SQL Server - updating distinct values - sql

Question

I have 3 columns of data:
Column1 has duplicate values eg a a b b c c
Column2 has all NULL values
Column3 has other data that is not really important
I want to update Column2 with a value eh Hello but only for 1 instance of each value for column1. Eg, a = Hello but the 2nd instance of Hello is NULL, same with b c and so on.
I can find the distinct value by using this:
select distinct Column1
from TABLENAME
But when I try to update a different column it breaks. What is wrong (probably a lot!!) with this:
update TABLENAME
set Column2 = 'Hello'
where (select distinct Column1 from TABLENAME)

Answer 1

You can try to use ROW_NUMBER window function make row number then only update with row number is 1.
update t1
set Column2 = 'Hello'
FROM (
select *,ROW_NUMBER() OVER(PARTITION BY Column1 ORDER BY Column3) rn
from TABLENAME
) t1
where rn = 1
Results:
| Column1 | Column2 | Column3 |
|---------|---------|---------|
| a | Hello | 1 |
| a | (null) | 2 |
| b | Hello | 3 |
| b | (null) | 4 |
| c | Hello | 5 |
| c | (null) | 6 |

Answer 2

Assuming it is your unique index, use Column 3.
UPDATE tablename SET column2 = 'Hello' WHERE column3 IN
(SELECT column3 from TableName GROUP BY column1)

Answer 3

You could also update only the rows that have odd numbers in Column 3.
UPDATE tablename SET column2 = 'Hello' WHERE column3 % 2 != 0

Answer 4

You can also use CROSS APPLY and CTE (Common Table Expression) to achieve this:
;with CTE AS
(SELECT t.Column1, t.Column2
FROM (SELECT DISTINCT Column1
FROM TABLENAME) x
CROSS APPLY(SELECT TOP 1 *
FROM TABLENAME
WHERE column1 = x.column1) t)
UPDTATE CTE
SET Column2 = 'Hello'
SELECT * FROM TABLENAME

Answer 5

You could use a window function as
UPDATE TT
SET Col = B
FROM
(
SELECT Col, ROW_NUMBER() OVER(PARTITION BY Col ORDER BY Col) RN
FROM T
)TT INNER JOIN
(
VALUES (1, 'Hello'), (2, NULL)
) TVC (A, B)
ON TT.RN = TVC.A;
Results:
+-------+
| Col |
+-------+
| Hello |
| NULL |
| Hello |
| NULL |
| Hello |
| NULL |
+-------+
Or using a CASE expression as:
UPDATE TT
SET Col = CASE WHEN RN = 1 THEN 'Hello' END
FROM
(
SELECT Col, ROW_NUMBER() OVER(PARTITION BY Col ORDER BY Col) RN
FROM T
)TT;
Online Demo