条件查询--LIKE模糊查询-- 排序查询--聚合函数--分组查询--分页查询

  3.条件查询  2.2.7.24         
== 查询年龄大于20岁
    SELECT * FROM student WHERE age >= 20;
== 查询年龄等于20岁
    SELECT * FROM student WHERE age = 20;
== 查询年龄不等于20岁
    SELECT * FROM student WHERE age !=20;
    SELECT * FROM student WHERE age <> 20;     
== 查询年龄大于等于20 并且 小于等于30
    不建议用&&    SELECT * FROM student WHERE age >= 20 && age <= 30;
    SELECT * FROM student WHERE age >= 20 AND age <= 30;
    SELECT * FROM student WHERE age BETWEEN 20 AND 30;
== 查询年龄22岁，19岁,25岁的信息(或者关系)  or ||
    SELECT * FROM student WHERE age = 22 OR age = 19 OR age = 25;
    SELECT * FROM student WHERE age IN (22,19,25);
== 查询英语成绩为null
    SELECT * FROM student WHERE english = null;
--不对的，null值不能使用=或者！=判断
    SELECT * FROM student WHERE english IS NULL;
  不是null
    SELECT * FROM student WHERE english IS NOT NULL;
 
  4.LIKE模糊查询  _:单个字符  %:多个字符
== 查询姓马的有哪些
    SELECT * FROM student WHERE NAME LIKE '马%'; 
     
  5.DQL查询语句
      排序查询
         语法：order by 排序字段1 排序方式1, 排序字段2 排序方式2...
         --排序方式，默认升序ASC  降序DESC       
      聚合函数，
         将一列数据作为一个整体，进行纵向的计算
          count:计算个数  --排除null值
            SELECT COUNT(NAME) FROM student;
             SELECT COUNT(IFNULL(english,0)) FROM student;
          max:计算最大值
             SELECT MAX(math) FROM student;
          min:计算最小值
          sum:计算和
          avg:计算平均值         
      分组查询
          == 按照性别分组，分别查询男，女同学的平均分
            SELECT sex,AVG(math) FROM student GROUP BY sex;
          == 按照性别分组，分别查询男，女同学的平均分，人数
            SELECT sex,AVG(math),COUNT(id) FROM student GROUP BY sex;  
          == 按照性别分组，分别查询男女同学平均分，人数，要求，分数低于70分的人，不参与分组
            SELECT sex,AVG(math),COUNT(id) FROM student WHERE math > 70 GROUP BY sex;   
          == 按照性别分组，分别查询男女同学平均分，人数，要求，分数低于70分的人，不参与分组,分组之后，人数要大于2个人
            SELECT sex,AVG(math),COUNT(id) FROM student WHERE math > 70 GROUP BY sex HAVING COUNT(id) > 2;   
          注意：1.where和having的区别
                where在分组之前进行限定，如果不满足条件，则不参与分组
                having在分组之后进行限定，如果不满足结果，则不会被查询出来  
                where后不可以跟聚合函数，having可以进行聚合函数的判断 
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      2.2.7.29           
      分页查询      limit(0,3)开始索引，每页显示条数
        公式：开始索引 = （当前页码 - 1）* 每页显示的条数
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