SQL XMLPI

What is SQL XMLPI?

Constructs an XML processing-instruction node from a target name and optional content string.

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Compatible dialects for SQL XMLPI: PostgreSQL, Oracle (XMLType), IBM Db2, SAP HANA. Not supported in MySQL, SQL Server, or SQLite.

SQL XMLPI Full Explanation

XMLPI builds an XML processing instruction (PI), a special node that begins with ''. In SQL/XML it lets you embed instructions (for example, style-sheet references or application hints) inside an XML document generated by SQL. The keyword accepts a required target identifier (the PI name) and an optional character expression for the body. If the body is omitted the PI is empty except for the target. The result is of the XML data type. New-line, question-mark, or the sequence '?>' inside the content is automatically escaped or raises an error, depending on the database. Because a PI belongs to the XML value space, result aggregation with other XML constructors (XMLELEMENT, XMLFOREST, etc.) is seamless. XMLPI is part of the SQL:2006 SQL/XML standard and is fully supported in PostgreSQL and several other engines that implement SQL/XML.

SQL XMLPI Syntax

XMLPI ( NAME target_identifier [, content_expression ] );

SQL XMLPI Parameters

target_identifier (identifier) - Required processing-instruction target (cannot contain whitespace, '|||' or 'xml' in any casing)
content_expression (string or character expression) - Optional text that becomes the body of the PI; must not include the sequence '?>'

Example Queries Using SQL XMLPI

-- Simple PI with no body
SELECT XMLPI(NAME php);

-- PI with body content
SELECT XMLPI(NAME php 'echo $x;');

-- Combine with other XML constructors
SELECT XMLELEMENT(NAME root, XMLPI(NAME xml-stylesheet 'type="text/xsl" href="style.xsl"'));

Expected Output Using SQL XMLPI

Each statement returns a single XML value, e
g
:
xsl"?>

Use Cases with SQL XMLPI

Embed version or style-sheet links when exporting query results as XML
Send application-specific directives to downstream parsers
Programmatically construct XML configuration files from relational data

Common Mistakes with SQL XMLPI

Forgetting the NAME keyword before the target identifier
Using a reserved word 'xml' as the target (disallowed by the standard)
Including the sequence '?>' in the content, which breaks well-formedness
Assuming XMLPI returns plain text; it returns XML type and must be cast if you need VARCHAR

First Introduced In

SQL:2006 (SQL/XML) standard; PostgreSQL 8.3

Frequently Asked Questions

What does XMLPI stand for?

XMLPI stands for XML Processing Instruction, a node type used to carry parser directives within an XML document.

Why does XMLPI need the NAME keyword?

The SQL/XML standard requires the NAME keyword to clearly distinguish the target identifier from the optional content argument.

How do I avoid errors with special characters in the content?

Escape or remove the sequence '?

>' and ensure the content does not contain illegal control characters. Standard string escaping rules apply.

Can XMLPI be used in a SELECT list and WHERE clause?

Yes in the SELECT list to construct XML output. In a WHERE clause it makes little sense because it always returns a new XML value rather than filtering rows.