Domain Key Normal Form DKNF is also called 6NF.
Our relations have no constraints other than domain constraints and key constraints.
Domain constraints
Domain constraints specify the possible values of the attribute.
E.g. The only colors of hair are black and white
E.g. The CGPA of a student is between 0 and 4.
Key constraints
Key constraints specify keys of some relation.
General constraints
General constraints or any other constraints which can be
expressed by the first-order logic.
E.g. If the first digit of a bank account is 2, then the balance of
the account is greater than >=4000.
| account | Balance | Type |
| 1-T4Tutorials-110 | ||
| 2-T4Tutorials-211 |
It’s logically wrong because Balance is nonprime attribute and determining the type. In this case, a transitive dependency is occurring.
Domain constraint: If Regular_Account, then the first digit of AccountNo will be 2 and balance < 4000
Domain constraint: If Executive_Account, then the first digit of AccountNo will be 1 and balance ³ 4000.
Solution: Separate table for Regular_Account and Executive_Account.
| account | Balance |
| 1-T4Tutorials-110 | |
| 2-T4Tutorials-211 |
Table: Regular_Account
| account | Balance |
| 1-T4Tutorials-110 | |
| 2-T4Tutorials-211 |
Table: Executive _Account
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