Getting started database days3

• Reference books 《MySql From entry to mastery 》
• Learning video 【 Han Shunping said MySQL】 Zero basis a week to learn MySQL -sql mysql course mysql video mysql introduction _ Bili, Bili _bilibili
• Mengxin beginner level , If there is any mistake, please give advice , Not very grateful

 

Catalog

Statistical function -count

  Aggregate function Sum

Aggregate function AVG

Aggregate function ——MAX/MIN 

Grouping statistics

Use group by Clause to group Columns [ First create the test table ]

  String function

 ​ edit

Mathematical functions

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Statistical function -count

Count Returns the total number of rows

       Select   count(*) | count( Name ) from Table name  

                                [WHERE where_definition]

example ： Count how many students there are in a class

 

  The total score of statistics is greater than 260 How many students are there

count(*) and count( Column ) The difference between

• count(*) Return the number of rows of records that meet the conditions
• count( Column ): Count the number of columns that meet the conditions , But it will be excluded as null The situation of

example ：

CREATE TABLE t10(
	`name` VARCHAR(20));
INSERT INTO t10 VALUES('tom');
INSERT INTO t10 VALUES('jack');
INSERT INTO t10 VALUES('mary');
INSERT INTO t10 VALUES(NULL);
SELECT * FROM t10;

Create a table t10, Use them separately count(*) and count(`name`) Inquire about

 

  One is 4 One is 3

  Aggregate function Sum

 Select  sum( Name ) {,sum( Name )...}   from Table name     

                [WHERE where_definition]

example ： Count the total score of a class

  Count the total scores of each subject of mathematics, English and Chinese in a class

Count the average score of a class in Mathematics  

 Sum Value only , If it is sum(`name`) It doesn't make sense

Aggregate function AVG

AVG The function returns a value that satisfies where The average of a column of conditions

Select  avg( Name ) {,avg( Name )...}  from Table name

                [WHERE where_definition]

example : Find the average math score of a class

 

Aggregate function ——MAX/MIN 

Select  max( Name ) {,avg( Name )...}  from Table name

                [WHERE where_definition]

example ： Ask for the highest and lowest scores of students in the class

Grouping statistics

Use group by Clause to group Columns [ First create the test table ]

SELECT   column1,column2 . column3... FROM table 

                        group  by   column  ( Filter )having ...

Continue to use our monster table

  Press image To query the average salary and maximum salary of monsters in groups

Filter （ Use having）, Only show that the average salary is less than 4000 Of

 

Show each image Average salary and maximum salary of each position in

  String function

 

example ： Returns the substring character set

  The substring character set is the default utf8

example ： Connection substring , Splice multiple columns into one column

  example :INSTR(string,substring)    return substring stay string Where in , No return 0( You can use the system when you don't have a watch DUAL surface )

 

  example ：UCASE(string)( Convert to uppercase ）

 

  Empathy LCASE(string) Convert to lowercase

  example :LEFT(string,length)    from string To the left of length Characters

  example :LENGHT(string) string length ( Press byte ）

 

  Here are monsters and monsters 001 The length of each of them is 9, Because a Chinese character here accounts for 3 Bytes (utf8)

  example ：replace(str,s1,s2) stay str of use s2 Replace s1

Here I will monster 003 Replace with monsters 004

  example :STRCMP(string1,string2) , Compare the size of two strings character by character

 

 

  example :SUBSTRING(str,a,b) from a Began to intercept str Of b Characters

  example ： Remove front-end or back-end spaces    LTRIM(string） RTRIM(string)   TRIM(string)

3 The result of both cases is the same , In the end " The database between my fingers was destroyed "

  practice : Display all employees in lowercase emp The name of the table

SELECT CONCAT(LACSE(SUBSTRING(ename,1,1),SUBSTRING(ename,2))   from     emp;

Mathematical functions

example :ABS(num） The absolute value

 

  example :BIN(num)  10 Turn into the system 2 Base number

  example :CEILING(num) Return ratio num The smallest big integer

  example :CONV(num1,n1,n2) take num1 from n1 Base to n2 Base number

  example :FLOOR(n) Get ratio n The smallest and largest integer

 

example :format(n1,n2)    Keep the decimal places ( rounding )

example :LEAST(n1,n2,n3...) For the minimum

  example :MOD(n1,n2)  Seeking remainder

  example ：RAND(N) Return random number , The scope is [0,1.0]  , If you want random numbers to remain constant , Then write the middle n

The above is today's video notes , In case of infringement, please contact me to delete .

                        Learning is like sailing against the current , move forward , or you 'll fall behind . Come on with Xiao Wu ！