sql基础之select语句（一）

sql语句是专门与数据库交互的语言，支持多种DBMS，不区分大小写！

一、检索语句

1.检索单列所有行数据：select user_name from users;

2.检索多个列所有数据：select user_id,user_name,user_sex from users;

3.检索所有列：select * from users;

4.检索不同的值：select distinct user_name from users;

5.检索指定前5行的数据：

    ①sqlServer/Access：select top 5 user_name from users;

    ②DB2：select user_name from users fetch first 5 rows only;

    ③Oracle： select user_name from users where rownum <= 5;

    ④Mysql：select user_name from users limit 5;

        1>返回从第五行起的5行数据：select user_name from users limit 5 offset 5;

6.排序语句：select user_age from users order by user_age;  

    ①使用order by 语句是，必须保证其是sql语句中的最后一条字句

    ②对多列进行排序：select user_age,user_name from users order by user_age desc,user_name;(user_age倒序，user_age正序)

7.where语句：select user_name from users where user_age = 18;

    ①范围查询：select user_name from users where user_age between 18 and 20;

    ②空值查询：select user_name from users where user_age is null;

    ③组合查询：select user_name from users where user_age = 18 and user_sex = '男';

    ④求值查询：select user_name from users where (user_age = 18 or user_age = 20) and user_sex = '男'

        1>在sql语句中，连接条件的执行顺序为：（）> and > or

        2>上述语句还可改为：select user_name from users where user_age in (18,20) and user_sex = '男'

8.通配符过滤查询：select * from users where user_name like '张%';(查询所有姓名为张开头的所有用户信息)