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use relational operators in switch

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问题 Is there a way to use relational operators (<,<=,>,>=) in a switch statement? int score = 95; switch(score) { case (score >= 90): // do stuff } the above example (obviously) doesn't work 回答1: No you can not. From jls-14.11 The type of the Expression must be char, byte, short, int, Character, Byte, Short, Integer, String, or an enum type (§8.9), or a compile-time error occurs. Relational operators (<,<=,>,>=) results in boolean and which is not allowded. All of the following must be true, or a

Why should I use document based database instead of relational database?

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Why should I use document based database like CouchDB instead of using relational database. Are there any typical kinds of applications or domains where the document based database is more suitable than the relational database? max Probably you shouldn't :-) The second most obvious answer is you should use it if your data isn't relational. This usually manifests itself in having no easy way to describe your data as a set of columns. A good example is a database where you actually store paper documents, e.g. by scanning office mail. The data is the scanned PDF and you have some meta data which

SQL selecting rows where one column's value is common across another criteria column

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问题 I have a cross reference table that looks like this: id document_id subject_id 1 8 21 2 5 17 3 5 76 4 7 88 5 9 17 6 9 76 7 2 76 It matches documents to subjects. Documents can be members of more than one subject. I want to return rows from this table where a given document matches all the subjects in a given set. For example, given the set of subjects: (17,76) I want to return only rows for documents which match all the subjects in that set (at least) somewhere in the cross reference table.

Use two or more relational operators in one sentence in python

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问题 How do two or more relational operators in a single sentence work? For example: 5 < 5 <= 3 > 10 回答1: Python supports double-ended comparisons. For example, 3 < x <= 7 is a check for 3 < x and x <= 7 (with x being evaluated just once). By extension, 5 < 5 <= 3 > 10 means (5 < 5) and (5 <= 3) and (3 > 10) , all of which are False , so the whole expression evaluates to False . 回答2: https://docs.python.org/2/reference/expressions.html#comparisons It's evaluated in order, so your expression

Database schema-related problem

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问题 I have a kind of theoretical question about databases. To make it more concrete I have thought up an example case. Suppose I have a store with products. I have lots of different products. Not every product has the same applicable properties. For instance, I could define the size of a harddisk in gigabytes, but cannot use that same property on a CPU, simply because it does not apply. What I want is a database where I can add properties to products dynamically. The only thing I can come up with

What is the difference between an entity relationship model and a relational model?

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I was only able to find the following two differences: The relationships in an E-R model are explicitly defined, while they are implicit in a relational model. Relational models require an intermediate table (often called a "junction table") to hold two foreign keys that implement the many-to-many relationship. And why do we use the relational model, when we have an E-R diagram ? You have it backwards. The relationships in an E-R model are explicitly defined, while they are implicit in a relational model. No. Each Relational Model (RM) database base table and query result represents an

What is the difference between an entity relationship model and a relational model?

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问题 I was only able to find the following two differences: The relationships in an E-R model are explicitly defined, while they are implicit in a relational model. Relational models require an intermediate table (often called a \"junction table\") to hold two foreign keys that implement the many-to-many relationship. And why do we use the relational model, when we have an E-R diagram ? 回答1: You have it backwards. The relationships in an E-R model are explicitly defined, while they are implicit in

What's the best practice for primary keys in tables?

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问题 When designing tables, I\'ve developed a habit of having one column that is unique and that I make the primary key. This is achieved in three ways depending on requirements: Identity integer column that auto increments. Unique identifier (GUID) A short character(x) or integer (or other relatively small numeric type) column that can serve as a row identifier column Number 3 would be used for fairly small lookup, mostly read tables that might have a unique static length string code, or a

What is Normalisation (or Normalization)?

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问题 Why do database guys go on about normalisation? What is it? How does it help? Does it apply to anything outside of databases? 回答1: Normalization is basically to design a database schema such that duplicate and redundant data is avoided. If some piece of data is duplicated several places in the database, there is the risk that it is updated in one place but not the other, leading to data corruption. There is a number of normalization levels from 1. normal form through 5. normal form. Each