rdbms | 易学教程

NoSql vs Relational database

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Recently NoSQL has gained immense popularity. What are the advantages of NoSQL over traditional RDBMS ? Not all data is relational. For those situations, NoSQL can be helpful. With that said, NoSQL stands for "Not Only SQL". It's not intended to knock SQL or supplant it. SQL has several very big advantages: Strong mathematical basis. Declarative syntax. A well-known language in Structured Query Language (SQL). Those haven't gone away. It's a mistake to think about this as an either/or argument. NoSQL is an alternative that people need to consider when it fits, that's all. Documents can be

How to get the byte size of resultset in an SQL query?

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问题 Is it possible to get the size in bytes of the results of an sql query in MySQL? For example: select * from sometable; ths returns 10000 rows. I don't want the rows but the size of the resultset in bytes. Is it possible? 回答1: select sum(row_size) from ( select char_length(column1)+ char_length(column2)+ char_length(column3)+ char_length(column4) ... <-- repeat for all columns as row_size from your_table ) as tbl1; char_length for enum , set might not accurate, please take note 回答2: To build

RMySQL dbWriteTable with field.types

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问题 I have a data frame, called df , that looks like this: dte, val 2012-01-01, 23.2323 2012-01-02, 34.343 The type on the columns is date and numeric. I would like to write this to a MySQL database using an already open connection. The connection works fine as I am able to query the db fine. I try to run the following: dbWriteTable(con, name="table_name", value=df, field.types=list("date", "double(20,10)")) This generates the error: Error in function (classes, fdef, mtable) : unable to find an

Why don't DBMS's support ASSERTION

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问题 So I recently learned about ASSERTION in my databases course, and my prof noted that major databases don't support it, even though it is in the SQL-92 standard. I tried googling to find out why, but there doesn't seem to be any discussion on the topic. So, why isn't ASSERTION supported by the vast majority of relational database packages? Is it soley a performance issue or is there something intrinsically hard about it? If you can, please note any database packages that implement it as well

Why are batch inserts/updates faster? How do batch updates work?

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问题 Why are batch inserts faster? Is it because the connection and setup overhead for inserting a single row is the same for a set of rows? What other factors make batch inserts faster? How do batch updates work? Assuming the table has no uniqueness constraints, insert statements don\'t really have any effect on other insert statements in the batch. However, during batch updates, an update can alter the state of the table and hence can affect the outcome of other update queries in the batch. I

IN vs ANY operator in PostgreSQL

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问题 What is the difference between IN and ANY operator in PostgreSQL? The working mechanism of both seems to be the same. Can anyone explain this with an example? 回答1: Logically , quoting the manual: IN is equivalent to = ANY . But there are two syntax variants of IN and two variants of the ANY construct. Details: How to use ANY instead of IN in a WHERE clause with Rails? The IN () variant taking a set is equivalent to = ANY() taking a set , as demonstrated here: postgreSQL - in vs any But the

Relational Database Design Patterns?

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问题 Design patterns are usually related to object oriented design. Are there design patterns for creating and programming relational databases? Many problems surely must have reusable solutions. Examples would include patterns for table design, stored procedures, triggers, etc... Is there an online repository of such patterns, similar to martinfowler.com? Examples of problems that patterns could solve: Storing hierarchical data (e.g. single table with type vs multiple tables with 1:1 key and

TRIGGERs that cause INSERTs to fail? Possible?

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问题 In cleaning up this answer I learnt a bit about TRIGGER s and stored procedures in MySQL, but was stunned that, while BEFORE INSERT and BEFORE UPDATE triggers could modify data, they seemingly couldn\'t cause the insert/update to fail (ie. validation). In this particular case I was able to get this to work by manipulating the data in such a way as to cause a primary key duplicate, which in this particular case made sense, but doesn\'t necessarily make sense in a general sense. Is this sort of

Normalization in database management system

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问题 I have a relation in a database: Emp_project(SSN,PNum,Hours,Ename,PName,Plocs) I have been asked to normalize the relation Emp_project . How do I normalize it? 回答1: A relation has an associated set of columns of any type and an associated set of rows. There is one value per column per row. Sometimes "normalize" is used to mean (1) "decompose each relation to smaller relations that are its projections and that join back to it". This is normalization to higher NFs (normal forms) than 1NF. This