SQL平均年龄比较函数返回null

Question

So, I'm working in MySQL, writing a function that averages the ages of women and men from a table and compares them, and returns which is greater. Dates are in the format of YYYY-MM-DD, and I'm using DATEDIFF(). The function appears to work correctly, but when I run the script, all I get is a message box with a null value instead of the response it should output.

USE tennis;

DROP PROCEDURE IF EXISTS AverAge;

DELIMITER //

CREATE PROCEDURE AverAge()
BEGIN
    DECLARE averWomen DECIMAL(3,2);
    DECLARE averMen DECIMAL(3,2);

    SELECT AVG(DATEDIFF(BIRTH_DATE,CURDATE()))
    INTO averWomen
    FROM PLAYERS
    WHERE sex = 'F';

    SELECT AVG(DATEDIFF(BIRTH_DATE,CURDATE()))  
    INTO averWomen
    FROM PLAYERS
    WHERE sex = 'M';

    IF averWomen > averMen THEN
        SELECT CONCAT('The average age of women is higher. It is ', averWomen) AS message;
    ELSEIF averMen > averWomen THEN
        SELECT CONCAT('The average age of women is higher. It is ', averMen) AS message;
    ELSE
        SELECT CONCAT('The average age of men and women is equal. It is ', averWomen, ' for women and ', averMen, ' for men.') AS message;
    END IF;
END//

DELIMITER ;

CALL AverAge();

Can someone spot where I'm messing up?

Edit

USE tennis;

DROP PROCEDURE IF EXISTS AverAge;

DELIMITER //

CREATE PROCEDURE AverAge()
BEGIN
    DECLARE averWomen DECIMAL(10,6);
    DECLARE averMen DECIMAL(10,6);

    SELECT AVG(EXTRACT(year FROM CURDATE()) - EXTRACT(YEAR FROM BIRTH_DATE))
    INTO averWomen
    FROM PLAYERS
    WHERE SEX = 'F';

    SELECT AVG(EXTRACT(year FROM CURDATE()) - EXTRACT(year FROM BIRTH_DATE))
    INTO averMen
    FROM PLAYERS
    WHERE SEX = 'M';

    IF averWomen > averMen THEN
        SELECT CONCAT('The average age of women is higher. It is ', averWomen) AS message;
    ELSEIF averMen > averWomen THEN
        SELECT CONCAT('The average age of women is higher. It is ', averMen) AS message;
    ELSE
        SELECT CONCAT('The average age of men and women is equal. It is ', averWomen, ' for women and ', averMen, ' for men.') AS message;
    END IF;
END//

DELIMITER ;

CALL AverAge();

So, this is what I did that works.

Answer 1

While you declared the variable averMen, you have not initialized it. The query which should be calculating averMen is calculating averWomen instead.

Try changing ...

SELECT AVG(DATEDIFF(BIRTH_DATE,CURDATE()))  
INTO averWomen
FROM PLAYERS
WHERE sex = 'M';

into

SELECT AVG(DATEDIFF(BIRTH_DATE,CURDATE()))  
INTO averMen
FROM PLAYERS
WHERE sex = 'M';

Answer 2

Your query is way too complicated. Why not just use conditional aggregation?

SELECT (CASE WHEN avg_f > avg_m
             THEN CONCAT('The average age of women is higher. It is ', avf_f) 
             WHEN avg_m > avg_f
             THEN CONCAT('The average age of men is higher. It is ', avg_m) 
             ELSE CONCAT('The average age of men and women is equal. It is ', avg_f, ' for both.')
        END) AS message
FROM (SELECT AVG(CASE WHEN sex = 'M' THEN DATEDIFF(BIRTH_DATE, CURDATE()) END) as avg_m,
             AVG(CASE WHEN sex = 'F' THEN DATEDIFF(BIRTH_DATE, CURDATE()) END) as avg_f
      FROM PLAYERS
     ) a

Why use five queries when you can just use one?