MySQL: order by sorting query, group by grouping query

order by query

That is, the query is performed in ascending or descending order according to the field. The default is ascending order, which can be specified according to the asc and desc suffixes.

Example:

Source data

Sort by age:

select* from user order by age;

Sort by name: will be sorted lexicographically

Specify multiple fields:
The meaning of the following statement is that if the names are the same, the age is sorted in ascending order

Use with where:

group by group query

When querying, if you want to know how old the user's age is in the table, you can normally use deduplication to query:

This can also be achieved using group by:

But if you want to know how many people there are in each age group, there is no way to deduplicate, but group by is still possible:

You can also remove unwanted data after grouping:

You can group by multiple fields:

Can be combined with order by query:

Performance issues

Actually, the queries of limit, order by, group by are all related to the index. In this table, the primary key is id, and other fields have no indexes, so when operating on other fields such as age, you can seeThe last column shows that the query needs to use filesort, that is, file sorting, which involves a lot of operations on the disk; and the index used to query the id is directly queried using the index.