2.查询数据表中的数据；
select selection_list select * /columns
from table_list from table1/table2
where primary_constraint
group by grouping_columns
order by sorting_colomns desc降序 select * from table order by id desc;
having second_constraint
limit count select * from table where id limit 1,4;
2 常用的统计函数：avg(字段),sum(字段),count(字段),min(字段),
select sum(字段2) as 别名 ，字段1 from table group by 字段1；
3 算数运算，字符串，以及逻辑运算构造表达式：select * (price*0.8) as '80%' from table;打八折bitscn.com