UPDATE does not add or delete rows, it only updates row data.
UPDATE without a WHERE clause will update all rows.
For this reason, you must be careful when doing an update.
The WHERE clause specifies the rows to be updated.

     UPDATE table
        SET column1 = [value,
                     | hostvariable,
                     | expression,
                     | NULL,
                     | specialregister,]
           [column2 = [value, ...
                     | hostvariable, ...
                     | expression, ...
                     | NULL, ...
                     | specialregister, ...]]
    [WHERE conditions];

> The SET value or expression must match the data type of the column and
replaces the existing value in that column.
> An expression can not cause an arithmetic error such as divide by zero.
> A value can not violate a CHECK constraint or a FOREIGN KEY constraint.