database-normalization

问题 While studying relational databases, I ran into this confusing page where the following snapshot summarizes my confusion: Why does the author say that Subject Table is in 1NF when student_id 401 and subject Math is repeated in the same way the blue depicts? This seems like a contradiction. 回答1: The key concept this page fails to mention is that the data contained in the row must not be duplicated, meaning that a single key value cannot have multiple values for a single column. The real

问题 While studying relational databases, I ran into this confusing page where the following snapshot summarizes my confusion: Why does the author say that Subject Table is in 1NF when student_id 401 and subject Math is repeated in the same way the blue depicts? This seems like a contradiction. 回答1: The key concept this page fails to mention is that the data contained in the row must not be duplicated, meaning that a single key value cannot have multiple values for a single column. The real

问题 I did a bit R&D on the fact tables, whether they are normalized or de-normalized. I came across some findings which make me confused. According to Kimball: Dimensional models combine normalized and denormalized table structures. The dimension tables of descriptive information are highly denormalized with detailed and hierarchical roll-up attributes in the same table. Meanwhile, the fact tables with performance metrics are typically normalized. While we advise against a fully normalized with

问题 1. A table is automatically in 3NF if one of the following holds: (i) If a relation consists of two attributes. (ii) If 2NF table consists of only one non key attribute. 2. If X → A is a dependency, then the table is in 3NF, if one of the following conditions exists: (i) If X is a superkey (ii) If A is a part of superkey I got the above claims from this site. I think that in both the claims, 2nd subpoint is wrong. The first one says that a table in 2NF will be in 3NF if we have all non-key

问题 I have been working on a database design and I'm stuck hitting a wall. I'm ending up with what I'm reading is not a normalized database structure but I'm having issues trying to find a "more correct" design and if this design is acceptable how do I execute it in Access? TLDR: If a table with a single column set as an auto number is an acceptable design, how do you go about inserting a record in it using Access? The segment of the database of concern is creating a structure for storing

问题 I have been working on a database design and I'm stuck hitting a wall. I'm ending up with what I'm reading is not a normalized database structure but I'm having issues trying to find a "more correct" design and if this design is acceptable how do I execute it in Access? TLDR: If a table with a single column set as an auto number is an acceptable design, how do you go about inserting a record in it using Access? The segment of the database of concern is creating a structure for storing

问题 I have seen many examples about fully functional dependencies, but they use to say that: x->y such that y shouldn't be determined by any proper subset of x, x has to be a key. But, what if y is determined by an attribute other than the proper subset or subset of x. Suppose that I have a students table which consists of rollno(primary key), name, phone no unique not null, email unique not null. As rollno is a primary key, let it be x and take name as y. now x->y, but phone or email also

问题 Suppose a relation schema R(A,B,C) and the FDs are {A -> B, B -> C} So the superkeys are {A}, {A,B} Now if we decompose it into 3NF it will be R1(A,B) with FD {A -> B} and R2(B,C) with FD {B -> C} Is it in BCNF ? I can't determine. Since B was not a superkey in R does {B -> C} in R2 violates BCNF ? 回答1: {AB} is a superkey, but it's not a candidate key. (It's not a minimal superkey.) The decomposition R 1 ( A B) R 2 ( B C) is in at least BCNF. Informally, a relation is in BCNF if every arrow

问题 I need to design database which would keep track of the following attributes: stdnum // student number postcode // postal code phone_number // student phone number city // student address: city Also listed are functional dependencies: stdnum -> postcode stdnum -> phone_number postcode -> city phone_number -> city I need to find lossless-join, dependency preserving, 3rd normal form decomposition of the attributes . I have tried different decompositions but there was no one that obeys all

问题 I need to design database which would keep track of the following attributes: stdnum // student number postcode // postal code phone_number // student phone number city // student address: city Also listed are functional dependencies: stdnum -> postcode stdnum -> phone_number postcode -> city phone_number -> city I need to find lossless-join, dependency preserving, 3rd normal form decomposition of the attributes . I have tried different decompositions but there was no one that obeys all