Built in functions for MySQL database operations

Preface ：MySQL Built in functions can be select、where、order by Use... In the back

One 、 Data preparation

--  establish student library ：student
create database student default character set utf8 collate utf8_general_ci;
/*  Create student table ：students,  Student number is Sid, The data type is varchar(10), Primary key ,  The name is name, The data type is varchar(10),  The gender is sex, The data type is varchar(1),  Age is age, The data type is int,  Class for class, The data type is varchar(10),  The ID card number is card, The data type is varchar(20),  The city is for city, The data type is varchar(20) */
create table students(
	Sid varchar(10) primary key,
	name varchar(10),
	sex varchar(1),
	age int,
	class varchar(10),
	card varchar(20),
	city varchar(20)
);
--  Insert data into the student table 
insert into students values
('001',' Wang Zhaojun ',' Woman ',30,'3 class ','634101199003157654',' Beijing '),
('002',' Zhugeliang ',' male ',29,'2 class ','110102199104262354',' Shanghai '),
('003',' Ruban master ',' male ',30,'1 class ','820102199003047654',' nanjing '),
('004',' White ',' male ',35,'4 class ','840202198505177654',' anhui '),
('005',' Big Joe ',' Woman ',28,'3 class ','215301199204067654',' tianjin '),
('006',' Sun shangxiang ',' Woman ',25,'1 class ','130502199506137654',' hebei '),
('007',' Hundred Li xuance ',' male ',39,'2 class ','140102198107277654',' shanxi '),
('008',' Little Joe ',' Woman ',25,'3 class ',null,' Henan '),
('009',' Hundred Li keep the promise ',' male ',31,'1 class ','',' hunan '),
('010',' Daji ',' Woman ',24,'2 class ','440701199607147654',' guangdong '),
('011',' Lian po ',' male ',30,'1 class ','110202199005017754',' Beijing '),
('012',' Sun Bin ',' male ',36,'3 class ','650102198401297655',' xinjiang ');
/*  Create a curriculum ：courses  The course number is Cid, The data type is varchar(10), Primary key ,  The course is called Cname, The data type is varchar(10) */
create table courses(
	Cid varchar(10) primary key,
	Cname varchar(10)
);
--  Insert data into the curriculum 
insert into courses values
('01',' Chinese language and literature '),
('02',' mathematics '),
('03',' English '),
('04',' biological '),
('05',' Politics '),
('06',' Geography '),
('07',' Physics ');
/*  Create a score table ：scores,  Student number is Sid, The data type is varchar(10), Student table foreign keys ,  The course number is Cid, The data type is varchar(10), Curriculum foreign key ,  The score is score, The data type is tinyint */
create table scores(
	Sid varchar(10),
	Cid varchar(10),
	score tinyint,
	foreign key (Sid) references students(Sid),
	foreign key (Cid) references courses(Cid)
);
--  Insert data into the score table 
insert into scores values
('001','01',49),
('001','02',75),
('001','03',54),
('001','04',66),
('001','05',59),
('001','06',94),
('002','01',58),
('002','02',57),
('002','03',74),
('002','04',90),
('002','05',55),
('002','06',64),
('003','01',83),
('003','02',73),
('003','04',50),
('003','05',67),
('003','06',87),
('004','01',97),
('004','02',50),
('004','03',56),
('004','04',69),
('004','05',72),
('004','06',51),
('005','01',54),
('005','02',66),
('005','03',61),
('005','05',49),
('005','06',86),
('006','01',97),
('006','02',77),
('006','03',66),
('006','04',76),
('007','01',49),
('007','02',40),
('007','03',99),
('007','04',86),
('007','05',78),
('007','06',55),
('008','01',82),
('008','02',72),
('008','03',98),
('008','05',47),
('008','06',68),
('009','01',84),
('009','02',56),
('009','03',74),
('009','04',64),
('009','05',79),
('009','06',97),
('010','02',85),
('010','03',85),
('010','04',90),
('010','05',93),
('010','06',61),
('011','01',85),
('011','02',63),
('011','03',71),
('011','04',58),
('011','05',80),
('011','06',66);

Two 、 String function

1. Splice string function -concat

grammar ：concat( Parameters 1, Parameters 2, Parameters 3....)
effect ： Concatenate all the parameters into a complete string
Be careful ： Parameters can be numbers , It could be a string

practice 1：

--  hold 12,34.5,‘abc1 in ’ Concatenate into a string 
select concat(12,34.5,'abc1 in ');

practice 2：

--  hold students In the table name and age The field data is spliced into a string and displayed 
select concat(name,age) from students;

2. Calculate the length of the string -length

grammar ：length( Parameters )
effect ： Calculate the length of the string
Be careful ： One utf8 The length of Chinese characters in the format is 3

practice 1：

--  Calculation ‘123abc "Avatar" ’ Length of string 
select length('123abc "Avatar" ');

practice 2：

--  Calculation students In the table name Length of field 
select length(name) from students;

3. Intercepting string

Be careful ： Here the length of Chinese characters and letters is 1; The intercept length can only be a positive integer
Left hand intercept
grammar ：left(' character string ', Interception length )
effect ： Intercepts the left part of a string of a specified length

practice 1：

--  Intercept ‘123abc "Avatar" ’ The left part of the string ‘123’
select left('123abc "Avatar" ',3);

practice 2：

--  Intercept students In the table name The length of the field is 9 The left part of the last name 
select left(name,1) from students where length(name)=9;

Right intercept
grammar ：right(' character string ', Interception length )
effect ： Intercepts the right part of a string of specified length

practice 1：

--  Intercept ‘123abc "Avatar" ’ The right part of the string ‘ "Avatar" ’
select right('123abc "Avatar" ',3);

practice 2：

--  Intercept students In the table name The length of the field is 12 The name of the right part of 
select right(name,2) from students where length(name)=12;

Intercept in the middle
grammar ：substring(' character string ', Starting position , Interception length )
effect ： Intercepts any part of a string of a specified length
Be careful ： The start position can be a positive integer 、0、 Negtive integer ; The start position is contained in the intercepted

practice 1：

--  Intercept ‘123abc "Avatar" ’ A string of ‘abc’ part 
select substring('123abc "Avatar" ',4,3);

practice 2：

--  Intercept students Tabular card Date of birth of the field 
select substring(card,7,8) from students;

practice 3：

--  Inquire about students Student information in the table , And according to the birthday from big to small ( Tips 1989 Annual ratio 1990 Big year )
select * from students order by substring(card,7,8);

4. Remove the space

Remove the space on the left
grammar ：ltrim(' character string ')
effect ： Remove the space to the left of the string

practice 1：

--  Remove strings ‘ 123abc "Avatar"  ’ The space on the left 
select ltrim(' 123abc "Avatar"  ');

Remove the space on the right
grammar ：rtrim(' character string ')
effect ： Remove the space to the right of the string

practice 1：

--  Remove strings ‘ 123abc "Avatar"  ’ The space on the right 
select rtrim(' 123abc "Avatar"  ');

Remove the space on both sides
grammar ：trim(' character string ')
effect ： Remove the space around the string

practice 1：

--  Remove strings ‘ 123abc "Avatar"  ’ Space on both sides 
select trim(' 123abc "Avatar"  ');

3、 ... and 、 Mathematical functions

1. rounding -round

grammar ：round( Number to process , Keep a few decimal places )
effect ： rounding , Keep several decimal places after the decimal point

practice 1：

-- 8.3579 rounding , Keep integer 
select round('8.3579',0);

practice 2：

--  Inquire about students Table average age of middle school students , And keep 2 Decimal place 
select round(avg(age),2) from students;

2. random number -rand

grammar ：rand()
effect ： Return a return from 0 To 1.0 A random fraction of

practice 1：

--  Return a return from 0 To 1.0 A random fraction of 
select rand();

practice 2：

--  from students A student is randomly selected from the table （ Random sorting takes the first ）
select * from students order by rand() limit 1;

Four 、 Date time function

1. The current date

grammar ：current_date()
effect ： View the current date （ Format ：yyyy-mm-dd）

practice 1：

--  View the current date 
select current_date();

2. current time

grammar ：current_time()
effect ： View current time （ Format ：HH-mm-ss）

practice 1：

--  View current time 
select current_time();

3. The current date + Time

grammar ：now()
effect ： View the current date + Time （ Format ：yyyy-mm-dd HH-mm-ss）

practice 1：

--  View the current date + Time 
select now();