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Double your dating the reference manual for

Once you've edited the file, just pop it into the sql/ folder in your source distribution, rename it to My Group Concat.cc, and then edit the make file and copy any section having to do with udf_and adjust it to My Group

Then, just type "make My Group Concat.so" and follow the instructions to install it into the server :-) By embedding an IF() function within a COUNT() function, you can gather several counts on the same table at once.

For numeric arguments, the variance and standard deviation functions return a aggregate functions do not work with temporal values.

(They convert the values to numbers, losing everything after the first nonnumeric character.) To work around this problem, convert to numeric units, perform the aggregate operation, and convert back to a temporal value.

If you do this, you need to change the IF() function to return 0 instead of NULL on false: SELECT SUM(IF(userid="steve",1,0)) AS mypostcount, COUNT(*) AS total FROM forumitem WHERE topicid=123; It isn't clear when the specific 'order by' for the group_concat function is needed. would it be needed if we added an explicit order by in the main sql statement for the concatenated column?

If the column is already indexed do we need to add order by to either the main sql statement or the group_concat function? ' GROUP BY' can be applied to more than 1 column at once.

This does not only save time but also allows you to achieve things that you thought were impossible.

Here is a far more efficient method of finding medians (uses temporary tables, but some things you can't do in one sql statement).

Here is athree-step process that has worked for me: CREATE TEMPORARY TABLE medians SELECT FROMdata x, data y GROUP BY HAVING((COUNT(*)-(SUM(SIGN(1-SIGN(y.val-x.val))))) I was trying to get the percentage of a number rows contained in a table in one query.

Normally you could do this with 2 queries: SELECT COUNT(*) as total FROM table WHERE poll=1231 and question=2SELECT COUNT(*) as result FROM table WHERE poll=1231 and question=2 and answer = 1and then you get the percentage getting (result/total)*100I looked over and over all the My Sql documentation, and finally I tried this and works (thanks to Tim Ferrer): SELECT (COUNT(DISTINCT T2.id)/COUNT(DISTINCT T1.id)*100) as percentage FROM table T1, table T2 WHERE T1.poll=1231 and T1.question=2 and T2.poll=1231 and T2.question=2 and T2.answer=1; That works for me.

For my particular application, I needed to find medians for each group of rows.

Here is a simple example (uses a temp table for the original data, so you can copy-paste it into a mysql term without needing to cleanup anything):====================================# setup example data CREATE TEMPORARY TABLE data (group_id INT, value FLOAT); INSERT INTO data VALUES (1,1),(1,5),(1,6),(2,1),(2,3),(2,4),(2,20);# get all data we need to search CREATE TEMPORARY TABLE temp_median (id INT AUTO_INCREMENT PRIMARY KEY) SELECT group_id, value FROM data ORDER BY group_id, value;# find the row-id of all median values CREATE TEMPORARY TABLE temp_median_ids SELECT ROUND(AVG(id)) AS id FROM temp_median GROUP BY group_id;# select all median values SELECT group_id, value FROM temp_median_ids LEFT JOIN temp_median USING (id);===================If you want to see both median values:# setup example data CREATE TEMPORARY TABLE data (group_id INT, value FLOAT); INSERT INTO data VALUES (1,1),(1,5),(1,6),(2,1),(2,3),(2,4),(2,20);# get all data we need to search CREATE TEMPORARY TABLE temp_median (id INT AUTO_INCREMENT PRIMARY KEY) SELECT group_id, value FROM data ORDER BY group_id, value;# find the row-id of all median values CREATE TEMPORARY TABLE temp_median_ids SELECT FLOOR(AVG(id)) AS id_1, CEILING(AVG(id)) AS id_2 FROM temp_median GROUP BY group_id;# select all median values SELECT group_id, value FROM temp_median_ids LEFT JOIN temp_median ON (id=id_1 OR id=id_2); Great extension indeed, except that the installation instruction is erroneous:instead of: CREATE FUNCTION group_concat RETURNS STRING SONAME "My Group Concat.dll"; do: CREATE AGGREGATE FUNCTION group_concat RETURNS STRING SONAME "My Group Concat.dll"; I've looked at the code for the Windows version of the GROUP_CONCAT plugin for My SQL 4.0 and it was basically a direct copy of the udf_plugin that comes with the distribution... If you simply take the My SQL_UDF.cpp, comment out:#include "stdafx.h"And then remove the '__declspec(dllexport)' from the function prototypes, and also remove the 'extern "C" __declspec(dllexport)' from all the function declarations, well, you've got a compilable module for your mysql server for unix :-)All the rest of that crap in the zip file musta been added by MSVC or whatever.

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