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SQL statement

2022-07-01 06:27:00 【HHYZBC】

Catalog

SQL What is a statement

SQL The classification of languages

Data processing functions （ Single line processing functions ）

Group function （ Multiline processing functions ）

Number of commonly used branch processing lines

Search engine

MyISAM Storage engine

InnoDB Storage engine

MEMORY Storage engine

Other commands

SQL What is a statement

SQL Statements are also called structured query languages , Programmers write SQL sentence , How to execute the database otherwise SQL sentence , Finally, the operation of adding, deleting, modifying and querying data in the database is completed . Learning database is actually right SQL Sentence learning .

SQL The classification of languages

SQL There are many languages , Categorize commands , It's easier to remember ,

Data processing functions （ Single line processing functions ）

Data processing functions are also called single line processing functions

Characteristics of single line processing functions ： One input corresponds to one output .

The opposite of a single line handler is ： Multiline processing functions .（ Features of multiline processing functions ： Multiple inputs , Corresponding 1 Outputs ！）, It will be written below .

  • Common single line processing functions
    • lower()
      • Convert lowercase
      • lower( The string being processed )
    • upper()
      • Convert a capital
      • upper( The string being processed )
    • substr()
      • Intercept
      • （substr( Intercepted string , Start subscript , Intercept length )）
      • The starting subscript here is 1.
    • concat()
      • String concatenation
      • concat（ character string 1, character string 2）
      • character string 2 You will receive a string 1 Behind
    • length()
      • Take the length
      • length（ data ）
    • trim()
      • Go to space
      • trim( The string being processed )
    • str_to_date()
      • Convert string to date
      • str_to_date(' String date ', ' Date format ')
      • mysql Date format for ：
                    %Y     year
                    %m month
                    %d Japan
                    %h     when
                    %i     branch
                    %s     second
      • If the format of string date is %Y-%m-%d, You may not use this function ,MySQL It will be converted automatically
    • date_format()
      • Date format is converted to string type
      • date_format( Date type data , ' Date format ')
    • format()
      • Set the thousandth
      • format( Numbers , ' Format ')
      • Format needs to be preceded by $ Number
    • round()
      • rounding
      • round( value , Reserve the number of digits )
      • The reserved digits can be negative numbers , If for -1 when , Will be kept to ten , And so on .
    • rand() 
      • Generate random number
      • The generated random numbers are all integers, and the part is 0 Decimals of .
      • If you need to take 100 Random number within , You can generate random numbers *100.
    • ifnull()
      • Can be null Convert to a specific value
      • ifnull( data , As what value )
      • ifnull Is an empty handler . Specialized in dealing with empty . Of all the databases , As long as there is NULL Participating mathematical operations , The end result is NULL.
    • now()
      • Get the current time

Group function （ Multiline processing functions ）

The characteristics of multiline functions ： Input multiple lines , The final output line .

Number of commonly used branch processing lines

  • count()   
    •   Count
  • sum()
    • Sum up
  • avg() 
    • Average
  • max()   
    • Maximum  
  • min() 
    • minimum value
  • Be careful ：
    • The grouping function must be grouped before use , Then we can use . If you don't group the data , The whole table defaults to a group of .
    • Grouping functions automatically ignore NULL, You don't need to tell me in advance NULL To deal with .
    • In the grouping function count(*) and count( Specific fields ) What's the difference? ？
      • count( Specific fields )： Indicates that all the statistics under this field are not NULL The total number of elements of .
      • count(*)： The total number of rows in the statistical table .（ Just one line of data count be ++）. Because every line of record cannot be NULL, One column in a row of data is not NULL
    • Grouping functions cannot be used directly in where clause .
      • Because the grouping function can only be used after grouping . And in the where What appears in , There is no grouping at this time .

Search engine

mysql Support nine storage engines , At present 5.5.36 Support 8 individual . Different versions support different situations .

have access to ：show engines See which search engines , The only search engines you need to know are 3 Kind of ：

MyISAM Storage engine

The tables it manages have the following characteristics ：
         Use three files to represent each table ：
             Format file — Definition of storage table structure （mytable.frm）
             Data files — Store the contents of table rows （mytable.MYD）
             Index file — Store indexes on tables （mytable.MYI）： The index is the catalogue of a book , Narrow the scanning range , A mechanism to improve query efficiency .
         Can be converted to compressed 、 Read only tables to save space
MyISAM Storage engine features ：
             Can be converted to compressed 、 Read only tables to save space
             This is the advantage of this storage engine ！！！！

MyISAM Transaction mechanism is not supported , Low security .

InnoDB Storage engine

This is a mysql The default storage engine , It is also a heavyweight storage engine .
    InnoDB Support transactions , Support automatic recovery mechanism after database crash .
    InnoDB The main feature of the storage engine is ： Very safe .

The tables it manages have the following main characteristics ：
        – Every InnoDB The table is in the database directory as .frm The format file represents
        – InnoDB Table space tablespace Used to store the contents of a table （ A tablespace is a logical name . Tablespaces store data + Indexes .）

        – Provides a set of log files used to record transactional activities
        – use COMMIT( Submit )、SAVEPOINT And ROLLBACK( Roll back ) Support transaction processing
        – Provide full ACID compatible
        – stay MySQL Provide automatic recovery after server crash
        – Many versions （MVCC） And row level locking
        – Support the integrity of foreign keys and references , Including cascading deletion and update

InnoDB The biggest feature is to support transactions ：
         To ensure data security . Not very efficient , And can't compress , Cannot convert to read-only ,
         It can't save storage space well .

MEMORY Storage engine

Use MEMORY The table that stores the engine , Its data is stored in memory , And the length of the line is fixed ,
     These two characteristics make MEMORY The storage engine is very fast .

MEMORY The tables managed by the storage engine have the following characteristics ：
        – In the database directory , Each table is marked with .frm Format file representation .
        – Table data and indexes are stored in memory .（ The goal is to , Quick query ！）
        – Table level locking mechanism .
        – Can not contain TEXT or BLOB Field .

    MEMORY The storage engine was formerly known as HEAP engine .

    MEMORY Engine benefits ： The query efficiency is the highest . No need to interact with the hard disk .
    MEMORY Engine faults ： unsafe , The data disappears after shutdown . Because the data and index are in memory .

about SQL The statement is , It's universal ,
             be-all SQL Statement to “;” ending .mysql It's gone “;” Don't execute ,“;” End of the said ！,
             in addition SQL Statement is case insensitive , Will do .

Be careful ： In all databases , Strings are uniformly enclosed in single quotes ,
             Single quotation marks are standard , Double quotation marks in oracle Not in the database . But in mysql
             Can be used in .

             Again ： All strings in the database are enclosed in single quotes . It's standard .
             Double quotation marks are not standard .

In the database null You can't use the equal sign to measure . Need to use is null
         Because in the database null Means nothing , It's not a value , So it can't be used
         The equal sign measures .

Other commands

Log in to the database

  • mysql -uroot -p

View currently used databases

  • select database();

Just look at the structure of the table , There's an order ：

  • desc Table name ;

Display current time

  • select now();

Exit database

  • exit/quit/ctr+d
  • Use any one

View all databases

  • show databases;

Create database

  • create database Database name charset= Coding format ;
  • The encoding format is generally utf8 that will do

Using a database

  • use Database name ;

Delete database

  • drop database Database name ;

View all tables in the current database

  • show tables;

View table structure

  • desc Table name ;

View creation table SQL sentence

  • show create table Table name ;

View creative Library SQL sentence

  • show create database Database name ;​​​​​​​​​​​​​​

View constraints 

  • select * from information_schema.TABLE_CONSTRAINTS where TABLE_NAME=" Table name ";
  • Indicates the constraint name in the specified table ,where It can be omitted , Indicates to view all constraint names
  • watermark,type_d3F5LXplbmhlaQ,shadow_50,text_Q1NETiBASEhZWkJD,size_20,color_FFFFFF,t_70,g_se,x_16
  • Red indicates the constraint name , Yellow indicates constraints  

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