string_agg function: This function is used for string aggregation or concatenation within groups of rows. It takes an expression (usually a column) and concatenates the values of that expression across rows in a group, separated by a specified delimiter. 

For example, suppose you have a table of orders and you want to concatenate the names of products for each order, separated by commas: 

SELECT order_id, string_agg(product_name, ', ') AS products 
FROM order_details 
GROUP BY order_id; 

In this query, string_agg(product_name, ', ') concatenates the product_name values for each order, separated by commas.

It's a useful function for creating comma-separated lists or combining text values within groups when working with SQL in PostgreSQL.