Database basic notes

This paper is finally updated at 687 Days ago, , The information may have developed or changed .

It covers

ACID,NULL,MVCC, constraint 、 conceptual model 、 Third normal form 、 Connect

1. ACID

Database management system Business (transaction) Four characteristics of （ Explain in sequence according to the acronym when analyzing ）： Atomicity （Atomicity）、 Uniformity （Consistency）、 Isolation, （Isolation）、 persistence （Durability） Transaction , It's a Operation sequence , These operations are either performed , Either not , It is an indivisible unit of work .（ A set of instructions or operations that perform a single logical function is called a transaction ）

Detailed explanation

1. Atomicity

Atomicity means that a transaction is a An indivisible unit of work , Either the operations in the transaction occur , Or none at all . May adopt “A towards B Transfer accounts ” This example is used to illustrate stay DBMS in , By default One SQL It's a separate business , The business is Automatic submission Of . Only explicit use start transaction Start a transaction , To put a block of code in a transaction .

2. Uniformity

Consistency means in Before and after a transaction , The database integrity constraint is not broken . This means that database transactions cannot be destroyed Integrity of relational data as well as Consistency in business logic . Such as A to B Transfer accounts , Whether the transaction operation of transfer is successful or not , The total deposits of both remain unchanged （ This is the consistency of business logic , The integrity of database relational constraints is better understood ）. Safeguard mechanism （ Also from two aspects ）： Database level Before and after a transaction is executed , The data will match your settings constraint （ Unique constraint , Foreign key constraints ,check constraint etc. ) And trigger settings ; Besides , The internal data structure of the database （ Such as B Tree index or double linked list ） Must be right . Business consistency is generally guaranteed by developers , It can also be transferred to the database level .

3. Isolation,

When multiple transactions are accessed concurrently , Transactions are isolated , A transaction should not affect the running effect of other transactions . In a concurrent environment , When Different transactions manipulate the same data at the same time when , Every transaction has Their complete data space . Changes made by concurrent firms must be isolated from changes made by any other concurrent firms . When transactions view data updates , The state of the data is either the state of another transaction before it was modified , Or another transaction changes its state , Transactions do not see data in the intermediate state . The most complex problems of transactions are caused by transaction isolation . Complete isolation is unrealistic , Complete isolation requires the database to execute only one transaction at a time , This can seriously affect performance . About the transaction isolation level in isolation （ The impact between transactions ）, See the corresponding blog

4. persistence

This is one of the best understood features ： persistence , It means after the transaction is completed , Changes made by the firm to the database are persisted in the database , It will not be rolled back .（ The completed transaction is Permanent part of the system , The impact on the system is permanent , This modification will remain unchanged even if there is a fatal system failure ） write ahead logging：SQL Server Used in WAL（Write-Ahead Logging） Technology to ensure the of transaction log ACID characteristic , Before data is written to the database , Write to the log first , Then change the log record to memory .

2. MVCC

3. Five constraints

　　1、 Primary key constraint （Primay Key Coustraint） Uniqueness , Non emptiness 　　2、 Unique constraint （Unique Counstraint） Uniqueness , Can be empty , But only one 　　3、 Check constraint （Check Counstraint） The range of data in this column 、 Format restrictions （ Such as ： Age 、 Gender, etc ） 　　4、 Default constraint （Default Counstraint） The default value of this data 　　5、 Foreign key constraints （Foreign Key Counstraint） You need to establish a relationship between the two tables and reference the columns of the main table

4. conceptual model

1. conceptual model ：
   • Definition ： The first level of abstraction from the real world to the information world , Determine the attribute relationship of domain entities, etc .
   • Representation ：E-R chart .
2. logical model ：
   • Definition ：
     • Transform a conceptual model into a data model .
     • According to the selected DBMS, take E-R The diagram is transformed into the corresponding logical model .
   • The essence ：E-R The transformation from graph to relational model is to solve how to transform entities and relationships between entities into relationships , And determine the attributes and codes of these relationships .
   • Transformation rules ：
     • An entity is transformed into a relationship , The attributes of entities are the attributes of relationships , The code of entity is the code of relation .
     • A connection is also transformed into a relationship , The attribute of the relation and the code of the entity to which the relation is connected are converted into the attribute of the relation , But the code of the relationship will change according to the type of connection , If it is ： 1：1 contact , The codes of both entities become the candidate codes of the relationship . 1：n contact ,n The code of the end entity becomes the code of the relationship . m：n contact , The combination of entity codes at both ends becomes the code of relationship .
3. The physical model ：
   • Definition ： The logical model corresponds to the machine implementation of the specific data model .
   • The essence ： A physical model is a description of a real database .
   • For example, some objects in relational database are tables 、 View 、 Field 、 data type 、 length 、 Primary key 、 Foreign keys 、 Indexes 、 constraint 、 Can it be empty 、 The default value is .

5. Three paradigms

• A normal form guarantees the atomicity of each column All field values in the database table are indecomposable atomic values
• Second normal form ： Make sure that a table describes only one thing Eliminate dependencies between non primary attributes
• Third normal form ---- Ensure that each column is directly related to the primary key Eliminate delivery dependency

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