SQL UPDATE, but only if the old value is null

Question

I have been using a sql like this to update a list of properties in my database:

update my_table set a = ?, b = ?, c = ?, d = ?,  where customer = ?

But I want to update a property with a new value ONLY if it does not have a value in the database already. How can I do that?

Answer 1

In MS SQL something like this (assuming non value means database NULL) should work:

update 
  my_table 
set 
  a = COALESCE(a, ?), 
  b = COALESCE(b, ?), 
  c = COALESCE(c, ?), 
  d = COALESCE(d, ?)
where 
  customer = ?

COALESCE() returns first non null value from its arguments.