What is the syntax for creating an SQL query that will generate an identical ID column if any of three additional columns have matching data?

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Assuming the table name is "my_table" and the column names are "column1", "column2", and "column3", the syntax for creating an SQL query that will generate an identical ID column if any of three additional columns have matching data is:

SELECT 
  CASE 
    WHEN COUNT(*) OVER (PARTITION BY column1, column2, column3) > 1 
    THEN row_number() OVER (PARTITION BY column1, column2, column3 ORDER BY id) 
    ELSE id 
  END AS identical_id,
  column1,
  column2,
  column3
FROM my_table;

This query will first check if there are any duplicate rows based on the combination of values in column1, column2, and column3. If there are duplicate rows, it will assign a unique ID to each group of duplicates using the rownumber() function, which will generate a sequential number for each row within each group. If there are no duplicate rows, it will simply use the existing ID column. The result will be a new column called "identicalid" that contains the unique IDs.