SQL set values of one column equal to values of another column in the same table

Question

I have a table with two DATETIME columns.

One of them is never NULL, but one of them is sometimes NULL.

I need to write a query which will set all the NULL rows

Answer 1

I would do it this way:

UPDATE YourTable SET B = COALESCE(B, A);

COALESCE is a function that returns its first non-null argument.

In this example, if B on a given row is not null, the update is a no-op.

If B is null, the COALESCE skips it and uses A instead.