Is it a normal practice when there’re 2 columns and if one is true the other must false, and vise versa

Yes, it's in 1NF.

You can't side-step the often hard work of determining all the candidate keys by hanging a number off the end of the table and saying, "There. I've got a primary key." One natural candidate key for this table is {Name, Bought from, Date bought}. Consider using "Time bought" instead of "Date bought".

Your definition of 2NF is wrong. Instead of

Second Normal Form: A relation that is in First Normal Form and every non-primary-key attribute is fully functionally dependent on the primary key.

you need something more like this.

Second Normal Form: a relation that is in First Normal Form, and every non-prime attribute is fully functionally dependent on every candidate key.

The term non-prime attribute doesn't mean quite what non-primary-key attribute means.

Your definition of 3NF is wrong, and it's wrong for the same reasons as your definition for 2NF was wrong.

Instead of this

Third Normal Form: A relation that is in First and Second Normal Form and in which no non-primary-key attribute is transitively dependent on the primary key.

you need something closer to this.

Third Normal Form: A relation that is in Second Normal Form and in which every non-prime attribute is nontransitively dependent on every candidate key. (There isn't a really good way to express all those negative in one sentence.)

If I understand your dilemma correctly, you have:

• Two tables, each of which can have a comment
• One of the tables is optional, i.e. it may not have an entry to correspond to the other
• For the second table, the comment is also optional, such that even if the second table has a record to match the first, the comment in the second may just "default" to a copy of the first.
• On the other hand, the second table may have a distinct comment after all.

If this is the case, your concern is that recording the "default" (duplicate) comment in the second table is wasteful or even dangerous since the data could get out of whack.

In this situation, you can use the SQL COALESCE function along with a LEFT OUTER JOIN to solve your problem.

Using the outer join lets you use a single SQL statement that pulls together the two tables (when there are records in each) or just pull data from the first (mandatory) table if the second (optional) record is missing. No complicated branching, just a single SQL select. The coalesce allows you to pick the first non-NULL value in a list of values. This is useful because it lets you take the first comment as a default if the second comment is NULL. The second comment can be NULL because either (a) there is no matching record in the second table or (b) the comment within the second table is NULL.

It seems like in your case there will always be an ISSUE entry. Sometimes this entry is created by a user and sometimes it is the result of an unsatisfactory STATUS_CHECK.

In this case you want to select:

...
COALESCE(I.comment, C.comment) as Remark
FROM ISSUE I LEFT OUTER JOIN STATUS_CHECK C
ON I.ID = C.ISSUE_ID
...

The net effect of this is that if you have a satisfactory status check, it will show the status check comment. If you have an unsatisfactory status check or an independently raised issue, it will show the (non-NULL) comment from the issue table.