SQL XMLSERIALIZE

What is SQL XMLSERIALIZE?

Converts an in-memory XML value to a character or binary string.

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Compatible dialects for SQL XMLSERIALIZE: PostgreSQL, Oracle, IBM Db2, DuckDB. Not natively supported in MySQL or SQL Server.

SQL XMLSERIALIZE Full Explanation

XMLSERIALIZE is part of the SQL/XML standard. It takes an XML data type value that was previously constructed or parsed in SQL and serializes it into a textual (CHAR, VARCHAR, CLOB) or binary (BINARY, VARBINARY, BLOB) representation. You can specify whether the serialization should treat the value as a DOCUMENT (exactly one top-level element) or as CONTENT (any sequence of nodes). Optional formatting clauses such as INDENT control pretty-printing. The function is deterministic, respects character set and encoding rules of the target data type, and preserves namespaces and entity references. XMLSERIALIZE is commonly paired with XMLPARSE (to go from string to XML) and XMLQUERY/XQUERY functions for manipulation. Be aware that very large XML documents may exceed the maximum length of the chosen target type, and that some dialects support only character targets.

SQL XMLSERIALIZE Syntax

XMLSERIALIZE(
    { DOCUMENT | CONTENT } xml_expression
    AS target_sql_type
    [ INDENT [SIZE integer] ]
)

SQL XMLSERIALIZE Parameters

DOCUMENT or CONTENT (keyword) - Defines whether the root must be a single element (DOCUMENT) or can be any XML node sequence (CONTENT).
xml_expression (XML) - The XML value to be serialized.
target_sql_type (SQL character or binary type) - Destination data type (e.g., VARCHAR(5000), CLOB, BLOB).
INDENT (optional keyword) - Requests pretty-printed output with line breaks and spaces.
SIZE (optional integer) - Number of spaces per indentation level when INDENT is used.

Example Queries Using SQL XMLSERIALIZE

-- Serialize a whole XML document to VARCHAR
SELECT XMLSERIALIZE(DOCUMENT xml_column AS VARCHAR(5000)) AS xml_text
FROM   invoices
WHERE  id = 42;

-- Serialize XML content with pretty printing into a CLOB
SELECT XMLSERIALIZE(CONTENT xml_data AS CLOB INDENT SIZE 2) AS formatted_xml
FROM   staging_xml;

-- Round-trip: string -> XML -> string
WITH raw AS (
  SELECT '<a><b>1</b></a>' AS txt
)
SELECT XMLSERIALIZE(DOCUMENT XMLPARSE(DOCUMENT txt) AS VARCHAR(100)) AS roundtrip
FROM   raw;

Expected Output Using SQL XMLSERIALIZE

Returns a character or binary string containing the textual XML representation
The result can be selected, stored, or sent to external systems just like any normal string column

Use Cases with SQL XMLSERIALIZE

Exporting XML stored in the database to applications that expect plain text.
Logging or auditing XML payloads.
Converting XML to a CLOB before full-text indexing.
Round-tripping XML data for validation during ETL workflows.

Common Mistakes with SQL XMLSERIALIZE

Omitting DOCUMENT when the XML value has exactly one root element, leading to dialect-specific defaults.
Selecting a target type that is too small, causing truncation errors.
Assuming INDENT is supported in all databases (some ignore it).
Forgetting that whitespace inserted by INDENT changes the byte length of the result.

First Introduced In

SQL:2003 (SQL/XML)

Frequently Asked Questions

What is the purpose of XMLSERIALIZE?

It turns an in-database XML value into a plain character or binary string so you can export, log, or further process the data.

Can XMLSERIALIZE pretty-print the output?

Yes. Use the INDENT clause, optionally followed by SIZE n, to add line breaks and spaces that make the XML easier to read.

Does XMLSERIALIZE validate the XML?

No. Validation occurs when the XML value is created. XMLSERIALIZE simply converts the in-memory representation to text.

How do DOCUMENT and CONTENT differ?

DOCUMENT enforces a single root element, matching well-formed XML documents. CONTENT allows multiple top-level nodes, useful for fragments.