Nested query and Having the speed of the query-CodePudding

This is the data
Demand is in the table to find all the repeat email
I wrote two SQL

 SELECT 
R.e mail 
The FROM 
(SELECT 
E.e mail, 
Count (e.e mail) as num 
The FROM 
Email e 
GROUP BY 
E.e mail) r 
Where 
R.n um & gt; 1

 SELECT 
E.e mail 
The FROM 
Email e 
GROUP BY 
E.e mail 
HAVING 
Count (e.e mail) & gt; 1

Compare two statement execution efficiency, found that nested query using time will be less than useful Having query

question: why is nested query time less than practical Having query

CodePudding user response:

This depends on the specific content and table structure, data
Normally, do not advocate using HAVING, still can use index way to speed up,