Sorting out common SQL interview questions and answers

How to optimize massive data SQL

According to the execution plan （explain） Set the appropriate index , Choose the right one type（system best ,all The worst ）

Database storage location

The data files and indexes of the database are stored in disk in ;
Each time you search for data, you need to read the index file to Memory in

MySQL And hive

OLAP： On line analytical processing , Corresponding data warehouse hive, Large amount of data , Not for efficiency
OLTP： Online data processing , Corresponding database , The amount of data is small , Pursuing efficiency

Aggregate functions and group by Field restrictions for simultaneous use

select  Field , Aggregate functions  
	from  Table name 
	where  Conditions 
	group by  Field  

here select The fields of are either contained in the aggregate function , Or included in group by in , Otherwise it will go wrong .

Nested use of aggregate functions

Aggregate functions cannot be nested directly , But you can nest subqueries that contain aggregate functions .
error ：

select max(count(*)) from ...

correct ：

select max(cnt) from(
	select count(*) as cnt from...
	)