MySQL SELECT IFNULL 然后将其与其他列进行数字比较

Question

我尝试了多种方法在选择时将 NULL 转换为数字 0：

SELECT
  ss.id AS staff_id,
  u.realname AS realname,
  ss.amount AS salary_amount,
  IF(s.paid_amount IS NOT NULL, s.paid_amount,0.00) AS paid_amount,
  (ss.amount-s.paid_amount)
FROM f_salary_staff ss
  JOIN user u ON (u.id=ss.user_id)
  LEFT JOIN (
    SELECT staff_id,
      month_year,
      SUM(amount) AS paid_amount
    FROM f_salary
    WHERE month_year='2020-02-29'
    GROUP BY staff_id,month_year
  ) s ON (ss.id=s.staff_id)

我使用了 IFNULL 但这根本不会将 0 转换为任何数字，因此选择甚至没有列出具有 NULL 值的那些。有没有办法在选择时将 NULL 值转换为数值并与其他列进行数值比较？

顺便说一句 .. 下面是两个表的创建表语句。

f_salary_staff

CREATE TABLE `f_salary_staff` (  `id` int(11) NOT NULL AUTO_INCREMENT, `store_id` int(11) NOT NULL,  `user_id` int(11) NOT NULL,  `date_enter` date DEFAULT NULL,  `amount` decimal(12,2) DEFAULT NULL,  `updated` datetime DEFAULT NULL,  `updated_by` int(11) DEFAULT NULL,  `created` datetime DEFAULT NULL,  `created_by` int(11) DEFAULT NULL,  `last_activated` datetime DEFAULT NULL,  `last_inactivated` datetime DEFAULT NULL,  `status` varchar(16) DEFAULT NULL,  PRIMARY KEY (`id`), KEY `store_id` (`store_id`),  KEY `user_id` (`user_id`),  CONSTRAINT `f_salary_staff_ibfk_1` FOREIGN KEY (`store_id`) REFERENCES `store` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,  CONSTRAINT `f_salary_staff_ibfk_2` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`) ON DELETE CASCADE ON UPDATE CASCADE )

然后是 f_salary

CREATE TABLE `f_salary` (  `id` int(11) NOT NULL AUTO_INCREMENT,  `staff_id` int(11) NOT NULL,  `amount` decimal(12,2) DEFAULT NULL,  `note` tinytext,  `ref_date` date DEFAULT NULL,  `month_year` date DEFAULT NULL,  `created` datetime DEFAULT NULL,  `created_by` int(11) DEFAULT NULL,  `updated` datetime DEFAULT NULL,  `updated_by` int(11) DEFAULT NULL,  `approved` datetime DEFAULT NULL,  `approved_by` int(11) DEFAULT NULL,  PRIMARY KEY (`id`),  KEY `staff_id` (`staff_id`),  CONSTRAINT `f_salary_ibfk_1` FOREIGN KEY (`staff_id`) REFERENCES `f_salary_staff` (`id`) ON DELETE CASCADE ON UPDATE CASCADE )

希望对需要了解表结构的人有所帮助

Answer 1

选择甚至没有列出具有 NULL 值的那些

因为您有一个LEFT JOIN，它被WHERE 子句转换为INNER JOIN。
将条件移至 ON 子句。
同样在左连接查询中，您选择列 staff_id 尽管它没有包含在应有的 GROUP BY 子句中。
改为：

SELECT ss.id AS staff_id, u.realname AS realname, ss.amount AS salary_amount, 
COALESCE(s.paid_amount, 0) AS paid_amount,
ss.amount - COALESCE(s.paid_amount, 0)
FROM f_salary_staff ss 
INNER JOIN user u ON (u.id=ss.user_id) 
LEFT JOIN (
  SELECT staff_id, ref_date, SUM(amount) AS paid_amount 
  FROM f_salary 
  WHERE month_year='2020-02-29' 
  GROUP BY staff_id, ref_date
) s ON (ss.id=s.staff_id) AND s.paid_amount > ss.amount

Answer 2

如果我能很好地理解您的需求，下面的查询应该可以工作：

SELECT t.*
FROM (
  SELECT 
    ss.id AS staff_id, 
    u.realname AS realname, 
    ss.amount AS salary_amount, 
    COALESCE(s.paid_amount, 0) AS paid_amount 
  FROM f_salary_staff ss 
  INNER JOIN user u ON (u.id=ss.user_id) 
  LEFT JOIN (
    SELECT staff_id, ref_date, SUM(amount) AS paid_amount 
    FROM f_salary 
    WHERE month_year='2020-02-29' 
    GROUP BY staff_id, ref_date
  ) s ON (ss.id=s.staff_id) 
) t
WHERE paid_amount > salary_amount

Answer 3

感谢 forpas 和 Gosfly .. 显然 COALESCE 或 IFNULL 甚至 IF 不能将 NULL 更改为 NULL 或 NOT NULL 以外的任何其他值 .. 结果仅用于显示，而不是它的真正价值 .. 所以我已将声明改为这样

SELECT
  ss.id AS staff_id,
  u.realname AS realname,
  ss.amount AS salary_amount,
  IFNULL(s.paid_amount,0.00) AS paid_amount,
  IF(s.paid_amount IS NULL,ss.amount,ss.amount-s.paid_amount) AS unpaid_amount
FROM f_salary_staff ss
  JOIN user u ON (u.id=ss.user_id)
  LEFT JOIN (
    SELECT staff_id,
      month_year,
      SUM(amount) AS paid_amount
    FROM f_salary
    WHERE month_year='2020-02-29'
    GROUP BY staff_id,month_year
  ) s ON (ss.id=s.staff_id)  
WHERE ss.amount != s.paid_amount OR s.paid_amount IS NULL

谢谢大家！