title: MySQL Query GROUP_CONCAT Over Multiple Rows
body: <p>I'm getting name and address data out of generic question / answer data to create some kind of normalised reporting database.</p>
  
  <p>The query I've got uses group_concat and works for individual sets of questions but not for multiple sets.</p>
  
  <p>I've tried to simplify what I'm doing by using just forename and surname and just 3 records, 2 for 1 person and 1 for another.  In reality though there are more than 300,000 records.</p>
  
  <p><strong><a href="http://sqlfiddle.com/#!2/28443/19" rel="nofollow">Example</a> of results with <code>qs.Id = 1</code>.</strong></p>
  
  <pre><code>QuestionSetId       Forename        Surname
  -------------------------------------------------------
              1       Bob             Jones
  </code></pre>
  
  <p><strong><a href="http://sqlfiddle.com/#!2/28443/18" rel="nofollow">Example</a> of results with <code>qs.Id IN (1, 2, 3)</code>.</strong></p>
  
  <pre><code>QuestionSetId       Forename        Surname
  -------------------------------------------------------
              3       Bob,Bob,Frank   Jones,Jones,Smith
  </code></pre>
  
  <p><strong>What I would like to see for <code>qs.Id IN (1, 2, 3)</code>.</strong></p>
  
  <pre><code>QuestionSetId       Forename        Surname
  -------------------------------------------------------
              1       Bob             Jones
              2       Bob             Jones
              3       Frank           Smith
  </code></pre>
  
  <p>So how can I make the 2nd example return a separate row for each set of name and address information?</p>
  
  <p>I realise the current way the data is stored is "questionable" but <strong>I cannot change the way the data is stored.</strong></p>
  
  <p>I can get <a href="http://sqlfiddle.com/#!2/28443/6" rel="nofollow">sets of individual answers</a> but not sure how to combine the others.</p>
  
  <p>My simplified Schema that I cannot change:</p>
  
  <pre><code>CREATE TABLE StaticQuestion (
    Id INT NOT NULL,
    StaticText VARCHAR(500) NOT NULL);
  
  CREATE TABLE Question (
    Id INT NOT NULL, 
    Text VARCHAR(500) NOT NULL);
  
  CREATE TABLE StaticQuestionQuestionLink (
    Id INT NOT NULL,
    StaticQuestionId INT NOT NULL,
    QuestionId INT NOT NULL,
    DateEffective DATETIME NOT NULL);
  
  CREATE TABLE Answer (
    Id INT NOT NULL,
    Text VARCHAR(500) NOT NULL);
  
  CREATE TABLE QuestionSet (
    Id INT NOT NULL,
    DateEffective DATETIME NOT NULL);
  
  CREATE TABLE QuestionAnswerLink (
    Id INT NOT NULL,
    QuestionSetId INT NOT NULL,
    QuestionId INT NOT NULL,
    AnswerId INT NOT NULL,
    StaticQuestionId INT NOT NULL);
  </code></pre>
  
  <p>Some example data for only forename and surname.</p>
  
  <pre><code>INSERT INTO StaticQuestion (Id, StaticText)
  VALUES (1, 'FirstName'),
         (2, 'LastName');
  
  INSERT INTO Question (Id, Text)
  VALUES (1, 'What is your first name?'),
         (2, 'What is your forename?'),
         (3, 'What is your Surname?');
  
  INSERT INTO StaticQuestionQuestionLink (Id, StaticQuestionId, QuestionId, DateEffective)
  VALUES (1, 1, 1, '2001-01-01'),
         (2, 1, 2, '2008-08-08'),
         (3, 2, 3, '2001-01-01');
  
  INSERT INTO Answer (Id, Text)
  VALUES (1, 'Bob'),
         (2, 'Jones'),
         (3, 'Bob'),
         (4, 'Jones'),
         (5, 'Frank'),
         (6, 'Smith');
  
  INSERT INTO QuestionSet (Id, DateEffective)
  VALUES (1, '2002-03-25'),
         (2, '2009-05-05'),
         (3, '2009-08-06');
  
  INSERT INTO QuestionAnswerLink (Id, QuestionSetId, QuestionId, AnswerId, StaticQuestionId)
  VALUES (1, 1, 1, 1, 1),
         (2, 1, 3, 2, 2),
         (3, 2, 2, 3, 1),
         (4, 2, 3, 4, 2),
         (5, 3, 2, 5, 1),
         (6, 3, 3, 6, 2);
  </code></pre>
  
  <p>Just in case SQLFiddle is down here are the 3 queries from the examples I've linked to:</p>
  
  <p>1: - working query but only on 1 set of data.</p>
  
  <pre><code>SELECT MAX(QuestionSetId) AS QuestionSetId,
         GROUP_CONCAT(Forename) AS Forename,
         GROUP_CONCAT(Surname) AS Surname
  FROM   (SELECT
              x.QuestionSetId,
              CASE x.StaticQuestionId WHEN 1 THEN Text END AS Forename,
              CASE x.StaticQuestionId WHEN 2 THEN Text END AS Surname        
          FROM     (SELECT (SELECT   link.StaticQuestionId
                            FROM     StaticQuestionQuestionLink link
                            WHERE    link.Id = qa.QuestionId
                                 AND link.DateEffective &lt;= qs.DateEffective
                                 AND link.StaticQuestionId IN (1, 2)
                            ORDER BY link.DateEffective DESC LIMIT 1) AS StaticQuestionId,
                           a.Text,
                           qa.QuestionSetId
                    FROM   QuestionSet qs
                              INNER JOIN QuestionAnswerLink qa ON qs.Id = qa.QuestionSetId
                              INNER JOIN Answer a ON qa.AnswerId = a.Id
                    WHERE  qs.Id IN (1)) x) y
  </code></pre>
  
  <p>2: - working query but undesired results on multiple sets of data.</p>
  
  <pre><code>SELECT MAX(QuestionSetId) AS QuestionSetId,
         GROUP_CONCAT(Forename) AS Forename,
         GROUP_CONCAT(Surname) AS Surname
  FROM   (SELECT
              x.QuestionSetId,
              CASE x.StaticQuestionId WHEN 1 THEN Text END AS Forename,
              CASE x.StaticQuestionId WHEN 2 THEN Text END AS Surname        
          FROM     (SELECT (SELECT   link.StaticQuestionId
                            FROM     StaticQuestionQuestionLink link
                            WHERE    link.Id = qa.QuestionId
                                 AND link.DateEffective &lt;= qs.DateEffective
                                 AND link.StaticQuestionId IN (1, 2)
                            ORDER BY link.DateEffective DESC LIMIT 1) AS StaticQuestionId,
                           a.Text,
                           qa.QuestionSetId
                    FROM   QuestionSet qs
                              INNER JOIN QuestionAnswerLink qa ON qs.Id = qa.QuestionSetId
                              INNER JOIN Answer a ON qa.AnswerId = a.Id
                    WHERE  qs.Id IN (1, 2, 3)) x) y
  </code></pre>
  
  <p>3: - working query on multiple sets of data only on 1 field (answer) though.</p>
  
  <pre><code>SELECT
      qs.Id AS QuestionSet,
      a.Text AS Answer
  FROM
      QuestionSet qs
          INNER JOIN QuestionAnswerLink qalink ON qs.Id = qalink.QuestionSetId
          INNER JOIN StaticQuestionQuestionLink sqqlink ON qalink.QuestionId = sqqlink.QuestionId
          INNER JOIN Answer a ON qalink.AnswerId = a.Id
  WHERE
          sqqlink.StaticQuestionId = 1 /* FirstName */
      AND sqqlink.DateEffective = 
              (SELECT DateEffective
                 FROM StaticQuestionQuestionLink
                WHERE StaticQuestionId = 1 
                  AND DateEffective &lt;= qs.DateEffective
             ORDER BY DateEffective
                 DESC 
                LIMIT 1)
  </code></pre>