mysql - SQL平均年龄比较函数返回null

Question

所以，我正在 MySQL 中工作，编写一个函数，从表中平均女性和男性的年龄并进行比较，并返回较大的值。日期的格式为 YYYY-MM-DD，我使用的是 DATEDIFF()。该函数似乎工作正常，但当我运行脚本时，我得到的只是一个带有空值的消息框，而不是它应该输出的响应。

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();

有人能发现我哪里搞砸了吗？

编辑

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();

所以，这就是我所做的有效的。

Answer 1

当您声明变量 averMen 时，您尚未初始化它。应该计算 averMen 的查询正在计算 averWomen。

尝试改变...

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

进入

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