Postgres XMLCONCAT() with subqueries

Question

I am trying to use Postgres to generate a single XML document as output.

Assume that I have a single relation, customers . It has the columns id , name and email .

I want XML that looks like this:

<customers>
    <customer>
        <id>0<id/>
        <name>Customer 1<name/>
        <email>customer1@gmail.com<email/>
    <customer/>
    <customer>
        <id>1<id/>
        <name>Customer 2<name/>
        <email>customer2@gmail.com<email/>
    <customer/>
    <!--and so on-->
<customers/>

So far, I can generate the XML for each customer, like this:

SELECT XMLELEMENT(NAME customer, 
           XMLELEMENT(NAME 'id', "id"),
           XMLELEMENT(NAME 'name', name), 
           XMLELEMENT(NAME email, email))
FROM customers;

However, concatenating this into a single XML document does not work. I have tried:

SELECT XMLELEMENT(NAME customers, XMLCONCAT((
    SELECT XMLELEMENT(NAME customer, 
               XMLELEMENT(NAME 'id', "id"),
               XMLELEMENT(NAME 'name', name), 
               XMLELEMENT(NAME email, email))
FROM customers;
)));

However, this fails with the message more than one row returned by a subquery used as an expression .

Answer 1

There are many ways of achieving this. An easy approach is to fetch the XML elements you want in aCTE and aggregate them with xmlagg

Sample Data

CREATE TEMPORARY TABLE customers (id int, name text, email text);
INSERT INTO customers VALUES 
(0,'Customer 1','customer1@gmail.com'),
(0,'Customer 2','customer2@gmail.com');

Query

WITH j AS (
  SELECT 
    XMLELEMENT(NAME customer, 
      XMLELEMENT(NAME id, id),
      XMLELEMENT(NAME name, name), 
      XMLELEMENT(NAME email, email)) AS customers
  FROM customers)
SELECT 
  XMLELEMENT(NAME customers, xmlagg(customers)) 
FROM j;

------------
<customers>
    <customer>
        <id>0</id>
        <name>Customer 1</name>
        <email>customer1@gmail.com</email>
    </customer>
    <customer>
        <id>0</id>
        <name>Customer 2</name>
        <email>customer2@gmail.com</email>
    </customer>
</customers>

Another alternative is to use a subquery

SELECT 
  XMLELEMENT(NAME customers, xmlagg((customers)))
FROM (SELECT 
  XMLELEMENT(NAME customer, 
    XMLELEMENT(NAME id, id),
    XMLELEMENT(NAME name, name), 
    XMLELEMENT(NAME email, email)) AS customers
FROM customers) j;

Answer 2

Add LIMIT 1 to the Subquery. This will remove the duplicate rows.