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

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 for column B equal to the values in column A.

I have tried this example but the SQL in the selected answer does not execute because MySQL Workbench doesn't seem to like the FROM in the UPDATE.

UPDATE YourTable
SET ColumnB=ColumnA
WHERE
ColumnB IS NULL 
AND ColumnA IS NOT NULL

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.

Here is sample code that might help you coping Column A to Column B:

UPDATE YourTable
SET ColumnB = ColumnA
WHERE
ColumnB IS NULL
AND ColumnA IS NOT NULL;

I don't think that other example is what you're looking for. If you're just updating one column from another column in the same table you should be able to use something like this.

update some_table set null_column = not_null_column where null_column is null