MySQL advanced (Advanced) SQL statement (I)

MySQL senior ( Advanced )SQL sentence （ One ）

Case study ：

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mysql -uroot -p
show databases;

create database plane;
use plane;
create table destination (
region char(20),
place_name char(20));

insert into destination values('southwest','chengdu');
insert into destination values('north China','beijing');
insert into destination values('southwest','kunming');
insert into destination values('north China','tianjin');
select * from destination;

create table info (
place_name char(20),
sales int(10),
date char(10));

insert into info values('chengdu','350','2021-02-10');
insert into info values('beijing','294','2021-02-10');
insert into info values('kunming','330','2021-02-10');
insert into info values('beijing','392','2021-02-16');
select * from info;

     

      
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One 、MySQL High level statements

1、SELECT------ Display all data of one or more fields in the table

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

     

      
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2、DISTINCT------ Don't show duplicate data

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

     

      
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3、WHERE------ Conditional query

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

     

      
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4、AND OR------ And or

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 grammar ：SELECT " Field " FROM " Table name " WHERE " Conditions 1" {[AND|OR] " Conditions 2"}+ ;# Priority judgment in parentheses 
 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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5、IN------ Display information about known values

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 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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6、BETWEEN------ Displays data in two ranges

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 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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7、 wildcard

• Usually, wildcards are used with LIKE Used together

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% ： 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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8、LIKE------ Match a pattern to find out what we want

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

     

      
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9、ORDER BY------ Sort by keyword

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 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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10、 function

（1） Mathematical functions

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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 
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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（2） Aggregate functions

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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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（3） String function

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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 

     

      
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• 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

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

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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 '123' from '1234567');

     

      
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• concat(x,y) The parameters that will be provided x and y Concatenate into a string

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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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• 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

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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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• length(x) Return string x The length of

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select region,length(place_name) from destination;

     

      
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• replace(x,y,z) The string z Alternative string x String in y

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select replace(region,'China',' ') from destination;

     

      
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• 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

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select upper(region) from destination;
select lower(region) from destination;

     

      
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• left(x,y) Return string x Before y Characters
• right(x,y) Return string x After y Characters

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select left(region,5) from destination;
select right(region,5) from destination;

     

      
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• repeat(x,y) The string x repeat y Time

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select repeat(region,2) from destination;
select repeat(123,2);

     

      
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• 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

11、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 .

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 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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12、HAVING------ Used to filter by GROUP BY Statement returns a record table ( result )

• 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 .

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 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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13、 Alias ------ Field alias , Table alias

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 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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14、 Subquery ： Connect tables

• Son WHERE Clause or HAVING Clause to insert another SQL sentence

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 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
 example 1：
select sum(sales) from info where place_name in (select place_name from destination where region = 'southwest');
# The following sentence is the simplified version of the above 
select sum(sales) from info where place_name in('chengdu','kunming');

     

      
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 example 2：
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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15、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 .

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 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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Two 、 Link query

1、inner join( It's equivalent )

• Only rows with equal join fields in two tables are returned

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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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2、left join( Left join )

• Returns records that include all records in the left table and join fields in the right table

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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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3、right join( Right link )

• Returns records that include all records in the right table and join fields in the left table

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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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3、 ... and 、CREATE VIEW View

• The difference between a view and a table is , There is the actual stored information in the form , And a view is a structure built on a table , It doesn't actually store data itself .
• The temporary table disappears automatically when the user exits or loses the connection to the database , And the view doesn't disappear .
• Views don't contain data , Just store its definition , Its purpose is to simplify complex queries . For example, you need to query several tables , But also to carry out statistical sorting and other operations , Write SQL Sentences can be cumbersome , Join several tables with views , Then query the view , Just like querying a table , Very convenient .

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 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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summary ：

• View tables are not like temporary tables , Automatically disappear after disconnection
• The view table saves select Statement query results , It doesn't store data itself
• When the structure of the view chart is the same as that of the original table storing data, you can modify or insert data , If not, you cannot modify or insert data ( For example, the results of multi table join queries )