Mysql aggregate function nested use operation

Purpose: Nested use of MySQL aggregate functions

Aggregate functions cannot be nested directly, for example: max(count(*))

But you can use nested subqueries, eg:

Note: The following as must be written

select max(total) from ( select count(*) as total from blog group by btype) as aa;

Supplementary knowledge: Tips for using the MAX() and count() functions in MySQL

1. max() function

In the case of considering improving database IO, you can create an index ===> create index index name on table name (column name);

2. count() function

Question: count(*) and count(column)

* Includes null data in the table

Tips: You can use this feature of count(*) to process instances that are counted according to different conditions:

For example:

Query the number of movies in 2006 and 2007 in one SQL statement ===>select count(release_year='2006' or null ) as 'Number of movies in 2006', count(release_year='2007' or null ) as 'Number of movies in 2007' from movies;

The above article on nested use of MySQL aggregate functions is all I have to share with you. I hope it can give you a reference. I also hope that you will support 123WORDPRESS.COM.