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8

1. Assuming that sku and item_number will always hold unique values I consider that you have come up to the answer by yourself, since an item_number is an optional attribute (or column at the implementation phase), i.e., it does not apply to all the product occurrences, therefore, from a logical point of view, it decidedly cannot (should not) be declared as ...


5

The compound PK of EmployeeId and AddressTypeId is exactly what you want, and you don't need anything else. The relationship is still a one-to-many relationship, as for one employee, you may have many address whether it is a home address or a work address. The bridge table is interesting if you may have the same address for multiple employees AND that a ...


4

Create a supertype entity CarBody for subtypes Sedan, SUV, ... and move all general information to that supertype. Your M-M relationship will be between CarBody and Feature. See this topic for subtyping in datamodeling Supertype/Subtype deciding between category: complete disjoint or incomplete overlapping .


3

The CASCADE clause works in the opposite direction. From the parent to the child table. When you delete a row in the parent table, the related rows in the child table are deleted as well.


2

What you are looking for is a 0-many relationship. This is usually referenced by a third table. Publication_Scientists : Publication_Id (int) | Scientist_Id (int) with 1:1 relationships back to the other two tables. Preferably using something like an enforced foreign key.


2

If your attribute item_number is unique, you can leave it in your original table even in case it can have null values. In fact the PostgreSQL manual says: For the purpose of a unique constraint, null values are not considered equal. So this could be right solution: CREATE TABLE product ( sku text PRIMARY KEY, name text ...


2

Error code 1064 : This means you have an error in your SQL syntax.( ; missed) Select Model, Variant, EngineSize, RegYear, Mileage, Price, Sector from sampledata ; update sampledata set RegYear = 2010 - RegYear where sector = 'Van Derived Cars' and mileage > 200 and regyear != -1 and price != 999999; Mysql error code 1175 : If you ...


2

The simplest and probably most common way would be to declare the (album_id, artist_id) pair of columns a unique composite key. There are two variations of this approach. First, you can keep the current structure of the album_artist table and simply add a unique constraint on the said two columns: ALTER TABLE album_artist ADD CONSTRAINT uq_album_artist ...


1

Normal forms are used to eliminate or at least reduce redundancy in data, as well as to eliminate the so called insertion/deletion anomalies. The BCNF eliminates redundancies and anomalies, but decomposing a relation in BCNF sometimes has the unpleasant effect of causing the loss of one or more functional dependencies during the process. For this reason ...



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