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Mysql database - Advanced SQL statement (I)

2022-07-03 22:00:00 【Seal nine FJ】

MySQL High level statements

Prepare two tables first

MySQL database —— senior SQL sentence ( One )_ Table name

One 、SELECT—— Display all data of one or more fields in the table 

 grammar ：SELECT " Field " FROM " Table name ";
 example ：
select place_name from destination;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Field _02

Two 、DISTINCT—— Don't show duplicate data 

 grammar ：SELECT DISTINCT " Field " FROM " Table name ";
 example ：
select distinct place_name from info;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ character string _03

3、 ... and 、WHERE------ Conditional query 

 grammar ：SELECT " Field " FROM " Table name " WHERE " Conditions ";
 example :
select place_name from info where sales > 300;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ character string _04

Four 、AND|OR—— And | or 

 grammar ：SELECT " Field " FROM " Table name " WHERE " Conditions 1" {[AND|OR] " Conditions 2"}+ ;
 example ：
select place_name from info where sales > 350 or sales < 300;
select place_name from info where sales > 300 and sales < 350;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ character string _05

5、 ... and 、IN—— Display information about known values 

 grammar ：SELECT " Field " FROM " Table name " WHERE " Field " IN (' value 1', ' value 2', ...);
 example ：
select * from info where place_name in ('beijing');
select * from info where place_name in ('beijing','kunming');

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Table name _06

6、 ... and 、BETWEEN—— Displays data in two ranges 

 grammar ：SELECT " Field " FROM " Table name " WHERE " Field " BETWEEN ' value 1' AND ' value 2';
 example ：
select * from info where sales between '300' and '350';

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Field _07

7、 ... and 、 wildcard —— Usually, wildcards are used with LIKE Used together 

% ： A percent sign means zero 、 One or more characters 
_ ： The underline represents a single character 
 example ：
'A_Z'： All with  'A'  At first , Another character of any value , And in  'Z'  String ending with . for example ,'ABZ'  and  'A2Z'  They all fit this pattern , and  'AKKZ'  It does not accord with  ( Because in  A  and  Z  There are two characters between , Not a character ).
'ABC%':  All with  'ABC'  Starting string . for example ,'ABCD'  and  'ABCABC'  All fit the pattern .
'%XYZ':  All with  'XYZ'  a null-terminated string . for example ,'WXYZ'  and  'ZZXYZ'  All fit the pattern .
'%AN%':  All contain  'AN' The string of this pattern . for example ,'LOS ANGELES'  and  'SAN FRANCISCO'  All fit the pattern .
'_AN%'： All the second letters are  'A'  And the third letter is  'N'  String . for example ,'SAN FRANCISCO'  In line with this pattern , and  'LOS ANGELES'  It doesn't fit the pattern .

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Table name _08

MySQL database —— senior SQL sentence ( One )_ Field _09

8、 ... and 、LIKE—— Match a pattern to find out what we want 

 grammar ：SELECT " Field " FROM " Table name " WHERE " Field " LIKE { Pattern };
 example ：
select * from info where place_name like '_ei%';

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Table name _10

Nine 、ORDER BY—— Sort by keyword 

 grammar ：SELECT " Field " FROM " Table name " [WHERE " Conditions "] ORDER BY " Field " [ASC, DESC];
#ASC  It's sorted in ascending order , Is the default sort method .
#DESC  It's sorted in descending order .
 example ：
select place_name,sales,date from info order by sales;
select place_name,sales,date from info order by sales asc;
select place_name,sales,date from info order by sales desc;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Table name _11

Ten 、 function

1） Mathematical functions 

abs(x)	           return  x  The absolute value of 
rand()	           return  0  To  1  The random number 
mod(x,y)	       return  x  Divide  y  The remainder after 
power(x,y)         return  x  Of  y  Power 
round(x)	       Return from  x  The nearest integer 
round(x,y)         Retain  x  Of  y  The rounded value of a decimal place 
sqrt(x)	           return  x  The square root of 
truncate(x,y)	   Return to digital  x  Truncated to  y  A decimal value , Do not round off 
ceil(x)	           Returns greater than or equal to  x  Minimum integer of 
floor(x)	       Returns less than or equal to  x  Maximum integer for 
greatest(x1,x2…)   Returns the maximum value in the collection 
least(x1,x2…)	   Returns the smallest value in the set 

SELECT abs(-1), rand(), mod(5,3), power(2,3), round(1.89);
SELECT round(1.8937,3), truncate(1.235,2), ceil(5.2), floor(2.1), least(1.89,3,6.1,2.1);

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Field _12

2） Aggregate functions 

avg()	 Returns the average value of the specified column 
count()	 Returns the value of the specified column  NULL  The number of values 
min()	 Returns the minimum value of the specified column 
max()	 Returns the maximum value of the specified column 
sum(x)	 Returns the sum of all values in the specified column 

#count(*) Including the number of rows of all columns , When it comes to statistical results , The value of... Is not ignored NULL
#count( Name ) The number of rows in the column that includes only the column name , When it comes to statistical results , Column values of NULL The line of 

 example :
select avg(sales) from info;

select count(place_name) from info;
select count(distinct place_name) from info;

select max(sales) from info;
select min(sales) from info;

select sum(sales) from info;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Field _13

3） String function 

trim()	 Returns a value with the specified format removed 
concat(x,y)	 The parameters that will be provided  x  and  y  Concatenate into a string 
substr(x,y)	 Get from string  x  No  y  A string starting at a position , Follow substring() Functions work the same 
substr(x,y,z)	 Get from string  x  No  y  The starting length of a position is  z  String 
length(x)	 Return string  x  The length of 
replace(x,y,z)	 The string  z  Alternative string  x  String in  y
upper(x)	 The string  x  All of the letters of the alphabet become capital letters 
lower(x)	 The string  x  All of the letters of the are changed into lower case letters 
left(x,y)	 Return string  x  Before  y  Characters 
right(x,y)	 Return string  x  After  y  Characters 
repeat(x,y)	 The string  x  repeat  y  Time 
space(x)	 return  x  A space 
strcmp(x,y)	 Compare  x  and  y, The value returned can be -1,0,1
reverse(x)	 The string  x  reverse 

 Add ： Such as sql_mode Open... Open PIPES_AS_CONCAT,"||" Treat as the concatenation operator of a string, not as the or operator , And string splicing function Concat Similar , This sum Oracle The database is used in the same way 

     
    
      
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  • trim() Returns a value with the specified format removed 


select TRIM ([ [ Location ] [ The string to remove ] from ]  character string );
#[ Location ]： The value of can be  LEADING ( At first ), TRAILING ( ending ), BOTH ( The beginning and the end ). 
#[ The string to remove ]： From the beginning of the string 、 ending , Or a string removed from the beginning and end . The default is space .
 example ：
select trim(leading '1234' from '12345678');

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Table name _14

  • concat(x,y) The parameters that will be provided x and y Concatenate into a string 
select concat(region, place_name) from destination;

select region || ' ' || place_name from destination;
select region || ' ' || place_name from destination where place_name = 'beijing';

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Table name _15

  • substr(x,y) Get from string x No y A string starting at a position , Follow substring() Functions work the same
  • substr(x,y,z) Get from string x No y The starting length of a position is z String 
select substr(place_name,3) from destination where place_name = 'beijing';
select substr(place_name,2,4) from destination where place_name = 'beijing';

     
    
      
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MySQL database —— senior SQL sentence ( One )_ character string _16

  • length(x) Return string x The length of 
select region,length(place_name) from destination;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Field _17

  • replace(x,y,z) The string z Alternative string x String in y
select replace(region,'China',' ') from destination;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Table name _18

  • upper(x) The string x All of the letters of the alphabet become capital letters
  • lower(x) The string x All of the letters of the are changed into lower case letters 
select upper(region) from destination;
select lower(region) from destination;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ character string _19

  • left(x,y) Return string x Before y Characters
  • right(x,y) Return string x After y Characters 
select left(region,5) from destination;
select right(region,5) from destination;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Table name _20

  • repeat(x,y) The string x repeat y Time 
select repeat(region,2) from destination;
select repeat(123,3);

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Field _21

  • space(x) return x A space 
select space(20);

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Field _22

  • strcmp(x,y) Compare x and y, The value returned can be -1,0,1

MySQL database —— senior SQL sentence ( One )_ Table name _23

  • reverse(x) The string x reverse

MySQL database —— senior SQL sentence ( One )_ Table name _24

11、 ... and 、GROUP BY—— Yes GROUP BY Summarize and group the query results in the following fields

  • Usually used in combination with aggregate functions
  • GROUP BY There is a principle , Namely SELECT In all the following columns , Columns that do not use aggregate functions , Must appear in GROUP BY Back .
 grammar ：SELECT " Field 1", SUM(" Field 2") FROM " Table name " GROUP BY " Field 1";
 example ：
select place_name,sum(sales) from info group by place_name order by sum(sales);

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Field _25

Twelve 、HAVING—— Used to filter by GROUP BY Statement returns a record table ***

  • Usually with GROUP BY Statements are used in conjunction with
  • HAVING The existence of sentences makes up for WHERE Keywords cannot be combined with aggregate functions . If you are SELECT Only the function bar , Then there is no need for GROUP BY Clause .
 grammar ：SELECT " Field 1", SUM(" Field 2") FROM " Table name " GROUP BY " Field 1" HAVING ( Function conditions );
 example ：
select place_name,sum(sales) from info group by place_name having sum(sales) > 400;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ character string _26

13、 ... and 、 Alias —— Field alias , Table alias 

 grammar ：SELECT " Table alias "." Field 1" [AS] " Field alias " FROM " Table name " [AS] " Table alias ";
 example ：
select A.place_name "name",sum(A.sales) "sum_sales" from info A group by name;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ character string _27

fourteen 、 Subquery ： Connect tables

  • Son WHERE Clause or HAVING Clause to insert another SQL sentence 
 grammar ：
# External query 
SELECT " Field 1" FROM " form 1" WHERE " Field 2" [ Comparison operator ]
# Internal query 
(SELECT " Field 1" FROM " form 2" WHERE " Conditions ");
# Can be symbolic operators , for example ：=、>、<、>=、<= ; It can also be a literal operator , for example  LIKE、IN、BETWEEN

select sum(sales) from info where place_name in (select place_name from destination where region = 'north');
# The following sentence is the simplified version of the above 
select sum(sales) from info where place_name in('beijing','tianjin');

select sum(a.sales) from info a where a.place_name in(select place_name from destination b where b.place_name = a.place_name);

     
    
      
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MySQL database —— senior SQL sentence ( One )_ character string _28

MySQL database —— senior SQL sentence ( One )_ Field _29

15、 ... and 、EXISTS—— It is used to test whether the inner query produces any results

  • Whether Boolean value is true or not
  • If any , The system will execute the SQL sentence . If not , The whole SQL Statement will not produce any results 
 grammar ：SELECT " Field 1" FROM " form 1" WHERE EXISTS (SELECT * FROM " form 2" WHERE " Conditions ");
 example ：
select sum(sales) from info where exists (select * from destination where region = 'southwest');

     
    
      
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MySQL database —— senior SQL sentence ( One )_ character string _30

Link query

1. inner join( It's equivalent )

  • Only rows with equal join fields in two tables are returned 
SELECT * FROM  surface 1  surface 1 Another name for  INNER JOIN  surface 2  surface 2 Another name for  on  surface 1 Alias . Field  =  surface 2 Alias . Field ;  # The field names here are the same 
 example ：
select * from destination a inner join info b on a.place_name = b.place_name;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ Table name _31

2. left join( Left join )

  • Returns records that include all records in the left table and join fields in the right table 
SELECT * FROM  surface 1  surface 1 Another name for  LEFT JOIN  surface 2  surface 2 Another name for  on  surface 1 Alias . Field  =  surface 2 Alias . Field ;  # The field names here are the same 
 example ：
select * from destination a left join info b on a.place_name = b.place_name;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ character string _32

3. right join( Right link )

  • Returns records that include all records in the right table and join fields in the left table 
SELECT * FROM  surface 1  surface 1 Another name for  RIGHT JOIN  surface 2  surface 2 Another name for  on  surface 1 Alias . Field  =  surface 2 Alias . Field ;  # The field names here are the same 
 example ：
select * from destination a right join info b on a.place_name = b.place_name;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ character string _33

CREATE VIEW View

  1. It can be treated as a virtual table or stored query

  2. Views are different from tables , A view is an architecture based on a table , It doesn't actually store data itself

  3. Trying to be different from the temporary table , The user will not disappear after exiting or disconnecting from the database

  4. Views don't contain data , Only store definitions , It can simplify complex queries , If you query several tables , When you also need to perform operations such as statistical sorting , Write sql Sentences are cumbersome , We can simplify queries with a common view 

 grammar ：CREATE VIEW " View table name " AS "SELECT  sentence ";
 example ：
create view new_info as select a.region region,a.place_name place_name,b.sales sales from destination a inner join info b on a.place_name = b.place_name;

select * from new_info;
drop view new_info;

     
    
      
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MySQL database —— senior SQL sentence ( One )_ character string _34

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