PostgreSQL – XMLAGG Function

The XMLAGG function in PostgreSQL is used to aggregate multiple XML elements into a single XML value. It is often used in combination with other functions like XMLELEMENT to construct XML documents or fragments. XMLAGG takes a set of XML values and concatenates them together.

Here’s the basic syntax of the XMLAGG function:

XMLAGG(expression ORDER BY expression, separator string)

where expression is the XML value or element that you want to aggregate. You can use functions like XMLELEMENT, XMLFOREST, or any other expression that produces XML values.

ORDER BY expression is optional. If you want to specify the order in which the XML values are concatenated, you can use the ORDER BY clause. The result will be sorted based on the specified expression before aggregation.

separator string is also optional. You can specify a separator string that will be inserted between the concatenated XML elements. If you don’t provide a separator, the elements will be concatenated without any spaces or separators.

Here’s an example of how you might use XMLAGG to concatenate XML elements:

SELECT XMLAGG(XMLELEMENT(NAME person, XMLFOREST(name, age, city))) FROM people;

In this example, the XMLELEMENT function is used to construct an XML element called “person” for each row in the “people” table, containing the “name,” “age,” and “city” columns. The XMLAGG function is then used to aggregate these XML elements into a single XML document. If you want to include a separator between these elements, you can add it as the third argument to XMLAGG.