mysql - SQL 连接两个表，但如果第一个表存在于第二个表中，则覆盖第一个表

Question

我无法让这个查询按照我想要的方式工作。我有两个数据几乎相同的表，但希望一个表覆盖另一个表(如果存在)。举个例子比尝试解释更容易:

有一个未调整的余额表:

以及每个余额调整的单独表格

所需的输出采用未调整的余额，并在其之上应用任何现有调整(如果 is_current=1)...本质上替换该行，但仍保留原始未调整的 current_balance。

所需的输出将是这样的:

这是我当前的查询，它没有按照我想要的方式工作...它正在翻转值并缺少 current_balance。我已经尝试了几个小时但无法到达任何地方:

SELECT
*
FROM
  (
    SELECT
      balance_adjustments.name,
      balance_adjustments.user_id,
      balance_adjustments.amount_owed,
      balance_adjustments.when_to_pay,
      balance_adjustments.current_balance
    FROM
      balance_adjustments
    WHERE
      balance_adjustments.when_to_pay = '2018-11-05'
      AND balance_adjustments.is_current = true
    UNION ALL
    SELECT
      unadjusted_balance.name,
      unadjusted_balance.user_id,
      unadjusted_balance.amount_owed,
      unadjusted_balance.when_to_pay,
      unadjusted_balance.current_balance
    FROM
      unadjusted_balance
      LEFT OUTER JOIN balance_adjustments ON balance_adjustments.user_id = unadjusted_balance.user_id
      AND balance_adjustments.name = unadjusted_balance.name
      AND balance_adjustments.when_to_pay = unadjusted_balance.when_to_pay
      AND balance_adjustments.is_current = true
    WHERE
      unadjusted_balance.when_to_pay = '2018-11-05'
      AND balance_adjustments.name IS NULL
  ) AS table1

一些额外的命令可以帮助任何人设置此场景以进行测试:

CREATE TABLE balance_adjustments
(
name varchar(30),
    user_id varchar(30),
    amount_owed float,
    when_to_pay datetime,
    current_balance float,
    is_current boolean
);


CREATE TABLE unadjusted_balance
(
name varchar(30),
    user_id varchar(30),
    amount_owed float,
when_to_pay datetime,
current_balance float
);


insert into balance_adjustments values ('ricardo', '82340001', 100.00, '2018-11-05', null, 1)
insert into balance_adjustments values ('ricardo', '82340001', 33.00, '2018-11-05', null, 0)

insert into unadjusted_balance values ('joseph', '82340000', 2400.00, '2018-11-05', 4049.00)
insert into unadjusted_balance values ('ricardo', '82340001', 899.00, '2018-11-05', 500.00)

感谢您的帮助

Answer 1

如果您只想将 amount_owed 替换为 balance_adjustments 表中的值，其中 is_current 为 1(我假设有仅这些值之一)，那么简单的 LEFT JOIN 和 COALESCE 就足够了。 COALESCE 确保 balance_adjustments 表中不匹配行的任何 NULL 值被 unadjusted_balances 中的原始值替换 table 。从您的问题中不清楚您是否也想替换 when_to_pay 字段，我假设您在此查询中已替换。如果不这样做，只需将 COALESCE(ba.when_to_pay, ub.when_to_pay) ASwhen_to_pay 替换为 ub.when_to_pay。

SELECT ub.name, ub.user_id, 
    COALESCE(ba.amount_owed, ub.amount_owed) AS amount_owed,
    COALESCE(ba.when_to_pay, ub.when_to_pay) AS when_to_pay,
    ub.current_balance
FROM unadjusted_balance ub
LEFT JOIN balance_adjustments ba ON ba.user_id = ub.user_id AND ba.is_current
ORDER BY ub.name

输出:

name        user_id     amount_owed     when_to_pay             current_balance
joseph      82340000    2400            2018-11-05 00:00:00     4049
ricardo     82340001    100             2018-11-05 00:00:00     500

Demo on dbfiddle