SQL Server 2019

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CREATE TABLE example (
  id         INT IDENTITY (1, 1),
  processed  INT
);
INSERT INTO
  example (processed)
VALUES
  (0),
  (0),
  (0),
  (0),
  (0),
  (0),
  (0),
  (0),
  (0),
  (0)
10 rows affected

SELECT * FROM example


id
processed


1
0

2
0

3
0

4
0

5
0

6
0

7
0

8
0

9
0

10
0


WITH
  sample AS
(
  SELECT TOP 50 PERCENT * FROM example WHERE processed = 0 ORDER BY newid()
)
UPDATE
  sample
SET
  processed = 5
5 rows affected

SELECT * FROM example


id
processed


1
5

2
0

3
0

4
5

5
5

6
5

7
5

8
0

9
0

10
0