普通查询： select * from user where id>10;
查询最大值：select id,max(salary),min(value),count(*) from userwhere id>10;
分组查询:select department, max(salary) as col from user group by department having col>2000;having与where的区别是后者用于子查询的；
限定返回的记录数：select * from user limit 5;
mysql联合查询：