1. 检索数据
    检索单个列：select column_name from table;
    检索多列: select column_name1, column_name2,... from table;
    检索所有列：select * from table;
    检索不同的行：select distinct column_name from table;
    限制结果：（返回不多于5行）select column_name from table limit 5;
    （返回从行5开始的5行）select column_name from table limit 5.5;
  2. 排序检索数据
    排序数据：select column_name from table order by column_name;
    按多列排序：select column_name1, column_name2, ... from table order by column_name1, column_name2, ...;
    指定排序方向: (默认为升序asc，降序desc) select column_name1, column_name2, ... from table order by column_name desc;
  3. 过滤数据
    使用where字句：select column_name from table where...;
    and操作符：(联结两个条件，返回满足所有条件的行) select column_name1, column_name2 from table where 条件1 and 条件2；
    or操作符：(返回满足任一条件的行) select column_name1, column_name2 from table where 条件1 or 条件2；
    in操作符：(指定条件范围，范围中的每个条件都可以进行匹配) select column_name1, column_name2 from table where column_name in (范围）order by column_name;
    not操作符：（否定其后面的任何条件）select column_name1, column_name2 from table where column_name not in () order by column_name;