SQL连接两个表，但是如果第二个表中存在第一个表，则覆盖第一个表

Question

I can't get this query to work the way I want it to. I have two tables with almost identical data but want one to override the other if it exists. An example would be easier than trying to explain:

There is an unadjusted balance table:

and a separate table for adjustments for each balance

the desired output takes unadjusted balances and applies any existing adjustments on top of it (if is_current=1)... essentially replacing the row but still keeping the original unadjusted current_balance.

the desired output would be something like this:

here is my current query that is not working how I want... it is flipping values and missing current_balance. i've been trying this for hours and can't get anywhere:

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

some additional commands to help anyone set this scenario up to test:

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)

thanks for any help

Answer 1

If you want to overwrite the unadjusted balance with balance adjustments then do do the following:

• Select all unadjusted balances.
• Left Join to adjustments so you can get any adjustments, making sure to filter out those who have is_current = 1.
• Use the sum of amount_owed in adjustments to get the overwrite amount.
• In order to default to the original if there is no adjustments, use coalesce outside of the sum, and have the original amount as the second parameter.

Coalesce will return the first value if it is not null, or the value of the next parameter otherwise. The result of the sum aggregate will be null if no rows are returned from the left join.

Query

SELECT ub.name
    , ub.user_id
    , COALESCE(SUM(ba.amount_owed), ub.amount_owed) AS amount_owed 
    , ub.when_to_pay
    , ub.current_balance
FROM @unadjusted_balance AS ub
LEFT JOIN @balance_adjustments AS ba ON ba.user_id = ub.user_id AND ba.is_current = 1
GROUP BY ub.name, ub.user_id, ub.amount_owed, ub.when_to_pay, ub.current_balance;

I wasn't quite sure if you could have more than one adjustment where is_current is equal to 1. If there is at most going to be one row, then omit the aggregate and group by and just pass ba.amount_owed as the first parameter in the coalesce.

Answer 2

If you just want to replace the amount_owed by the value from the balance_adjustments table where is_current is 1 (and I am assuming there is only one of these values), then a simple LEFT JOIN and COALESCE will suffice. The COALESCE ensures any NULL values from unmatched rows in the balance_adjustments table get replaced by the original value from the unadjusted_balances table. It's not clear from your question whether you want to replace the when_to_pay field as well, I have assumed you have in this query. If you don't, just replace COALESCE(ba.when_to_pay, ub.when_to_pay) AS when_to_pay with 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

Output:

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