SQL query

I have come across a query that a developer wrote to update a few rows in table, the query did update the two desired rows but also updated the rest of the table with the column value as 'false'.

Update tableA set col1 = null and col2 in (1,2);

The query updated col1 to null for the rows where col2 was either 1 or 2, rest of rows were also updated for col1 to 'false'.

 The above was run without where clause.

Could the experts throw some light on this?

Regards

Vikas Sharma