Question

In SQL, how do update a table, setting a column to a different value for each row?

I want to update some rows in a PostgreSQL database, setting one column to a number from a sequence, where that column has a unique constraint. I hoped that I could just use:

update person set unique_number = (select nextval('number_sequence') ); 

but it seems that nextval is only called once, so the update uses the same number for every row, and I get a ‘duplicate key violates unique constraint’ error. What should I do instead?

Answer 1

Don’t use a subselect, rather use the nextval function directly, like this:

update person set unique_number = nextval('number_sequence');