Addition, deletion, modification and query of MySQL table (Advanced)

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  Catalog

🥬 newly added

🥬 Aggregate query

🥬 Group query

🥬 Conditions of the query

🥬 The joint query

 🥬 Self join

🥬 Merge query

🥬 Summary

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🥬 newly added

insert into B select * from A;// take A The information of the table is added to B Go in the table 

🥬 Aggregate query

count;// The total data returned to the query 

sum;// The total data returned to the query （ Only meaningful for numbers ）

Only meaningful for numbers

avg/max/min;// Return the average value of query data / Maximum / minimum value ( Only meaningful for numbers ）

🥬 Group query

select * from  Table name  group by  Grouping conditions ;

  Here is to perform grouping first , Then execute the aggregation function of each group according to the Group .

🥬 Conditions of the query

having;

group by After the clauses are grouped , When the grouping results need to be conditionally filtered , You can use having.where Is performed before grouping , If you want to filter the results after grouping , You need to use having(having collocation group by Use ）.

for example ： Find the average salary for each role , Except Wu Jiu .（ Here is the use of where, Specify conditions before grouping , First remove Wu Jiu , Find the average salary in groups .

  Find the average salary for each role , Keep only the average salary 10000 Following , Here is to use having. You need to find out the average salary before you can filter .

🥬 The joint query

 The first way to write it ：select * from  Table name 1, Table name 2;
 The second way ：select * from  Table name 1 join  Table name 2 on  Conditions ;

The joint query （ More important ） It is a multi table query , The previous queries are all single table queries . Core operations in multi table query --- The cartesian product .

The operation of Cartesian product is to combine each record of two tables separately , Get a new set of records .

  The above records are not all the results we want , We can get the results we want through screening .

that join on Followed by conditions and use where What's the difference with the conditions ？

from Multiple tables where It's called “ Internal connection ".
Use join on Writing , It can represent inner connection , It can also represent external connection .
select Name from surface 1 inner join surface 2 on Conditions ;inner join Said is " Internal connection " among inner It can be omitted .
select Name from surface 1 left join surface 2 on Conditions ; The left outer join .
select Column from surface 1 right join surface 2 on Conditions ; Right connection .

 🥬 Self join

Self join refers to join itself in the same table for query .

for example ： Show all “ Chinese language and literature ” Score better than “ mathematics ” High score information .

First of all, we need to know the Chinese and mathematics courses course_id, Find these two courses first . Then compare them .

select s1.student_id,s1.score,s2.score from score as s1,score as s2 where s1.student_id=s2.student_id and s1.course_id=3 and s2.course_id=1 and s1.score>s2.score;

🥬 Merge query

union;// This can automatically remove the weight 
union all;// This cannot be removed automatically 

  This operator is used to get the union of two result sets .

for example ： Inquire about id Less than 3, Or the name is “ english ” Curriculum .

select * from course where id<3 union select * from course where name=' english '; 

Or use or To achieve

select * from course where id<3 or name=' english ';

🥬 Summary

That's what we have today , If you have any questions, you can leave a message in the comment area