数据库----单表查询

select语句

    select [distinct] *|{字段名，字段名2，字段名3，····}
        from 表名
        [where 条件表达式1]
        [group by 字段名 [having 条件表达式2]]
        [order by 字段名 [asc|desc]]
        [limit [offset] 记录数]

建立表student 添加20记录

        create table student(
        id int(3) primary key auto_increment,
        name varchar(20) not null,
        grade float,
        gender char(2)
        );
        添加20条记录
        insert into student(name,grade,gender) values
        ('赵四',100,'男'),
        ('王五',95,'男'),
        ('夏敏',12,'男'),
        ('赵六',13,'男'),
        ('李四',15,'男'),
        ('李明',85,'女'),
        ('小明',66,'男'),
        ('小王',50,'男'),
        ('小李',59,'男'),
        ('小丽',87,'男'),
        ('大王',95,'男'),
        ('李强',96,'男'),
        ('小红',35,'女'),
        ('小白',78,'女'),
        ('吴勇',96,'男'),
        ('八戒',12,'男'),
        ('莎莎',69,'女'),
        ('刘备',98,'男'),
        ('孔明',95,'男'),
        ('鲁智深',77,'男');

1.查询所有记录

    select * from 表名
    eg:select * from student；   

2.查询所有字段

    select 字段名1，字段名2，字段名3···· from 表名；
    eg：select name,grade,gender from student;

3.查询指定的字段

    select 字段名1，字段名2，字段名3···· from 表名；（字段指的是部分字段）
    eg：select name from student;

4.关系运算符的查询

• 指定查询条件对数据进行过滤
  select *|字段名1，字段名2·· from 表名 where 条件表达式
  eg:select * from student where id=1;
  eg:select * from student where id=1;
  eg:select * from student where id>5;
  eg:select * from student where id!=2;
  eg:select * from student where name=’刘备’;
  eg:select * from student where grade=98;

5.带in关键字的查询

• 判断某个字段是否在指定集合中
  select *|字段名1，字段名2·· from 表名 where 字段名 [not] in(元素1，元素2···)；
  eg:select * from student where id in(1,2,3);
  eg:select * from student where gender in(‘男’);
  eg:select * from student where grade in(100,95,96);
  eg:select * from student where id not in(1,2,3);
  eg:select * from student where grade not in(100,95);

6.带between and关键字的查询

• 判断某个字段的值是否在指定的范围内
  select *|字段名1，字段名2···· from 表名 where 字段名 [not] between 值1 and 值2；
  eg：select * from student where id between 1 and 4;
  eg:select * from student where grade between 90 and 100;
  eg:select * from student where grade between 10 and 30;
  eg：select * from student where id not between 1 and 4;
  eg:select * from student where grade not between 100 and 90;

7.空值查询

• 判断字符串是否为空值
  select *|字段名1，字段名2，··· from 表名 where 字段名 is [not] null；
  eg:select * from student where id is not null;
  eg：select * from student where id is null;

8.带distinct关键字的查询

• 去掉查询记录中重复的数据
  select distinct 字段名 from 表名；
  eg:select distinct grade from student;
  eg：select distinct name from student;
  eg:select distinct id from student;

9.distinct关键字可以作用于多个字段

    select distinct 字段名1，字段名2··· from 表名；
    eg：select distinct gender,name from student;

10. 带like关键字的查询

• 对字符串进行模糊查询
  select *|字段名1，字段名2··· from 表名 where 字段名 [not] like ‘匹配字符串’;
  eg:select * from student where name like ‘小明’;
  eg:select * from student where name not like ‘%小%’;
• 百分号%通配符 可以匹配任意长度的字符串，包括空字符串
  eg:select * from student where name like ‘%备’
  eg:select * from student where name like ‘刘%’
  eg:select * from student where name like ‘%小%’;
• 下划线_通配符 可以匹配任意的长度的字符串，包括空字符串
  eg：select * from student where name like ‘小_’;
• 添加一条数据 insert into student(name,grade,gender) values(‘sun%er’,95,’男’);
  eg:select * from student where name like ‘%\%%’;

11.带and关键字的查询

    select *|字段名1，字段名2··· from 表名 where 条件表达式1 and 条件表达式2[···and 条件表达式n]；
    eg：select * from student where id=1 and id=2
    eg:select * from student where id>1 and grade<60;

12.带or关键字的多条件查询

    select *|字段名1，字段名2··· from 表名 where 条件表达式1 or 条件表达式2[···or 条件表达式n]
    eg：select * from student where id>1 or grade <60;