Summary of 2021 computer level III database

Hello everyone , I meet you again , I'm your friend, Quan Jun .

 For convenience   The next big questions are put here 

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The first big question ——ER Diagrams and relational patterns

Related references ： （ One ） What is? ER chart ？

（1） Concept 
ER chart ： Entity relation diagram , Brief notes E-R chart , Refers to the entity 、 Relationship 、 Three basic concepts of attributes summarize the basic structure of data , So as to describe the conceptual pattern of static data structure 
（2） elements 
3 elements ： Entity 、 Properties and relationships 
（3） Express 　
** Entity ** type ：  Use a rectangle to represent ,** Inside rectangle ** Specify the entity name ; 　　 
** attribute **：  use ** ellipse ** Rounded corners or rectangles represent , Connect with the corresponding entity ; Multivalued attributes are connected by two lines ; 
 Underline the name of the primary attribute ; 　　 
** contact **： use ** The diamond ** Express , Write the contact name in the diamond box , Connect with related entities , At the same time, the type of connection is marked beside the undirected edge  
（4） Relationship 
 stay E-R The diagram should clearly show 1 To many ,1 Yes 1 Relationships and many to many relationships . 　　
1 Yes 1 The relationship is written in the direction of the connection between two entities 1; 　　 
1 The many to many relationship is 1 The party who wrote 1, More than one party to write N; 　　 
 Many to many relationships are written in the direction of the connection between two entities N,M

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For the requirements analysis phase , Example title ：

ER chart

Relationship model

The second big question ——SQL Fill in the blanks

1） Use FOR or AFTER The trigger defined by the top selection is the post trigger , Only in The actions in the statement that triggered the trigger execution have been successfully executed , And after all constraint checks have been successfully completed , To execute the trigger . 2） Use INSTEAD OF The trigger defined by the top selection is the front trigger . In the trigger of this mode , Appoint Execute trigger Instead of executing what triggers the trigger to execute SQL sentence , Instead of the operation that triggers the statement . Two special temporary worksheets can be used in trigger statements ：INSERTED Table and DELETED surface . These two tables are created by the user data During the change operation of ,SQL Server Automatically created and managed . among INSERTED Tables are used to store INSERT and UPDATE A copy of the line affected by the statement . and DELETED Tables are used for storage DELETE and UPDATED A copy of the line affected by the statement . The syntax for creating triggers is ： CreateTRIGGER trigger_name ON { table | view } { { { FOR |AFTER | INSTEAD OF } { [ Insert ] [ , ] [ Update ] } AS [{ IF Update (column ) [{ AND | or } Update ( column )] […n ] | IF ( COLUMNS_UpdateD ( ) { bitwise_operator } updated_bitmask) { comparison_operator } column_bitmask […n ] }] sql_statement […n ] } }

The third big question —— case analysis

【 Their thinking 】 This question examines Knowledge of partition tables, database operating environment and parameter adjustment . （1） Partitioned table is to divide the data in the table into different subsets in a horizontal way , These subsets of data are stored in one or more filegroups in the database . Reasonable use of partitions will greatly improve the performance of the database . Whether to create a partitioned table mainly depends on the current and future data size of the table , It also depends on the operation characteristics of the data in the table . Usually , If a large table meets the following conditions at the same time , Is more suitable for partitioning ：① The table contains （ Or will contain ） A large amount of data used in many different forms ,② The data is segmented , For example, data is separated by years . However , A large amount of data is not the only condition for creating partitioned tables **, If a large number of data in the table are frequently used data , And they operate in the same way , It's best not to use partition tables **. When the operation on data only involves some data but not all data , Consider creating partition tables . （2） about CPU、 Memory 、 Disk and other hardware devices are used more than when idle 90%, It indicates that hardware resources are scarce . If the utilization rate is less than 90% It indicates that the hardware resources are sufficient . For partition tables , If for a table Current segment data , A common operation is to add 、 modify 、 Delete 、 Inquire about , For other data , There is almost no operation , Or the operation is limited to query , Then you can partition the table . Use RAID when , You can put the basic table and the index based on the table on different disks , When accessing the base table in this way , Disk drives that store data and indexes work in parallel , You can get faster file reading and writing speed ; Allied , Log files and data objects （ surface 、 Index, etc. ） It can also be stored on different disks to improve the performance of the system I/O performance .**RAID1 be relative to RAID0 To improve the reading speed , The reliability of the system is enhanced , But the writing efficiency has not improved .** stay RAID in , If price is not taken into account , Actually RAID10 Best fit , It's a RAID0 And RAID1 The combination of , It inherited RAID0 Fast and RAID1 The safety of the , At the same time, the reading and writing speed is improved . But the topic mainly tests reading ability , therefore RAID1 Than RAID0 More suitable for .

answer ： [1] On the login time column in the login log table Create the right partition function in months , Divide the login log table into 12 Zones , Each partition corresponds to the value of one month of the year . because Operations are limited to query , Creating partition tables can effectively improve query efficiency . [2] Adoption scheme 2. about CPU、 Memory 、 Disk and other hardware devices are used more than when idle 90%, It indicates that hardware resources are scarce . If the utilization rate is less than 90% It indicates that the hardware resources are sufficient . In this question , Memory and disk utilization are higher than 90%, and CPU The utilization rate of is not higher than 50%, explain ** Memory 、 Lack of hard disk resources , Need to adjust , and CPU We have sufficient resources and do not need to adjust .** Therefore, when adjusting the hardware , The capacity of memory and the access efficiency of hard disk should be improved .RAID1 be relative to RAID0 To improve the reading speed , The reliability of the system is enhanced , But the writing efficiency has not improved .

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