Question

I’m trying to update a field in SQL Server 2005 from one table to another field in another table, and I’m getting this whole violation of UNIQUE KEY constraint and I can’t seem to figure out where it went wrong with my script. Here’s what I have so far:

UPDATE table1
SET column1 = (SELECT table2.column2
FROM         table2 INNER JOIN
                  table1 ON table1.id = table2.id)

I’m quite new to SQL and would definitely love some input in regard to this situation. Thank you so much!

Answer 1

This syntax seems a little cleaner, but you will need to validate that the values in table2.column2 are unique.

UPDATE t1 
  SET column1 = t2.column2
    FROM dbo.table1 AS t1
    INNER JOIN dbo.table2 AS t2
    ON t1.id = t2.id;

Assuming table1.column1 has a unique constraint, and that id is the primary key in each table, you can check for potential violations first. This will tell you if there are dupes in column2 in the source table:

SELECT column2
  FROM dbo.table2
  WHERE id IN (SELECT id FROM dbo.table1)
  GROUP BY column2
  HAVING COUNT(column2) > 1;

And this will tell you if there are single values in table2 that will conflict with other rows that aren’t common between the two tables:

SELECT t2.column2
  FROM dbo.table2 AS t2
  WHERE EXISTS (SELECT 1 FROM dbo.table1 AS t1
    WHERE t1.column1 = t2.column2
    AND t2.id <> t1.id);