SQL learning notes (04) - data update and query operations

One 、 Data update operation

1. Add data （insert Insert statement ）

The inserted values and columns should maintain a one-to-one correspondence

--  Mode one ： Use multiple insert Insert statement 
-- insert into  Table name ( Name 1, Name 2, Name 3.....) value( value 1, value 2, value 3...);
insert into students (id,name) values (0,' Zhang Fei ');
insert into students (id,name) values (1,' zhaoyun ');

--  Mode two ： Pass one insert Insert multiple data 
-- insert into  Table name  value( value 1, value 2, value 3...);
insert into heros value(0,' Sun shangxiang ',' Woman ',20),(1,' Luna ',' Woman ',18);

--  Mode three ：insert into  Table name ( Name 1, Name 2, Name 3.....) value( value 1, value 2, value 3...),( value 1, value 2, value 3...);
insert into heros(id,name,sex,age) value(0,' Sun shangxiang ',' Woman ',20),(1,' Luna ',' Woman ',18);

2. Delete data （delete Delete statements ）

delete and drop The difference between ：delete Only delete the data in the table , The watch is still there ;drop Is to delete the entire table

2-1. Physical delete

--  Format 1 ： Delete the entire data in the table 
-- delete from  Table name ;
delet from students;

--  Format two ： Delete data according to conditions 
delete from  Table name  where  Conditions ;
and   The conditions are met at the same time 					delete from  Table name  where  Conditions 1 and  Conditions 2 and  Conditions 3....
or    Multiple conditions only need to meet one of them 		delete from  Table name  where  Conditions 1 or   Conditions 2 or   Conditions 3....

2-2. Logical deletion

It refers to setting a field to identify that the current record has been deleted is_delete Field to identify ,1 Said to delete ,0 Indicates not deleted

update students set is_delete=1 where id=8; 

2-3. Other ways to delete data

-- truncate table Table name （ Clear all the data in the table , But the table structure will be preserved , The value of the self increasing field will change from 1 Start .)
truncate tablestudens;

-- drop table Table name （ Delete data table , Including data and table structure )
drop table students;

3. Modifying data

--  Format 1 ：
update  Table name  set  Name = value ;

--  Format two ：
update  Table name  set  Name 1= value 1, Name 2= value 2...;

--  Format three ： Conditional updates 
update  Table name  set  Name = value  where  Conditions ;
update  The employee table  set  Wages =12000 where  full name = Xiao Zhang ";

 

Two 、 Data query operation

1. Query basic syntax structure

• Query the values of some fields

select  Field name 1, Field name 2 .... from  Table name 	( Query the information of a part of the field )

--  Look up the names in the student table 、 Gender 、 Age data 
select name,sex,age from students;

• Take the alias （as keyword ）

--  Aliasing tables 
-- select Alias . Field name 1, Alias . Field name 2 ..... from  Table name  as  Alias 
select s.name,s.sex,s.age from students as s;

-- Alias field 
-- select Field name 1 as  Alias 1, Field name 2 as  Alias 2 .... from Table name ;
select name as  full name , sex as  Gender ,age as  Age  from students;

• duplicate removal （distinct keyword ）

select distinct  Field name 1, Field name 2 .... from  Table name ;

-- According to the age of the students 
select distinct age from students;

2. Conditions of the query

2-1. Logical operators

• and（ At the same time meet the conditions ）,or（ One of the conditions ）,not（ Does not meet this condition ）

--  Grammatical structure 
select  Name  from  Table name  where  Conditions 1  Logical operators   Conditions 2  Logical operators   Conditions 3....;

--  Example 
-- 1. Check the weather in Shanghai or Beijing 
select * from weather where city=' Shanghai ' or city=' Beijing ';
-- 2. Check the weather except Shanghai 
select * from weather where not city=' Shanghai ';
select * from weather where city!=' Shanghai ';

2-2. Comparison operator

• Comparison operator ： Greater than (>)、 be equal to (=)、 Less than (<)、 Greater than or equal to (>=)、 Less than or equal to (<=)、 It's not equal to (<> perhaps !=)

-- 1. The query age is less than 18 Year old girl 
select * from students where age < 18;
-- 2. The query result is greater than or equal to 85 Students with scores above 
select * from students where grade >= 85;
-- 3. Query the names of all girls of gender 
select * from students where sex <> ' male ';
select * from students where sex != ' male ';

2-3. Range queries

• between …and…： The data that meets a certain range of query results is within a certain range

select  Name  from  Table name  where  Name  between  Starting value  and  End value ;

--  Query score in 85-90 Between the students 
select * from students where grade between 85 and 90;
select * from students where grade >= 85 and grade <= 90;

• not between … and…： Data that is not within a certain range

select  Name  from  Table name  where  Name  not between  Starting value  and  End value ;

--  The query score is not 85-90 Between the students 
select * from students where grade not between 85 and 90;

• in： Query data in a discontinuous range , As long as one of them is satisfied, there will be query results

--  Check the weather in Shenzhen, Guangzhou or Shanghai 
select * from weather where city in(' Shenzhen ',' Guangzhou ',' Shanghai ');
select * from weather where city = ' Shenzhen ' or city = ' Guangzhou ' or city = ' Shanghai ';

2-4. Fuzzy query

• Fuzzy query ：like keyword
• % It stands for Zero or more character
• _ It stands for A character

-- 1. Look up the student surnamed Wang 
select * from students where name like ' king %';

-- 2. Look up the student whose last name is Wang and whose first name is 
select * from students where name like ' king _';

-- 3. The query name contains ' rain ' Of the students 
select * from students where name like '% rain %';

-- 4. Query students whose names are two words 
select * from students where name like '__';

-- 5. The inquiry number is in 5 The students at the end 
select * from students where name like '%5';

2-5. Null value query

null and '' Empty characters are different

--  The judgment is empty ：select  Name  from  Table name  where  Name  is null;
--  Query the information that the student subject is empty 
select * from students where subject is null;

--  Judgment is not empty ：select  Name  from  Table name  where  Name  is not null;
--  Query the information that the student subject is not empty 
select * from students where subject is not null;

Sort （order by）

• The default is ascending （ From small to large ）
• asc Arrange... In ascending order （ From small to large ）
• desc Arrange... In descending order （ From big to small ）

--  Format ：select  Name  from  Table name  order by  Name 1 asc, Name 2 desc;

--  Example ∶ Check the records of all students in the grade sheet , It is required to be arranged in ascending order of scores , The same results , Arrange in descending order according to the student number 
select * from  League tables  order by  achievement  asc, Student number  desc;

Grouping and aggregation functions

Aggregate functions

• sum（ Name ）        Sum a column
• avg （ Name ）        Average a column
• max（ Name ）        Maximize a column
• min （ Name ）        Find the minimum value for a column
• count（*）         Count the number of tuples ( See how many records there are in a table )
• count（ Name ）           Count the number of values in a column

except count(*) Outside , When other functions operate again , Ignore null values (null)

--  Inquire about '1 class ' The sum of the students' scores 
select sum(grade) from students where class='1 class ';

--  Inquire about '1 class ' The student's average score 
select avg(grade) as  average  from students where class='1 class ';

--  Inquire about '1 class ' Students get the highest scores 
select max(grade) as  The highest  from students where class='1 class ';

--  Inquire about '1 class ' The minimum age of students 
select min(age) as  Minimum age  from students where class='1 class ';

--  Statistics name How many numbers are there 
select count(name) from students;
--  Count the number of tuples 
select count(*) from students;

Group query

• Group the query results

 Format ：select  Name , Aggregate functions  from  Table name  group by  Name ;

--  Check the number of people of different genders 
select sex,count(*) from students group by sex;
--  Check the number of students in each class 
select class,count(*) from students group by class;
--  Query the number of people of different genders in each class 
select class,sex,count(*) from students group by class,sex;

• Data filtering after grouping
• Treat the grouped data as a table data , And then through having Condition to filter the current table data .
• select Name from Table name group by Name having Conditions ( In general, aggregate functions are used as conditions );

Be careful :1.having It has to be with group by fixed collocation ;
   2.having You can write aggregate functions later , however where You can't write .

--  Check the total number of girls 
select count(*) from students where sex=' Woman ';
select sex,count(*) from students group by sex having sex=' Woman ';

--  Query the total number of girls in each class 
select class,sex,count(*) from students group by class,sex having sex=' Woman ';

--  Query that the number of records of different genders in all classes is greater than 1 Information about 
select class,sex,count(*) from students group by class,sex having count(*)>1;

Paging query

From the database 0 Start index

select  Name  from  Table name  limit [ Specify the row to start the query ,] Total rows queried ;

--  for example ： Query the student ID in the student form from 3 OK, let's start , Inquire about 7 That's ok 
select sno from students limit 2,7;