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    <p>I see something like this:</p> <p><img src="https://i.stack.imgur.com/m6YPR.png" alt="enter image description here"></p> <p><code>questions</code> is the list of questions to be answered. <code>question_type</code> is an enumeration that indicates what type of answer is expected (e.g. lookup from <code>question_choices</code>, a date, a number, text, etc.) - whatever types of data you expect to be entered. This, along with the other columns in this table, can drive your input form.</p> <p><code>question_answers</code> contains a list of predefined answers to questions (such as a predefined list of religions, or hair color, or eye color, etc.). This can be used to build a drop-down list of values on your input form.</p> <p><code>users</code> is pretty self explanatory.</p> <p><code>user_characteristics</code> contains a list of my answers to the questionnaire. The <code>weight</code> column indicates how important it is to me that someone looking for me have this same answer. The <code>question_choices_id</code> would be populated if the answer came from a select list built from the <code>question_choices</code> table. Otherwise <code>question_choices_id</code> would be NULL. The converse is true for the <code>value</code> column. <code>value</code> would be NULL if the answer came from a select list built from the <code>question_choices</code> table. Otherwise <code>value</code> would contain the user's hand crafted answer to the question.</p> <p><code>user_preferences</code> contains answers to the questionnaire for who I am looking for. The <code>weight</code> column indicates how important it is to me that the person I am looking for have this same answer. The <code>question_choices_id</code> and <code>value</code> columns behave the same as in the <code>user_characteristics</code> table.</p> <p>SQL to find my match might look something like:</p> <pre><code>SELECT uc.id ,SUM(up.weight) AS my_weighted_score_of_them ,SUM(uc.weight) AS their_weighted_score_of_me ,SUM(up.weight) + SUM(uc.weight) AS combined_weighted_score FROM user_preferences up JOIN user_characteristics uc ON uc.questions_id = up.questions_id AND uc.question_choices_id = up.question_choices_id AND uc.value = up.value AND uc.users_id != up.users_id WHERE up.users_id = me.id GROUP BY uc.id ORDER BY SUM(up.weight) + SUM(uc.weight) DESC ,SUM(up.weight) DESC ,SUM(uc.weight) DESC </code></pre> <p>For performance reasons, an index on user_characteristics(id, question_id, question_choices_id, value, and user_id) and an index on user_preferences(id, question_id, question_choices_id, value, and user_id) would be advisable.</p> <p>Note that the above SQL will return one row for EVERY user except the one making the request. This certainly is NOT desirable. Consequently, one might consider adding <code>HAVING SUM(up.weight) + SUM(uc.weight) &gt; :some_minimum_value</code> - or some other way to further filter the results.</p> <p>Further tweaks might include only returning people who value an answer as much or more than I do (i.e. their characteristic weight is >= my weight preference weight.</p>
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