Mysql – How to create a user-defined aggregate function

aggregatefunctionsMySQL

I need an aggregate function that MySQL doesn't provide.

I would like it to be in MySQL's flavor of SQL (that is, not in C).

How do I do this? What I'm stuck on is creating an aggregate function — the docs don't seem to mention how this is done.

Examples of desired usage of a product function:

mysql> select product(col) as a from `table`;
+------+
| a    |
+------+
|  144 |
+------+
1 row in set (0.00 sec)

mysql> select col, product(col) as a from `table` group by col;
+-----+------+
| col | a    |
+-----+------+
|   6 |   36 |
|   4 |    4 |
+-----+------+
2 rows in set (0.01 sec)

Best Answer

According to the documentation http://dev.mysql.com/doc/refman/5.5/en/adding-udf.html it's only possible to write aggregate functions in C. Sorry!

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Mysql – find and insert row to another table using thesql trigger

This should do the trick for you:

DELIMITER $$
DROP TRIGGER IF EXISTS `employee_INSERT` $$
CREATE TRIGGER `employee_INSERT` 
AFTER INSERT ON `employee`
FOR EACH ROW
BEGIN
    INSERT INTO employee_tools (Id, Tool)
    SELECT new.Id, tools.Tool_Name 
            FROM tools
            WHERE tools.Division = new.Division;
END $$
DELIMITER ;

Mysql – ORA-00937 when calculating a sub-value in an aggregate select statement

Your additional calculated value:

(select count(t1.num) from T t1)

Is a scalar subquery, which is a dynamic rather than static expression. As such it's treated the same as a column as far as the aggregate is concerned and needs to be included in the group by clause to avoid the ORA-00937: not a single-group group function error

However, oracle does not allow subqueries as part of the group by clause and trying to include the scalar subquery and/or the whole case statement:

group by (case (select count(*) cnt from t t1) when 0 then 1 else 0 end)

just results in an ORA-22818: subquery expressions not allowed here error.

The only ways around this are to either convert your scalar subquery to an aggregate value like so:

max(case (select count(*) cnt from t t1) when 0 then 1 else 0 end)

(case max((select count(*) cnt from t t1)) when 0 then 1 else 0 end)

or rewrite your query to move the unaggregated scalar subquery out of the aggregated query:

select (case (select count(*) cnt from t t1) when 0 then 1 else 0 end) * sum
  from (select sum(t3.num) sum from t t3) t2;

or precompute your scalar subquery so it can be used in the group by clause:

select case t1.cnt when 0 then 1 else 0 end * sum(t2.num)
  from t t2
     , (select count(*) cnt from t) t1
 group by case t1.cnt when 0 then 1 else 0 end

Best Answer

Related Solutions

Mysql – find and insert row to another table using thesql trigger

Mysql – ORA-00937 when calculating a sub-value in an aggregate select statement

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