understanding of sql

select * from a left join b ; --"The result is
select in the order of table b min(a),b,c from s;-->>This writing is wrong, it shows the smallest value of column a, and s The values ​​of column b and column c in the first row of the table;
select sname,age from s order by id desc,age asc;--> first sort in descending order of ID, if age is the same, sort in ascending order of age