SQL Server 2016

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create table records (
  id int identity(1, 1),
  amount int not null
);

create table loadedRecords (
  id int identity(1, 1),
  amount int not null,
  recordID int
);

insert into records (amount) values (400), (400), (400), (500);
insert into loadedRecords (amount) values (400), (500), (400), (400);
8 rows affected

UPDATE loadedRecords
SET recordID = r.id
FROM loadedRecords
  JOIN records as r
    ON r.amount = loadedRecords.amount
WHERE loadedRecords.recordID IS NULL
4 rows affected

select * from loadedRecords;


id
amount
recordID


1
400
1

2
500
4

3
400
1

4
400
1