SQL XMLFOREST

What is SQL XMLFOREST?

Generates an XML fragment by turning each supplied expression into a separate child element.

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Compatible dialects for SQL XMLFOREST: Oracle, PostgreSQL, IBM Db2, SQL Server (via FOR XML), MariaDB (XML plugin), any database that implements SQL/XML

SQL XMLFOREST Full Explanation

XMLFOREST is part of the SQL/XML standard. It takes one or more SQL expressions and returns a single XML value that contains a sequence (forest) of XML elements. For every expression supplied, the function creates an element whose name is either the column alias or, if none is provided, the column name. Null expressions are omitted from the output, keeping the XML compact. The order of elements in the result matches the order of arguments in the call. XMLFOREST is typically used with XMLSERIALIZE or direct client consumption to convert relational row data into well-formed XML that can be exchanged with external systems. Because it is evaluated per row, using it in a SELECT list returns one XML fragment per row. When the target column is already of type XML, the result can be stored directly; otherwise, it can be cast to text. Performance may be affected by large result sets, so indexing or limiting row counts is advised when generating large XML payloads.

SQL XMLFOREST Syntax

XMLFOREST (
  expr1 [AS alias1],
  expr2 [AS alias2],
  ...
)

SQL XMLFOREST Parameters

exprN - any SQL expression that resolves to a scalar value
aliasN - optional identifier used as the XML element name; defaults to the expression or column name if omitted

Example Queries Using SQL XMLFOREST

-- Basic example in PostgreSQL
SELECT XMLSERIALIZE(
  XMLFOREST(id AS "UserId", name AS "UserName") AS text
) AS user_xml
FROM users
WHERE id = 1;

-- Combine with XMLAGG to build a full document
SELECT XMLSERIALIZE(
  XMLELEMENT(NAME "Users",
    XMLAGG(XMLFOREST(id AS "UserId", name AS "UserName"))
  ) AS text
) AS full_doc
FROM users;

Expected Output Using SQL XMLFOREST

First query returns a single row with a text column containing:1AliceSecond query wraps all user elements inside a root tag, returning one XML document representing the entire table

Use Cases with SQL XMLFOREST

Converting relational rows to XML for REST or SOAP responses
Exporting data to XML files for regulatory reporting
Passing XML to middle-tier applications that expect hierarchical data
Building XML configuration snippets from database tables
Preparing XML payloads for message queues or service buses

Common Mistakes with SQL XMLFOREST

Forgetting to provide an alias, leading to unintuitive element names
Assuming null columns will appear in the output (they are skipped)
Omitting XMLSERIALIZE when the client expects plain text instead of the native XML type
Using XMLFOREST on very large result sets without XMLAGG can create memory pressure

First Introduced In

SQL:2003 (SQL/XML functional extensions)

Frequently Asked Questions

What does SQL XMLFOREST do?

SQL XMLFOREST takes one or more expressions and turns each into a sibling XML element, returning the combined XML fragment.

How do I give custom element names?

Supply an alias after the expression, for example `XMLFOREST(empno AS "EmpId")`, to control the element name.

Are NULL values included in the output?

No. If an expression evaluates to NULL, XMLFOREST omits that element entirely, preventing empty tags.

Can I aggregate multiple XMLFOREST results into one document?

Yes. Wrap XMLFOREST inside XMLAGG, then optionally surround it with XMLELEMENT to add a root node, and finally serialize with XMLSERIALIZE.