier.
• Performance with very large datasets. STRING_AGG() can sometimes be slow with ample data.
• You want a concatenated value for each row. Consider FOR XML PATH instead.

So weigh the tradeoffs when deciding between STRING_AGG() and other methods.

Summary

The STRING_AGG() function is a useful tool for aggregating data into a single concatenated string value in PostgreSQL. Key points about STRING_AGG():

• Concatenates values from multiple rows into a string
• Requires GROUP BY to specify the groups
• Skips NULL values in the result
• Allows sorting concatenated values with ORDER BY
• Avoids needing to manually concatenate in application code

In this comprehensive article, we have explored the fundamental concepts of utilizing the STRING_AGG() function in PostgreSQL, providing practical examples and real-world use cases. When it comes to efficiently aggregating string data in PostgreSQL, STRING_AGG() emerges as an invaluable ally in your database toolkit, streamlining your data manipulation processes and enhancing your data management capabilities.