Database three paradigms, easy to understand_PHP tutorial

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Official definition: First Normal Form (1NF): The fields in the database table have a single attribute and cannot be subdivided.
My understanding: Needless to say the first normal form, as long as the relational database satisfies the first normal form

Official definition: Second Normal Form (2NF): There is no partial functional dependence of non-key fields on any candidate key field in the database table
My understanding: In the second normal form, A or B in the combined primary key (AB) [Note: also called composite primary key] cannot be combined with other fields. To solve this problem, the usual approach is that we do not need to combine the primary key. Adding an ID as a single primary key can satisfy the second normal form. If you do not want to add an ID, please ensure that A or B in the composite primary key (AB) cannot be duplicated with other fields.
For example: the second normal form is not satisfied, and the combination of A and field C in the composite primary key is repeated
+----------------+----------+-----------+
pk pk row
+----------------+----------+-----------+
A A B B C
+----------------+----------+-----------+
A A D D C
+----------------+----------+-----------+
A A A C
+----------------+----------+-----------+

Change to this to satisfy the second normal form (but not the third normal form, field A and field C are repeated combinations):
+---------+----------------+-----------+-------------- -----+
pk row row
+---------+----------------+-----------+-------------- -----+
1 1 A A B B C
+---------+----------------+-----------+-------------- -----+
2 +---------+----------------+-----------+-------------- -----+
3 3 A A E E C
+---------+----------------+-----------+-------------- -----+

Official definition: Third Normal Form (3NF): Based on the second normal form, if there is no transitive functional dependence of non-key fields on any candidate key field in the data table, it conforms to the third normal form.
My understanding: In the third normal form, there cannot be combinatorial duplication between fields
For example: the third normal form is not satisfied, the combination of field A and field C is repeated
+---------+----------------+-----------+-------------- -----+---------------+
pk row row
+---------+----------------+-----------+-------------- -----+---------------+
1 1 A A B B C C F
+---------+----------------+-----------+-------------- -----+---------------+
2 +---------+----------------+-----------+-------------- -----+---------------+
3 3 A A E E C C K
+---------+----------------+-----------+-------------- -----+---------------+

Change this to satisfy the third normal form:

Table 1
+---------+----------------+-----------+
pk row row
+---------+----------------+-----------+
1 1 A A B
+---------+----------------+-----------+
2 +---------+----------------+-----------+
3 3 A A E
+---------+----------------+-----------+
and Table 2
+---------+-------------------+----------------+
pk row
+---------+-------------------+----------------+
1 1 C C F
+---------+--

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