Database three paradigm, easy to understand _php tutorial

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Official definition: First paradigm (1NF): A field in a database table is a single attribute and cannot be divided.
My understanding: The first paradigm does not have to say, as long as the relational database satisfies the first paradigm

Official definition: Second paradigm (2NF): A partial function dependency of a non-critical field in a database table on any of the candidate key fields
My understanding: In the second paradigm, the combination of the primary key (AB) "Annotated: Also called composite primary key" inside a or B and other fields can not have a combination of repetition, in order to solve this problem, it is common practice that we do not have to combine the primary key, add an ID, as a single primary key to meet the second paradigm. If you do not want to add an ID, please satisfy the combination of a or b in the combined primary key (AB) and the other fields cannot exist.
For example, the combination of a and field C in a composite primary key is not satisfied with the second normal form.
+------------+-----------+-------------------+
PK PK Row
+------------+-----------+-------------------+
A B C
+------------+-----------+-------------------+
A D C
+------------+-----------+-------------------+
A E C
+------------+-----------+-------------------+

Instead of satisfying the second paradigm (but not satisfying the third paradigm, field A and field C are combined duplicates):
+---------+------------+-----------+-------------------+
PK Row, Row row
+---------+------------+-----------+-------------------+
1 A B C
+---------+------------+-----------+-------------------+
2 A D C
+---------+------------+-----------+-------------------+
3 A E C
+---------+------------+-----------+-------------------+

Official definition: The third paradigm (3NF): On the basis of the second paradigm, if there is no non-critical field in the data table the transfer function dependency on either of the candidate key fields conforms to the third paradigm.
My understanding: In the third paradigm, there can be no combination of fields and fields
If the third normal form is not satisfied, field A and field C are combined repeatedly
+---------+------------+-----------+-------------------+---------------+
PK Row row, row row
+---------+------------+-----------+-------------------+---------------+
1 A B C F
+---------+------------+-----------+-------------------+---------------+
2 A D C G
+---------+------------+-----------+-------------------+---------------+
3 A E C K
+---------+------------+-----------+-------------------+---------------+

Instead, this satisfies the third paradigm:
Table 1
+---------+------------+-----------+
PK Row Row
+---------+------------+-----------+
1 A B
+---------+------------+-----------+
2 A D
+---------+------------+-----------+
3 A E
+---------+------------+-----------+
and table 2
+---------+-------------------+------------+
PK Row Row
+---------+-------------------+------------+
1 C F
+---------+--

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