SQL (basic 2)

DQL Basic query

● DQL（Data Query Language） Data query language query is one of the most frequently used operations , You can query data from a table , You can also query data from multiple tables .

● Basic query syntax ： select  Query list   from   Table name ;

● characteristic ：

• The query list can be ： Fields in the table 、 Constant 、 expression 、 function
• The result of the query is a virtual table

Query result processing ：

Specific column query :select column1,column2 from table

All columns query : select * from table

Arithmetic operator :+ - * /

Exclude duplicate lines : select distinct column1,column2 from table

Query function ：select function ; / for example version()

Character functions

length()： Gets the number of bytes of the parameter value

char_length() Get the number of characters of the parameter value

concat(str1,str2,.....)： String concatenation

upper()/lower()： Capitalize a string / A lowercase letter

substring(str,pos,length)： Intercepting string Location slave 1 Start

instr(str, Specify the characters )： Returns the index of the first occurrence of the substring , If we can't find a way back 0

trim(str)： Remove spaces or substrings before and after the string ,trim( Specify substring from character string )

lpad(str,length, Fill character )： The left padding with the specified character will str Fill to the specified length

rpad(str,length, Fill character )： Right padding with the specified character will str Fill to the specified length

replace(str,old,new)： Replace , Replace all substrings

INSERT INTO basketmember( name , Birthday , height , weight , Location )
VALUES(' Dwayne . wade ','1982-1-17',1.93,96,'null'),
      (' lebron . James ','1984-12-30',2.03,113,' striker '),
      (' Kobe . Bryant ','1978-8-23',1.92,99,' defender '),
      (' Deco . Dirk Nowitzki ','1978-6-19',2.13,111,'null'),
      (' Iris . Paul ','1985-5-6',1.82,79,' defender '),
      (' Tony . Parke ','1982-5-17',1.87,83,' defender '),
      (' Kevin . Garnett ','1981-7-14',2.12,113,'null'),
      (' Paul . Pierce ','1977-10-13',2.00,106,' striker '),
      (' Michael . Jordan ','1963-2-17',1.98,98,' striker '),
      (' Dwight - Howard ','1985-12-18',2.10,120,' Center-forward '),
      (' Yao Ming ','1980-9-12',2.29,140,' Center-forward '),
      (' Shaquille . O'Neill ','1972-3-6',2.15,147,' Center-forward ') 

  This example is the case we used today !!!

 SELECT  Number  , name ,  Location  FROM basketmember WHERE  Number =2   # Select a fixed column in the table 

 SELECT*FROM basketmember ORDER BY( weight )# Sort the selected columns 

SELECT*FROM basketmember LIMIT 0,2# Select the selected column （ initial , end ） That's ok 

SELECT DISTINCT  name , Birthday  FROM basketmember# Remove duplicate data , Duplicate data means that each column has the same value 

 SELECT  Number , height + weight  FROM basketmember# Add some columns 

SELECT LENGTH( name )FROM basketmember# Output in bytes 

 

SELECT CHAR_LENGTH( name )FROM basketmember# Output in string 

SELECT UPPER( name )FROM basketmember# Turn capitalization 

  SELECT LOWER( name )FROM basketmember# Turn lowercase 

 SELECT INSTR( name ,'i')FROM basketmember# Returns the position where the specified character first appears in the character 

SELECT SUBSTRING( name ,1,2)FROM basketmember# Intercept the string content from the beginning to the end 

 SELECT TRIM( name )FROM basketmember# Remove the space before and after 

SELECT TRIM('i'FROM  name )FROM basketmember# Remove the string before and after the column name 

SELECT LPAD( name ,8,'a')FROM basketmember# Add characters to the front until they meet the given character length 

 

SELECT RPAD( name ,8,'b')FROM basketmember# Add characters to the end of a given character length 

 SELECT REPLACE( name ,' Yao Ming ',' Liu Ziwen ')FROM basketmember# Replace （ used , new ）

  Logical processing ：

case when Conditions then result 1 else result 2 end; There can be multiple when

ifnull( Detected value , The default value is ) The function detects whether it is null, If null, Then return the specified value , Otherwise return to

The value of the original

if function ：  if else Of effect if( Conditions , result 1, result 2)

 SELECT  name , Birthday ,
      (CASE WHEN  height >=2.00 THEN' Giant '
      WHEN  height <=1.99 AND  height >=1.8 THEN' secondary ' ELSE' Not medium ' END
      )AS height 
      FROM basketmember#when sentence 

SELECT  Number , name ,IFNULL( height ,' Not entered yet ')FROM basketmember# To determine if there is null, Fill in default 

SELECT  Number , name ,IF( height >=2.00,' tall ',' Low man ')AS height FROM basketmember#if Judgment statement 

Mathematical functions

• round( The number )： rounding
• ceil( The number )： Rounding up , return >= The smallest integer of this parameter
• floor( The number )： Rounding down , return <= The maximum integer of the parameter
• truncate( The number , Keep the number of decimal places )： truncation , Truncate to several digits after the decimal point
• mod( Divisor , Divisor )： Remainder , The divisor is positive , It's positive ; The divisor is negative , It's negative
• rand()： Get random numbers , return 0-1 Decimal between

Date function

• now()： Returns the current system date + Time
• curdate()： Returns the current system date , Not including time
• curtime()： Return current time , No date included You can get the specified part , year 、 month 、 Japan 、 Hours 、 minute 、 second
• YEAR( Date column ),MONTH( date Column ),DAY( date Column ) , HOUR( date Column ) ,MINUTE( date Column )SECOND( date Column )
• str_to_date： Convert the character of date format to the date of specified format
• date_format： Convert date to string
• datediff(big,small)： Returns the number of days between two dates

 

Group function

function ： For statistical use , Also known as aggregate function or statistical function or group function

classification ：sum Sum up 、avg Average 、max Maximum 、min minimum value 、count Count （ Non empty ）

1.sum,avg Generally used to deal with numerical type max,min,count Can handle any type of

2. All the above grouping functions are ignored null value

3.count General use of functions count（*） Used to count lines

4. The field requirement to query with grouping function is group by Fields after

SELECT SUM( weight )FROM basketmember# The sum of the 

SELECT AVG( weight )FROM basketmember# Average 

 

SELECT MAX( weight )FROM basketmember# Maximum 

 

 SELECT MIN( weight )FROM basketmember# minimum value 

 

Conditions of the query

Use WHERE Clause , Filter out rows that do not meet the criteria ,WHERE Clause in the wake of FROM Clause .

grammar :select < result > from < Table name > where < Conditions >

Compare          =, != or <>, >, <, >=, <=

Logical operations

• and And
• or or
• not Not

SELECT *FROM basketmember WHERE  height  BETWEEN 1.80 AND 2.00# In a   Within the interval , Include boundary values 

Conditions of the query  

Fuzzy query

LIKE ： Whether it matches a pattern Generally used with wildcards , You can judge character type or numeric type .

wildcard ： % Any number of characters , contain 0 Characters _ Any single character

between and Between the two , Contains a critical value ;

in Determine whether the value of a field belongs to in An item in the list

IS NULL（ Empty ） or IS NOT NULL（ Not empty ）

 SELECT *FROM basketmember WHERE  name  LIKE(' a surname %')# Keyword matches any character