Collation of MySQL error prone knowledge points (to be updated)

Mysql Sorting out fallible knowledge points （ To be updated ）

Note： Here, according to CSDN Mysql The skill tree Organized error prone questions , May refer to MySQL This one is enough ,MySQL Detailed study tutorial （ Recommended collection ）,MySQL Novice tutorial

One 、 Use of database

• Initial account （create + grant）, Reference resources mysql Create new users and authorize
• mysql Sign in , Log on to the remote server , Reference resources MySQL Two login methods ,MySQL Login tutorial
  Note：
  • If Joe Has been deployed on the development machine ssh service , So he No pass mysql -h Joe Development machine address -u joe-p mysql stay Use on any computer in the LAN .
  • mysql -h IP Address -u user -p password -P port Sign in .
• Create and delete databases （create database Database name perhaps drop ...）
• Create table （create）
• Self increasing field （id INT PRIMARY KEY AUTO_INCREMENT）
• Delete table （drop table Table name ）, Reference resources MySQL This one is enough
• Database coding , Reference resources MySql Change the database code to UTF8,show create database grammar
• MySQL Storage engine , Ensure that the storage engine of the data table is innodb, Reference resources MySQL Three common storage engines
  Note：
  • InnoDB Support transactions and foreign keys , There are better Concurrency capability . to update （ Delete ） Operating frequency Also high , Or to ensure the integrity of the data , such as OA Automatic office system .
  • MyISAM Transaction and foreign key are not supported , Simple structure , Can be compressed into Read only status . Mainly read write insertion Applications for , Like the blog system 、 News portal .
  • Memory The engine saves the data in memory , Restart will lose data , Read fast, fast , For a Session table and cache surface .
  • Temporary tables are not necessarily used Memory engine ：
    • Memory temporary table Adopted yes memory Storage engine
    • Disk temp table Adopted yes myisam Storage engine ( Disk temporary tables can also be used innodb Storage engine , adopt internal_tmp_disk_storage_engine Parameter to control which storage engine to use ,
      from mysql5.7.6 After that, it defaults to innodb Storage engine , The previous version defaults to myisam Storage engine

Two 、 data type

• Implicit type conversion of values , Reference resources MySQL Implicit transformation of
  Note：
  • If string starts with the a number , And all are numbers , Then the numerical result of conversion is the whole string ; Part of it is numbers , The result of the number conversion is until the first character that is not a number . such as varchar str = "123dafa" // The conversion to a number is 123
  • DECIMAL(P,D)：P Is the number of significant digits precision ,P The scope is 1〜65,D Is said The number of digits after the decimal point ,D The range is 0~30,MySQL requirement D Less than or equal to (<=)P such as DECIMAL(4,2), The value is 23.46.
  • For example, for int Type of x and y,select id, (x^2 + y^2)/2 as result from points; Output result The type is decimal type .
• Time default （default constraint , This question datetime and timestamp Will do ), Reference resources MySQL Default constraint ,mysql datetime And timestamp difference
• The text field ,MySQL in varchar and char difference ,MySQL Middle face VARCHAR and TEXT How to choose ？,mysql How many bytes does a character occupy
  Note：
  • varchar The length of the type is variable , and char The length of the type is fixed , When defined as char(10) when , Even if the inserted content is abc 3 Characters , It will still occupy 10 Characters , Which includes 7 Storage space of empty characters .
  • char Maximum length 255 Characters ,varchar Maximum length 65535 Characters ;varchar Type of The search efficiency is relatively low , and char The search efficiency of type is relatively high , Therefore, variable length is used varchar, Fixed length use char.
  • MySQL 5.0.3 A change in the version includes VARCHAR The maximum length of the field is from 255 Add characters to 65,535 Characters . This makes VARCHAR The type is more similar to TEXT. Although the two data types share Maximum length is 65,535 Characters , but VARCHAR Medium VAR Indicates that you can set the maximum size to 1 To 65,535 Any value between . TEXT Field The maximum fixed size is 65,535 Characters .
  • MEDIUMTEXT Up to 16 MB String ,LONGTEXT Up to 4 GB String .
• Binary fields （ There is something wrong with this topic , You should use mediumBlob）, Reference resources MySQL Details of various types （ See the comments ）,Blob Data type and application ,MySQL：BLOB And TEXT And its maximum storage limit
  Note：
  • blob Used to store binary numbers , The maximum number of bytes is 65535, That is, the maximum file size is 65K; and varchar The maximum number of characters is 65535.
  • char,varchar,text Is based on characters ; and blob Is based on bytes , There is no character set .

3、 ... and 、 Data modification

• insert data （ Pay attention to field constraints ）
• Update data
• Delete data

Four 、 Data query

5、 ... and 、 Database composition

6、 ... and 、 Query advanced

7、 ... and 、 Built in functions

8、 ... and 、 Aggregation and grouping

Nine 、 Subquery

Ten 、 Link query

11、 ... and 、 Indexes