title: string_agg No function matches the given name
body: <p>I have relational database and would like to use <code>string_agg()</code> since it seems fit to my need.<br><br>
  <strong>I want :</strong><br></p>
  
  <pre><code>product_id | quiz_id
  -----------+----------
           1 | 1,6
           2 | 2,7
           3 | 3,8
           4 | 4
  </code></pre>
  
  <p><strong>Here is my database.</strong></p>
  
  <pre><code>    select quiz_id , product_id, lastmodified from dugong.quiz;
   quiz_id | product_id |         lastmodified          
  ---------+------------+-------------------------------
         1 |          1 | 2015-11-11 14:46:55.619162+07
         2 |          2 | 2015-11-11 14:46:55.619162+07
         3 |          3 | 2015-11-11 14:46:55.619162+07
         4 |          4 | 2015-11-11 14:46:55.619162+07
         5 |          5 | 2015-11-11 14:46:55.619162+07
         6 |          1 | 2015-11-11 14:46:55.619162+07
         7 |          2 | 2015-11-11 14:46:55.619162+07
         8 |          3 | 2015-11-11 14:46:55.619162+07
  </code></pre>
  
  <p><strong>My attempt :</strong> <br>
  Refer to document.
  <a href="https://stackoverflow.com/questions/43870/how-to-concatenate-strings-of-a-string-field-in-a-postgresql-group-by-query">How to concatenate strings of a string field in a PostgreSQL &#39;group by&#39; query?</a>
  <a href="http://www.postgresql.org/docs/current/static/sql-expressions.html#SYNTAX-AGGREGATES" rel="noreferrer">http://www.postgresql.org/docs/current/static/sql-expressions.html#SYNTAX-AGGREGATES</a></p>
  
  <pre><code>select product_id , string_agg(quiz_id, ',' order by lastmodified) from dugong.quiz;
  ERROR:  function string_agg(integer, unknown) does not exist
  LINE 1: select product_id , string_agg(quiz_id, ',' order by lastmod...
                              ^
  HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
  </code></pre>
  
  <p><br>
  <strong>Postgres version :</strong> <br>
  PostgresApp 9.4.4.1</p>
  
  <p><strong>Update :</strong> <br>
  @code-monk
  It still error.</p>
  
  <pre><code>select product_id , string_agg(quiz_id::int, ',' order by lastmodified) from dugong.quiz;
  ERROR:  function string_agg(integer, unknown) does not exist
  LINE 1: select product_id , string_agg(quiz_id::int, ',' order by la...
                              ^
  HINT:  No function matches the given name and argument types. You might need to add explicit type casts.
  </code></pre>
  
  <p><strong>Question :</strong><br>
  What is wrong with my query?</p>