Srtange result from mysql function - mysql

Question

I create a function that returns a value, selected in a periods table like this:
CREATE TABLE `periods` (
`period` DATE NOT NULL,
`threshold` DOUBLE NOT NULL,
PRIMARY KEY (`period`)
)
COLLATE='latin1_swedish_ci'
ENGINE=InnoDB;
+------------------------+
| periods |
+------------+-----------+
| period | threshold |
+------------+-----------+
| 2013-11-01 | 5 |
+------------+-----------+
| 2013-12-01 | 1 |
+------------+-----------+
| 2014-01-01 | 5 |
+------------+-----------+
| 2014-02-01 | 5 |
+------------+-----------+
And function create:
DELIMITER $$
CREATE DEFINER=`root`#`localhost` FUNCTION `GET_THRESHOLD`(`PERIOD` VARCHAR(10))
RETURNS double
LANGUAGE SQL
NOT DETERMINISTIC
READS SQL DATA
SQL SECURITY DEFINER
COMMENT 'RETURN THRESHOLD'
BEGIN
DECLARE RESULT DOUBLE;
SELECT threshold INTO RESULT FROM periods WHERE period = PERIOD LIMIT 1;
RETURN RESULT;
END
$$
DELIMITER ;
but the function returns a value the Wrong
mysql>SELECT GET_THRESHOLD('2013-12-01')
-> 5
someone can help me?

Answer 1

You parameter has the same name as a column. That's a no-no. Try this:
DELIMITER $$
CREATE DEFINER=`root`#`localhost` FUNCTION `GET_THRESHOLD`(`V_PERIOD` VARCHAR(10))
RETURNS double
LANGUAGE SQL
NOT DETERMINISTIC
READS SQL DATA
SQL SECURITY DEFINER
COMMENT 'RETURN THRESHOLD'
BEGIN
DECLARE RESULT DOUBLE;
SELECT threshold INTO RESULT FROM periods WHERE period = V_PERIOD LIMIT 1;
RETURN RESULT;
END
$$
DELIMITER
The statement:
WHERE period = PERIOD
is comparing the column value to itself. So, it chooses all rows that have a non-NULL column value. Not very interesting.
It is good practice to always prefix variables and arguments with something to distinguish them from columns in tables.