MySQL details - aggregation and grouping

1. Aggregate functions

Aggregate functions	function
count()	Count the number of rows of records that meet the conditions
max()	Find the maximum value of records that meet the conditions
min()	Find the minimum value of records that meet the conditions
sum()	Count the sum of the values of the records that meet the conditions
avg()	Count the average of the values of the records that meet the conditions

Be careful :

• count Use of functions :
  count(*)、count(id)、count(1).
  count(*) It can count the total number of records , and count(id) It's statistics id The number of records with non null value of column , That is, their values may be different . Extended reading : MySQL:count(*),count( Field ),count(1) The difference between

• In the query statement of the aggregate function , You cannot query fields other than aggregate functions and grouping fields , Because it doesn't make sense ( Generally, the database will report an error ).

  select max(age) from stu√
  select max(age),name from stu×

• Aggregate functions generally cooperate with group by Use it together

• Aggregate function is matching group by when , Group fields can be queried additionally .

  select max(age), gender from stu group by gender√

 

 

2. Group query

  Grouping query is to make grouping statistics on the original data .

select  Group column name 1,  Group list 2,  Aggregate functions 1,  Aggregate functions 2 from  Table name 
group by  Group column name 1,  Group column name 2;

From the format we can see , Groups can be grouped according to multiple fields .

 

• The condition screening after grouping

select  Group column name ,  Aggregate functions  from  Table name 
[where  Conditions 1]
group by  Group column name 
[having  Conditions 2( You can judge by the value of the aggregate function )];

We can use where Filter by conditions , It can also be used. having Filter by conditions . however The location of the two and the logic of the code execution are different !

Be careful :

• select The fields that appear in , Must be in a group .
• having The conditional judgment fields appearing in the clause , It must also be in the group .

Example :

-- 1
select gender, max(age) from student
where gender=' male '
group by gender;

-- 2
select gender, max(age) from student
group by gender
having gender=' male ';

For example 1,where It's right from and select The structure of , After this layer of screening , Re execution group by Grouping .

For example 2,having It's right group by Screening , stay group by After execution , And then screen .

SQL The execution order of each keyword in the query :

from > on > join > where > group by > with > having > select > distinct > order by > limit