常见SQL整理

基础语法

查询

全表查询
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select * from student
选择查询
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select name,age from student
别名
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select name as 学生姓名,age as 学生年龄 from student
常量和运算
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select name,score,score*2 as double_score from student
条件查询--where
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select name,score from student where name='鱼皮'
条件查询--运算符
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select name,age from student where name != '热dog'
条件查询--空值
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select name,age,score from student where age is not null
条件查询--模糊查询
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select name,score from student where name not like '%李%'
条件查询--逻辑运算
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select name,score from student where name like '%李%' or score>500
去重
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select distinct class_id,exam_num from student
排序
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select name,age,score from student order by score desc, age asc
截断和便宜
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select name,age from student order by age limit 1,3
条件分支
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select name, case when (age>60) then '老同学' when (age>20) then '年轻' else '小同学' end as age_level from student order by name
时间函数
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select name, date() as 当前日期 from student
字符串处理
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select id,name,upper(name) as upper_name from student where name='热dog'
聚合函数
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select sum(score) as total_score,avg(score) as avg_score,max(score) as max_score,min(score) as min_score from student
单字段分组
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select class_id,avg(score) as avg_score from student group by class_id
多字段分组
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select class_id,exam_num,count('*') as total_num from student group by class_id,exam_num
having子句
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select class_id,sum(score) as total_score from student group by class_id having total_score>150
关联查询--cross join
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select s.name as student_name, s.age as student_age, c.id as class_id, c.name as class_name from student as s cross join class as c
关联查询--inner join
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select s.name as student_name,s.age as student_age,c.id as class_id,c.name as class_name,c.level as class_level from student as s inner join class as c on s.class_id = c.id
关联查询--outer join
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select s.name as student_name,s.age as student_age,c.id as class_id,c.name as class_name,c.level as class_level from student as s left join class as c on s.class_id = c.id
子查询
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select name,score,class_id from student where class_id in (
    select distinct id from class
)
子查询exists
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select name,age,class_id from student where not exists (
    select 1 from class where student.class_id=class.id
)
聚合查询
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select name,age,score,class_id from student
union all
select name,age,score,class_id from student_new
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