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    <p>You cannot set optional parameters in MySQL stored procedures.<br> You can however set optional parameters in a MySQL UDF. </p> <p>You do know that MySQL has an <a href="http://dev.mysql.com/doc/refman/5.1/en/group-by-functions.html#function_avg" rel="nofollow noreferrer">AVG aggregate function</a>?</p> <p><strong>Workaround</strong> If you can face the ugliness of this workaround here's samplecode that uses a comma separated string with values as input and returns the average.</p> <pre><code>DELIMITER $$ CREATE FUNCTION MyAvg(valuestr varchar) RETURNS float BEGIN DECLARE output float; DECLARE arg_count integer; DECLARE str_length integer; DECLARE arg float; DECLARE i integer; SET output = NULL; SET i = LENGTH(valuestr); IF i &gt; 0 THEN BEGIN SET arg_count = 1; WHILE i &gt; 0 DO BEGIN IF MID(valuestr, i, 1) SET i = i - 1; END; END WHILE; /* calculate average */ SET output = 0; SET i = arg_count; WHILE i &gt; 0 DO BEGIN SET arg = SUBSTRING_INDEX( SUBSTRING_INDEX(valuestr, ',' , i) , ',', -1 ); SET output = output + arg; SET i = i - 1; END; END WHILE; SET output = output / arg_count; END; END IF; RETURN output; END $$ DELIMITER ; </code></pre> <p>Use concat_ws to feed the function.</p> <pre><code>SELECT MyAvg(CONCAT_WS(',',100,200,300,500)) AS test; </code></pre> <p>You can also <a href="http://dev.mysql.com/doc/refman/5.1/en/adding-functions.html" rel="nofollow noreferrer">write an UDF</a> in <a href="https://stackoverflow.com/questions/1914776/mysql-aggregate-udf-user-defined-function-in-c">C(++)</a> or <a href="https://stackoverflow.com/questions/5894258/creating-a-udf-for-mysql-in-delphi">Delphi/Lazarus</a></p>
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