using a user variable in where clause without subquery - mysql

Question

I have the following scenario:
+-----------+
| my_column |
+-----------+
| A |
| B |
| C |
| D |
| E |
+-----------+
I have simplified my_function bellow for this example;
DROP FUNCTION IF EXISTS my_function;
CREATE FUNCTION my_function(
phrase VARCHAR(255),
column_value VARCHAR(255)
)
RETURNS FLOAT(20,10)
READS SQL DATA
SQL SECURITY INVOKER
BEGIN
IF(column_value = 'A') THEN RETURN 1.0000000000;
ELSEIF(column_value = 'B') THEN RETURN 0.7500000000;
ELSEIF(column_value = 'C') THEN RETURN 0.7500000000;
ELSEIF(column_value = 'D') THEN RETURN 0.5000000000;
ELSEIF(column_value = 'E') THEN RETURN 0.0000000000;
END IF;
END;
Here is my main stored procedure:
DROP PROCEDURE IF EXISTS my_procedure;
CREATE PROCEDURE my_procedure(
IN phrase VARCHAR(255)
)
READS SQL DATA
SQL SECURITY INVOKER
BEGIN
SET #phrase = phrase;
SET #query = "
SELECT
my_column,
#score_var := my_function(?,my_column) as score,
#score_var
FROM my_table
ORDER BY score DESC;
";
PREPARE stmt FROM #query;
EXECUTE stmt USING #phrase;
DEALLOCATE PREPARE stmt;
END;
Now if I call my_procedure
call my_procedure('anything');
The result is:
+-----------+--------------+------------+
| my_column | score | #score_var |
+-----------+--------------+------------+
| A | 1.0000000000 | 1 |
| B | 0.7500000000 | 0.75 |
| C | 0.7500000000 | 0.75 |
| D | 0.5000000000 | 0.5 |
| E | 0.0000000000 | 0 |
+-----------+--------------+------------+
But if I add WHERE #score_var > 0.5 inside of the query in my_procedure, the result is:
+-----------+--------------+------------+
| my_column | score | #score_var |
+-----------+--------------+------------+
| A | 1.0000000000 | 1 |
| C | 0.7500000000 | 0.75 |
| E | 0.0000000000 | 0 |
+-----------+--------------+------------+
Expected result ´> 0.5´:
+-----------+--------------+------------+
| my_column | score | #score_var |
+-----------+--------------+------------+
| A | 1.0000000000 | 1 |
| B | 0.7500000000 | 0.75 |
| C | 0.7500000000 | 0.75 |
+-----------+--------------+------------+
I have seen some answers that use a subquery, but my question is: can (in this case) I not use a subquery?
Alternative approaches are also welcome.

Answer 1

When you read and write a user variable in the same statement, the behavior is documented as "undocumented". In other words the result is unpredictable unlesss you read and understand the source code of your MySQL version.
However - I think you are complicating things here unnecessarily. I don't see a reason to use a prepared statement, neither to use u user variable. Your procedure body could be just:
SELECT
my_column,
my_function(phrase, my_column) as score,
FROM my_table
HAVING score > 0.5
ORDER BY score DESC
Also your function could be written with less code duplicatin:
RETURN
CASE column_value
WHEN 'A' THEN 1.0000000000
WHEN 'B' THEN 0.7500000000
WHEN 'C' THEN 0.7500000000
WHEN 'D' THEN 0.5000000000
WHEN 'E' THEN 0.0000000000
END