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MySQL advanced SQL statement (1)

2022-07-04 01:51:00 【kiroct】

select High level statements

Case study ： Start with a database

use kgc;
create table location (Region char (20) ,Store_Name char(20));
insert into location values('East','Boston');
insert into location values('East','New York');
insert into location values('West','Los Angeles');
insert into location values('West','Houston');



location form 
create table store_info (Store_Name char(20) ,Sales int (10),Date char(10));
insert into store_info values('Los Angeles','1500', '2020-12-05');
insert into store_info values ('Houston','250','2020-12-07');
insert into store_info values('Los Angeles','300', '2020-12-08');
insert into store_info values ('Boston','700','2020-12-08');




SELECT—— Displays all data records for one or more fields in the table 

 grammar : SELECT " Field " FROM " Table name ";

SELECT Store_Name FROM Store_Info;

     
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mysql senior SQL sentence （1）_select Advanced statement of

mysql senior SQL sentence （1）_select Advanced statement of _02

mysql senior SQL sentence （1）_select Advanced statement of _03

DISTINCT : Do not display duplicate data records

 grammar :SELECT DISTINCT " Field " FROM " Table name ";

SELECT DISTINCT Store_Name FROM Store_Info;

WHERE—— Conditional query 

 grammar : SELECT " Field " FROM " Table name " WHERE " Conditions ";
SELECT Store_Name FROM Store_Info WHERE Sales > 1000;

AND OR  :  And or 
 grammar : SELECT " Field " FROM " Table name " WHERE " Conditions 1" { [ANDIOR] " Conditions 2"}+ ;
SELECT Store_Name FROM Store Info WHERE Sales > 1000 OR (Sales < 500 AND Sales > 200);


IN—— Data records showing known values 
 grammar : SELECT " Field " FROM " Table name " WHERE " Field " IN (' value 1', ' value 2', ...);
SELECT * FROM Store_Info WHERE Store_Name IN ('Los Angeles','Houston');



BETWEEN—— Display data records in the range of two values 
 grammar : SELECT " Field " FROM " Table name " WHERE " Field " BETWEEN ' value 1' AND ' value 2';
SELECT * FROM Store_Info WHERE Date BETWEEN '2020-12-06' AND '2020-12-10';

 wildcard —— Usually, wildcards are used with LIKE Used together 
% : A percent sign means zero 、 One or more characters 
_ : The underline represents a single character 
'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 .

LIKE—— Match a pattern to find the data record we want 
 grammar : SELECT " Field "FROM " Table name "WHERE" Field " LIKE { Pattern };
SELECT * FROM Store_Info WHERE Store_Name like '%os%';


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 .
SELECT Store_Name, Sales,Date FROM Store_Info ORDER BY Sales DESC;

     
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mysql senior SQL sentence （1）_select Advanced statement of _04

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 
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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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 

SELECT avg (Sales) FROM Store_Info;
SELECT count (Store_Name) FROM Store_Info;
SELECT count (DISTINCT Store_Name) FROM Store_Info;

SELECT max (Sales) FROM Store_Info;
SELECT min (Sales) FROM Store_Info;
SELECT sum(Sales) FROM Store_Info;


SELECT count (name) FROM City;
SELECT count(*) FROM City;
#count(*) Including the number of rows of all columns , When it comes to statistical results , Column values of... Are 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 


     
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mysql senior SQL sentence （1）_select Advanced statement of _08

mysql senior SQL sentence （1）_select Advanced statement of _09

mysql senior SQL sentence （1）_select Advanced statement of _10

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 

SELECT concat (Region,Store_Name) FROM location WHERE Store_Name = 'Boston';


# Such as sql_mode Open the PIPES_AS_ CONCAT,"||" Treat as the concatenation operator of a string, not as the or operator , And string splicing function Concat Similar , This library is used in the same way 
SELECT Region || ' ' || Store_Name FROM location WHERE Store_Name = 'Boston';

SELECT substr(Store_Name,3) FROM location WHERE Store_ Name = 'Los Angeles';
SELECT substr(Store Name,2,4) FROM location WHERE Store Name = 'New York';

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 .
SELECT TRIM (LEADING 'Ne' FROM 'New York');
SELECT Region,length(Store_Name) FROM location;
SELECT REPLACE(Region,'ast','astern') FROM location;


     
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mysql senior SQL sentence （1）_select Advanced statement of _11

Group by Use ！

Yes group by The query results of the following fields are summarized and grouped , 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 .

HAVING: Used to filter by GROUP BY Statement returned recordset , 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 ) ;
SELECT Store_Name, SUM(Sales) FROM Store_Info GROUP BY Store_Name HAVING SUM(Sales) > 1500;

 Alias : Field alias table alias 
 grammar : SELECT " Form alias ". " Field 1" [AS] " Field alias " FROM " Table name " [AS] " Form alias ";
SELECT A.Store Name Store, SUM(A.Sales) "Total Sales" FROM Store_Info A GROUP BY A.Store_Name;

 Subquery : Connect tables , stay WHERE Clause or HAVING Clause to insert another SQL sentence 
 grammar : SELECT " Field 1" FROM " form 1" WHERE " Field 2”[ Comparison operator ]
(SELECT " Field 1" FROM " form 2"WHERE" Conditions ");
 
 # Can be symbolic operators , for example =、>、<、>=、<= ; It can also be a literal operator , for example LIKE、 Seek group  IN、BETWEENSELECT SUM(Sales) FROM Store Info WHERE Store_Name IN (SELECT Store_Name FROM location WHERE Region = 'West') ;
 
SELECT SUM(A.Sales) FROM Store_Info A WHERE A.Store_ Name IN (SELECT Store_Name FROM location B WHERE B.Store_ Name = A.Store_Name);

 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 ");
SELECT SUM(Sales) FROM Store_Info WHERE EXISTS (SELECT * FROM location WHERE Region = 'West');


---- Link query ----
location form 
 Region        Store_Name
 East          New York
 East          Boston
 West          Los Angeles
 West          Houston
................................

UPDATE Store_Info SET store_name= 'Washington' WHERE sales=300;


Store_Info  form 
Store_Name       Sales   Date
..................................
Los Angeles      1500  2020-12-05
Houston          250   2020-12-07 
Washington       300   2020-12-08
Boston           700   2020-12-08
.................................

inner join ( Internal connection ): Only rows with equal join fields in two tables are returned 
left join( Left connection ): Returns records that include all records in the left table and join fields in the right table 
right join( The right connection ): Returns records that include all records in the right table and join fields in the left table 
SELECT * FROM location A INNER JOIN Store_Info B on A.Store_Name = B.Store_Name;
SELECT * FROM location A RIGHT JOIN Store_Info B on A.Store Name = B.Store_Name;
SELECT * FROM location A,Store_Info B WHERE A.Store_Name = B.Store_Name;
SELECT A.Region REGION,SUM(B.Sales) SALES FROM location A,Store_Info B
WHERE A.Store_Name = B.Store_Name GROUP BY REGION;


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mysql senior SQL sentence （1）_select Advanced statement of _14

mysql senior SQL sentence （1）_select Advanced statement of _15

create view : View , It can be thought of as a virtual table or a storage query .

The difference between a view and a table is , There are actually stored data records in the form , The view is a framework based on tables , It itself does not actually store data records . 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 - It can generally simplify complex queries . For example, you need to query several tables , But also to carry out statistical sorting and other operations SQL Sentences can be cumbersome , Join several tables with views , Then query the view , It's the same as querying a table , Very convenient .

 grammar : CREATE VIEW " View table name " AS "SELECT  sentence ";
CREATE VIEW V_REGION_SALES AS SELECT A.Region REGION, SUM(B.Sales) SALES FROM location A INNER JOIN Store_Info B ON A.Store_Name = B.Store_Name GROUP BY REGION;

SELECT * FROM V_REGION_SALES;
DROP VIEW V_REGION_SALES;

     
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mysql senior SQL sentence （1）_select Advanced statement of _16

A small summary ：

truncate (3.1415926,4)
substr ('abc123',2)

order by
group by
having

inner join
left join
right join

create view V_xx as select.......
 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 )

     
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