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SQL快速入门
2022-07-06 09:29:00 【狗蛋儿l】
基础查询
查询多列
查询所用语句 SELECT 字段名 FROM 表名;
答案
select device_id, gender, age, university from user_profile;
查询所有列
*号是选取所有列名
答案:
SELECT * FROM user_profile
简单处理查询结果
查询结果去重
去重查询DISTINCT 要放到列名的前面
答案:
SELECT DISTINCT university FROM user_profile
查询结果限制返回行数
LIMIT显示前面两行。
答案:
SELECT device_id FROM user_profile LIMIT 0,2;
将查询后的列重新命名
改变列标题(取别名) 语法: 列名 | 表达式 [ AS ] 新列名 或: 新列名=列名 | 表达式
答案:
SELECT device_id as user_infors_example
from user_profile
LIMIT 2
条件查询
基础排序
通过order by 进行排序:
asc : 升序
desc : 降序
答案:
select device_id,age
from user_profile
order by age asc;
查找后多列排序
答案:
SELECT device_id, gpa, age
FROM user_profile
ORDER BY gpa, age;
查找后降序排列
答案:
select device_id,gpa,age
FROM user_profile
ORDER BY gpa desc,age desc
基础操作符
查找学校是北大的学生信息
在where条件中增加条件为‘北京大学’即可。
答案:
select device_id,university
from user_profile
where university='北京大学'
查找年龄大于24岁的用户信息
答案:
select device_id,gender,age,university from user_profile
where age>24
查找某个年龄段的用户信息
确定范围 用BETWEEN…AND 和 NOT BETWEEN…AND是逻辑运算符,可以用来查找属性值在或不在指定范围内的元组,其中BETWEEN后边指定范围的下限,AND后边指定范围的上限。BETWEEN…AND…的格式为:列名 | 表达式 [ NOT ] BETWEEN 下限值 AND 上限值
答案:
select device_id,gender,age
FROM user_profile
WHERE age>=20 and age<=23
查找除复旦大学的用户信息
答案:
select device_id, gender, age, university
from user_profile
where university != '复旦大学'
用where过滤空值练习
答案:
SELECT `device_id`,`gender`,`age`,`university`
from user_profile
where not age is NULL
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