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Database System concepts reading notes-entity-contact model bitsCN.com Database System concepts reading notes-entity-contact model Preface To review the message system architecture I
most widely used data model.Based on the object data Model (object-based). The object-oriented data model can be seen as an extension of the E-R model with the addition of concepts such as encapsulation, methods (functions), and object identifiers.Semi-structured data Model (semistructured). A semi-structured data model allows data items of the same type to contain data descriptions for different sets of properties.
system that guarantees this feature is called guaranteed serializable. Consider a dispatch s containing i,j two instructions, only I and J are all read instructions, the execution order of two instructions is insignificant, and as long as there is at least one write instruction, I and J are conflicting.If the dispatch s can be converted into S1 through a series of non-conflicting instruction exchanges, then the S1 is called the conflict equivalence o
Database System Concepts Reading Notes-object-contact model preface in order to review the message system architecture I wrote, I need to re-read the first three chapters of database system co
Label:What is it? Silbershartz, the latest version of the sixth edition Database system Concepts distinguishes the concept of a data base from the database vendor (e.g., Oracle,mysql, etc.). In addition, many colleges and universities choose as the undergraduate, the postgraduate
reflected on the real table. Because allowing the view to appear anywhere the real table can appear, you can insert data for the previous faculty view:INSERT into faculty values (30765, ' Green ');However, instructor.salary can only insert null, instructor.salary if the provision is not allowed to be null, the operation will be blocked because of a violation of consistency. In addition, considering that the view is associated with multiple tables, the nesting of views, and so on, the problem be
exclusive lock when the other transaction does not hold a lock on a data item (whether a shared or exclusive lock).9.2 Time StampFor the same data item, the system holds two timestamp, maximum read timestamp, and maximum write timestamp.When access is not possible, the offending transaction will be aborted and will be assigned to an up-to-date timestamp to start again.9.3 Multi-version and Snapshot isolationEach transaction starts with a copy of its
, relatively independent tuples are composed of several attributes, and many of the properties of the tuples are arranged horizontally.tuples correspond to several parallel, relatively independent entities in an entity set, and several attribute groups of each entity are the properties of tuples Properties : in a relationship, each vertical column is called an attribute, and the property corresponds to the entire information of one property of the entity setRelationship ModeIn a relationship,
A data dictionary or system directory: A system database that describes the data information in a database and is a data or metadata data.Tuple-> row or record property-> the number of column or field tuples-> the number of cardinality attributes-> degreesA collection key-> a unique identifier for a-> table field-> A v
Principles of database systems (I)-basic concepts of database systems and concepts of Database Systems Concept Model: (1) express the overall logical structure of the data, which is a comprehensive description of the data involved in the entire application project by the
Some important concepts: ① Data: Data is the symbol that describes things. Data is everywhere ② database: The database is where the data is stored. A database is a collection of data and database objects. The so-called database ob
section I, Related Concepts 1. Data: A basic object stored in a database and a symbolic record describing things. 2. Database: Databases are a collection of large amounts of data that are stored in a computer, organized, and shareable over a long period of time. 3. DBMS: A database management
a little bit of database-----Day 1-Some basic concepts of the database ----Reprint Please specify the Source: Coder-pigIntroduction to this section:This section mainly introduces some basic concepts of the names of some databases, so that you have a basic understanding of the datab
Learn about database ----- Day 1-some basic concepts of database ----- day1- One database every Day ----- Day 1-some basic concepts of the database ---- Reprinted with the source: coder-pig This section introduces: This secti
Some basic concepts often asked in database interview 1, Super Key, candidate key, primary key, foreign key Super key: The set of attributes in a relationship that uniquely identifies a tuple is called a hyperlink to a relational pattern. A property can be used as a super key, and multiple attributes can be grouped together as a super key. The Super key contains candidate keys and primary keys. Candidate
Operating System Concepts Learning Note 6 system callsSystem callssystem calls, usually written in C or C + +, may be provided as assembly-language instructions for the underlying tasks, such as requiring direct access to hardware.An example of a system call:To write a simple program that reads data from one file and c
/init $ ORACLE_SID.ora.If multiple databases are created on the same server, there must be multiple database instances at the same time. At this time, you can repeat the definition process (Export ORACLE_SID = orcl) To select different instances. You can also use the oraenv command line to switch different ORACLE_SID to start different instances through the operating system)4
Operating System Concepts Learning Note 5 Operating System Management brief process managementA program that is in execution is called a process.The process requires a certain amount of resources (including CPU time, memory, files, I/O devices) to complete the task. These resources can be assigned to a process when the process is created, or it can be assigned to
Tags: construction part sof hidden unit identification von Neumann why commandin this chapter, we begin the introduction of Linux system, we first understand the structure of the disk and the concept of file system, in the foundation of the previous article, the main introduction of the use of Linux commands and other concepts, knowledge is trivial, I hope that t
are used to store and manipulate the logical structure of the data. According to the classification of information.A database may contain several data tables, each table is composed of rows and columns , record one data, the data table adds a row, each column consists of a field name and a collection of field data, the column is called a field ,Each column also has its own multiple properties , such as whether to allow Nulls, default values, lengths,
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