1NF says " Each column type is unique " - Database
Programming Language (MCQ) questions
Q. What type of form says “ Each column type is unique ” ?
- Published on 26 Aug 15
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-Posted on 03 Nov 15
- A technique of organising data is known as database Normalisation.
- It is a systematic approach of decomposing tables to eliminate data redundancy and undesirable characteristics like Insertion, Update and Deletion Anamolies.
- There is a multi-step process followed that puts data into tabular form by removing duplicated data from the relation tables.
- It is mainly used for two purpose:
1. Eliminating the reduntant(useless) data.
2. Ensuring data dependencies make sense i.e data is logically stored.
- In this form, no two Rows of data must contain repeating group of information i.e each set of column must have a unique value, such that multiple columns cannot be used to fetch the same row.
- Every table in the database should be organized into rows, and each row should have a primary key that distinguishes it as unique.
- The Primary key is usually a single column, but sometimes a combination of more than one column is used to create a single primary key.
- In this form there must not be any partial dependency of any column on primary key.
- It means that for a table that has concatenated primary key, each column in the table that is not part of the primary key must depend upon the entire concatenated key for its existence.
- A table fails Second normal form when any column depends only on one part of the concatenated key.
- This normal form is used in database normalization.
- It is the next level of normalization after Boyce–Codd normal form (BCNF).
- Whereas the second, third, and Boyce–Codd normal forms are concerned with functional dependencies, 4NF is concerned with a more general type of dependency known as a multivalued dependency.
- A Table is in 4NF if and only if, for every one of its non-trivial multivalued dependencies X \⇒ Y, X is a superkey—that is, X is either a candidate key or a superset thereof.
- This normal form is a higher version of the third normal form.
- It deals with certain type of anamoly that is not handled by 3NF.
- A 3NF table which does not have multiple overlapping candidate keys is said to be in BCNF.
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