SQL 상용 어구 DQL 언어

select * from students;  --  students      
select stuid,name,age from students;  -- students    stuid,name,age       
select name as   ,age   ,stuid from students; --         ，as       
select * from students where stuid = 20;  --  students  stuid 20    
select * from students where stuid > 20;  --  students  stuid  20    
select * from students where stuid <> 20;  --  students  stuid   20    
select * from students where stuid != 20;  --  students  stuid   20    
select * from students where age>10 and gender='f';  --  students      10        
select * from students where age >= 20 and age <= 30;  --  students        20     30  
select * from students where age between 20 and 30;  --        
select * from students where name like 's%';  --  students  name  s        
select * from students where name like '%yu%';  --  students  name    yu      
select distinct age  from students ;  --  students  age  ，         
select * from students where classid is null; --  students  classid        
select * from students where classid is not null; --  students  classid         
select count(stuid) from students;  --  students  stuid         
select max(age) from students;   --  students         
select min(age) from students;   --  students         
select avg(age) from students;   --  students         
select gender,avg(age) as     from students group by gender;  --  students      ，       
select classid,avg(age) as     from students group by classid having classid >3;  --  students  classid  3 ， classid           
select classid,gender,avg(age)  from students group by classid,gender;  --  students   classid gender         。       
select * from students order by age desc;  --  students ， age       ，     
select * from students order by -classid desc;  --  students ， classid      ，  null     ，     
select classid,sum(age) from students group by classid order by -classid desc;  --  students ，   classid             ，      
select classid,sum(age) from students group by classid order by -classid desc limit 3;  --  students ，   classid             ，      ，      
select classid,sum(age) from students group by classid order by -classid desc limit 2，3;  --  students ，   classid             ，      ，           
select * from students where classid in (1,3,5);  --  students classid 1,3,5  

select stuid,name,age,gender from students union select * from teachers;  --     stuid,name,age,gender                   
select * from teachers union all  select * from teachers;  --        ，   ，  all        
select * from students cross join teachers;  --                    ，                ，      
select * from students inner join teachers on students.teacherid=teachers.tid;  --        ID       id     ，     
select s.stuid,s.name,s.age,t.tid,t.name,t.age from students as s inner join teachers as t on s.teacherid=t.tid; --         id，     ，    id，     ，          ，       ，       ，        ID       ID     
select s.stuid,s.name,s.age,t.tid,t.name,t.age from students as s left outer join teachers as t on s.teacherid=t.tid;  --                             ，      id，     ，    id，     ，         null，      
select s.stuid,s.name,s.age,t.tid,t.name,t.age from students as s right outer join teachers as t on s.teacherid=t.tid;  --                             ，      id，     ，    id，     ，         null，      
select s.stuid,s.name student_name,s.age student_age,t.tid,t.name teacher_name,t.age teacher_age from students as s inner join teachers as t on s.teacherid=t.tid and s.age > 30; --         id，     ，    id，     ，          ，       ，       ，        ID       ID     ，         30   
select s.stuid,s.name,s.age,t.tid,t.name,t.age from students as s left outer join teachers as t on s.teacherid=t.tid where t.tid is null;  --       ，     tid      
select s.stuid,s.name,s.age,t.tid,t.name,t.age from students as s right outer join teachers as t on s.teacherid=t.tid where s.teacherid is  null;  --       ，     teacher      
select s.stuid,s.name,s.age,t.tid,t.name,t.age from students as s full outer join teachers as t on s.teacherid=t.tid;  --             ，  mysql   
select s.stuid,s.name,s.age,t.tid,t.name,t.age from students as s left outer join teachers as t on s.teacherid=t.tid union select s.stuid,s.name,s.age,t.tid,t.name,t.age from students as s right outer join teachers as t on s.teacherid=t.tid ;  --mysql          ，     ，             
select * from (select s.stuid,s.name s_name,s.teacherid,t.tid,t.name t_name from students s left outer join teachers t on s.teacherid=t.tid union select s.stuid,s.name,s.teacherid,t.tid,t.name from students as s right outer join teachers as t on s.teacherid=t.tid) as a where a.teacherid is null or a.tid is null;  --                         
select st.name,co.course,sc.score from students as st inner join scores as sc on st.stuid=sc.stuid inner join courses as co on sc.courseid=co.CourseID;  --      ID       ID       ，             id          ，             ，                ，      

select * from students where age >(select avg(age) from students);  --                  select       

seletct e.name,l.name from emp as e left join emp as l on e.leaderid=l.id;  --           ，      ID   ID