SQL Update row values from another row if value does not match - sql

Question

SQL rookie here. I'm trying to update a specific row of values using values from another row in the same table. However, I want the initial values to only update if it's null or does not match the new value.
My current unfinished script:
DECLARE #Record1 VARCHAR(50) = 'ID1',
#Record2 VARCHAR(50)='ID2'
UPDATE R1
SET r1.column1 = COALESCE(r1.column1, r2.column1),
r1.column2 = COALESCE(r1.column2, r2.column2),
r1.column3 = COALESCE(r1.column3, r2.column3),
r1.column4 = COALESCE(r1.column4, r2.column4)
FROM TABLE1 r1
INNER JOIN TABLE1 r2 ON r1.ID1 = #Record1 AND r2.ID2 = #Record2
I'm currently using SQL Server 2016 Management Studio.
EDIT:
After reading Sgeddes's reply, I realize my question was very dumb. Of course replacing everything in Record1 from Record2 makes the most sense.
I guess what I was fishing for was if there way a way to place a condition on each column to update. For example, if I wanted to update Record1's columns 1 & 2 only if NULL, but replace everything in columns 3 & 4 from Record2.

Answer 1

You would use a left join . . . carefully:
UPDATE R1
SET r1.column1 = coalesce(r1.column1, r2.column1),
r1.column2 = coalesce(r1.column2, r2.column2),
r1.column3 = coalesce(r1.column3, r2.column3),
r1.column4 = coalesce(r1.column4, r2.column4)
FROM TABLE1 r1 LEFT JOIN
TABLE1 r2
ON r2.ID2 = #Record2
WHERE r1.ID1 = #Record1 ;
Note that the condition on r1 goes in the where clause so it actually filters rows. The condition on t2 stays in the on clause, so all rows are returned, even when there are no matches in r2.