cardinality

问题 So, I've read a whole lot of answers here on stackoverflow, but I'm still confused about the whole concept thereof. Specifically, I've gone over this article (including all the ones it references), but can't seem to find a solid grasp on the concept (or perhaps it is my confusion between cardinality (n:m, etc.) and identities): Still Confused About Identifying vs. Non-Identifying Relationships My issue is this: I know that identifying relationships imply that the primary key of a child entity

问题 I have added a FULLTEXT index to one of my MySQL database tables as follows: ALTER TABLE members ADD FULLTEXT(about,fname,lname,job_title); The problem is that using phpmyadmin I can see the cardinality of my new index is only 1 . Does this mean the index will never be used? I have run a analyze table command but it didn't seem to do anything. analyze table members The respective types of the index fields are varchar(100), varchar(100), text, varchar(200) and the engine used is MyISAM and the

问题 I have a MySQL database table that's quite large with 2.5million rows and growing. To speed up queries I've added an index to one of columns. When I set the index manually, through PHPMyAdmin for example, it's cardinality is around 1500 which seems about right and my queries run without issue. The problem then comes after a few queries, (especially on an INSERT but not limited to), have been run, the cardinality of that index drops to 17 or 18, and queries run extremely slow. Sometimes it

问题 How does SQL Server determine whether a table column has low cardinality? The reason I ask is because query optimizer would most probably not use an index on a gender column (values 'm' and 'f'). However how would it determine the cardinality of the gender column to come to that decision? On top of this, if in the unlikely event that I had a million entries in my table and only one entry in the gender column was 'm', would SQL server be able to determine this and use the index to retrieve

问题 With the following entity relationship structure I'm struggling to figure out if the relationship between the LOAN and ITEM entities is valid? The weak entity of LOAN uses a partial key of 'loan_dateLeant' and the primary keys from CUSTOMER and ITEM to form LOANs primary key. However LOAN has a 'one to many' relationship with ITEM as a loan can consist of more than one item. But surely this means that if more than one item is loaned, then the loan record will have two item_id values for part

问题 I have a data frame called thetas containing about 2.7 million observations. > str(thetas) 'data.frame': 2700000 obs. of 8 variables: $ rho_cnd : num 0 0 0 0 0 0 0 0 0 0 ... $ pct_cnd : num 0 0 0 0 0 0 0 0 0 0 ... $ sx : num 1 2 3 4 5 6 7 8 9 10 ... $ model : Factor w/ 7 levels "dN.mN","dN.mL",..: 1 1 1 1 1 1 1 1 1 1 ... $ estTheta : num -1.58 -1.716 0.504 -2.296 0.98 ... $ trueTheta : num 0.0962 -3.3913 3.6006 -0.1971 2.1906 ... $ estError : num -1.68 1.68 -3.1 -2.1 -1.21 ... $ trueAberSx:

问题 Usually when we say "all of my children are female" we intend to imply "and there's at least one of them". The famed pizza tutorial (V1.3) addresses this on page 100 saying that it's usually an error to have a universal restriction (owl:allValuesFrom) without an existential restriction (owl:someValuesFrom). owl:someValuesFrom is sort of a back-handed way of saying "and there's at least one". Is there any logical, performance or aesthetic reason not to instead use "owl:minCardinality"? 回答1:

问题 I keep coming across these terms: high cardinality and low cardinality in Cassandra . I don't understand what exactly they mean. what effects they have on queries and what is preferred. Please explain with example since that will be be easy to follow. 回答1: The cardinality of X is nothing more than the number of elements that compose X. In Cassandra the partition key cardinality is very important for partitioning data. Since the partition key is responsible for the distribution of the data

问题 I'm quite new to relational databases and have been having a lot of trouble recently trying to understand an entity relationship diagram I've been given. Here it is: Solicitor ERD (ERD is for a made up solicitor company) Basically my task is to take this ERD and write an SQL script to create the database, obviously filling in the tables with data I can make up. The SQL syntax isn't the thing I'm having a hard time with, it's simply understanding the cardinality in the diagram. To me the terms

问题 I have a UML diagram of what I need my database schema to look like. This diagram includes roles and cardinality. How can I enforce cardinality in Postgres? 回答1: Cardinality is enforced by using constraints. http://www.postgresql.org/docs/8.1/static/ddl-constraints.html 来源： https://stackoverflow.com/questions/3135206/cardinality-in-postgresql