SQL 平均年龄比较函数 returns null

Question

所以，我在 MySQL 工作，编写了一个函数，计算 table 中女性和男性年龄的平均值并进行比较，returns 哪个更大。日期的格式为 YYYY-MM-DD，我使用的是 DATEDIFF()。该函数似乎可以正常工作，但是当我运行脚本时，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();

有人能看出我哪里搞砸了吗？

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，这就是我所做的工作。

Answer 1

虽然你声明了变量averMen，但你还没有初始化它。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';

Answer 2

您的查询方式太复杂了。为什么不直接使用条件聚合？

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

为什么只能使用一个查询而使用五个查询？

SQL 平均年龄比较函数 returns null

SQL average age comparison function returns null

mysql

sql

datetime

function