[SQL] MySQL query statement execution sequence analysis

-- Example : select distinct s.id  from T t join  S s on t.id=s.id where t.name="Yrion" group by t.mobile having count(*)>2 order by s.create_timelimit 5;

1：from

The first step is to choose from The table following the key words , This is also sql The first step of execution : Indicates which table to execute from the database .
Example is given to illustrate : In this case, first find the table from the database T

2:join on

join Is the table to be associated ,on It's the connection condition . adopt from and join on Select the database table to execute T and S, Produce Cartesian product , Generate T and S The merged interim table Temp1.on: Determine the binding relationship of the table , adopt on Create a temporary middle table Temp2.
Example is given to illustrate ： Find the watch S, Generate temporary middle table Temp1, Then find the watch T Of id and S Of id The same parts make up the table Temp2,Temp2 It contains T and S id All equal data

3:where

where Means screening , according to where The following conditions are filtered , According to the value of the specified field ( If there is and The connector will be federated ) From temporary middle table Temp2 To filter the required data .
If no data is found at this stage , Will return to the client directly , Not going down . Otherwise, this process will generate a temporary intermediate table Temp3.
Be careful ： stay where Aggregate functions and column aliases cannot be used in , Aggregate functions mainly include (min\max\count\sum Such as function )
Example is given to illustrate : stay temp2 Found... In temporary table set T Tabular name="Yrion" The data of , After finding the data, it will be a temporary middle table Temp3,temp3 It contains name As a "Yrion" All table data for
4:group by
group by It's grouping , Yes where Condition filtered temporary table Temp3 Group by fixed fields , Create a temporary middle table Temp4, This process is just a change in the order of the data , And the amount of data doesn't change , The data in the table exists as a group .
Example is given to illustrate : stay temp3 In the table data mobile Grouping , Find out mobile The same data , And put it together , produce temp4 A temporary table .
grammar ：

group by  Name 1, Name 2.... Name n

Be careful group by Column aliases can be used after , No function can be used after that

5:Having

For the temporary middle watch Temp4 Aggregate , This can be count Wait for the count , And then the middle table Temp5, You can use... At this stage select Alias in
Example is given to illustrate : stay temp4 The number of items found in the temporary table is greater than 2 The data of , If it is less than 2 To be abandoned directly , Then generate a temporary middle table temp5
Be careful ：having You can use the function after , as well as select Columns and column aliases , No one reported wrong.

6:select

Select the data to be queried from the grouped and aggregated tables , If * It will be resolved to all data , The middle table will be generated Temp6
Example is given to illustrate ： At this stage is right temp5 In the temporary aggregation table S In the table id To produce Temp6, here temp6 It only includes s Tabular id Column data , also name=“Yrion”, adopt mobile The number of groups is greater than 2 The data of
7:Distinct
distinct De duplicate all data , If there is min、max Function will perform field function calculation and then perform de duplication , And then a temporary watch Temp7
Example is given to illustrate : This stage is right for temp5 The data in is de duplicated , engine API Will call the de duplication function to filter the data , In the end, just keep id The first piece of data , And then a temporary middle table temp7
Be careful ：Distinct * and Distinct Name 1 The two are different

8:order by

Will be based on Temp7 To arrange in order or reverse order , Then insert the temporary middle table Temp8, This process is more resource consuming
Example is given to illustrate ： This paragraph will bring all temp7 The data in the temporary table is based on the creation time (create_time) Sort , There will be no column or row loss in this process
Be careful ：order by Then you can use the function , Query all fields or aliases in the table

9:limit

limit On the middle watch Temp8 paging , Create a temporary middle table Temp9, Return to the client .
Example is given to illustrate : stay temp7 Ordered data in , Then insert the first five into Temp9 In this temporary table , Finally back to the client
ps: In fact, this process is not absolutely so , middle mysql There will be some optimizations to achieve the best results , For example 6、7 Operation in 3 Wait after operation
author ： Hui Jing

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