Simple sql function with SELECT - mysql

Question

I try to understand how functions work. I can make the equivalent in procedure but I can't create a simple function with select.
element is UNIQUE and
thing is PRIMARY
CREATE DEFINER=`root`#`localhost`
FUNCTION `get_element_by_thing`(`thing` VARCHAR(255))
RETURNS VARCHAR(255)
CHARSET utf8
NOT DETERMINISTIC
READS SQL DATA
SQL SECURITY DEFINER
DECLARE #return_element VARCHAR(255);
SET #return_element = (
SELECT
`element`
FROM
`table1`
WHERE
`thing` = thing
);
RETURN #return_element;
I use the phpmyadmin interface.

Answer 1

1) Don't declare user-defined variables.
The name of a local variable in MySQL stored program does not start with an at sign #. As an example:
DECLARE stored_program_local_variable VARCHAR(255);
SET stored_program_local_variable = 'somevalue';
The name of a user-defined variables start with an at sign #. (The at sign character is what distinguishes user-defined variables from other identifiers.) It's not valid to declare a user-defined variable in a stored program. To create a user-defined variable, just assign a value to it. For example:
SET #user_defined_variable = 'somevalue';
2) If we don't need to persist variables beyond the scope of a stored program, we typically use local variables, which exist only for the duration of the stored program execution. (Which is different behavior than user-defined variables which are at the session level.)
3) Use the SELECT ... INTO syntax to retrieve scalar values into user-defined or local variables. https://dev.mysql.com/doc/refman/5.7/en/select-into.html
Try:
DELIMITER $$
CREATE DEFINER=`root`#`localhost`
FUNCTION `get_element_by_thing`(`thing` VARCHAR(255))
RETURNS VARCHAR(255)
...
BEGIN
DECLARE return_element VARCHAR(255) ;
SELECT t1.element
INTO return_element
FROM table1 t1
WHERE t1.thing = thing
LIMIT 1 ;
RETURN return_element ;
END $$
DELIMITER ;
Note: with ambiguous identifiers (i.e. routine parameter and column with the same name in a SQL statement, the routine parameter takes precedence over the column name. Qualify the column reference with the table name or table alias so it's not ambiguous. I prefer to assign routine parameters (and local variables) names that do not match column names.
If for some reason you need to assign a value to a user-defined variable in a SQL statement, you can use the := assignment operator. This is also valid outside the context of a stored program.
SELECT #user_defined_variable := t.somecolumn
FROM mytable t
WHERE somecondition
ORDER BY someexpression
LIMIT 1