Database hyperphone (II)

Multi table join query

One 、 Join query structure

1.where、inner join

1. where The join statement used in clause , In database language , It's called implicit connection .inner join……on The join produced by a clause is called an explicit join .（ other join Parameters are also explicit connections ）

2. where and inner join The resulting connection , There is no essential difference , It turns out the same thing .

3. If two sides are not equal , Then it cannot be connected

2. Specific internal and external connections

1. (inner) join： Internal connection , Understood as a “ Connect effectively ”, The data in both tables will be displayed
2. left(outer) join：（ Left ） External connection , Display qualified data rows , And the unqualified data rows in the left table . At this time, the right data row will be easy NULL fill
3. right(outer) join：（ Right ） External connection , Display qualified data rows , And the unqualified data rows in the right table . At this time, the left data row will be easy NULL fill
4. full(outer)join: Display qualified data rows , And the data rows of the left table and the right table that do not meet the conditions . At this time, data lines that lack data will be easy NULL fill
5. cross join: No special requirements , Any table can be cross queried

3. Use AS Precautions for

< Source table name > [ AS ] < Table alias >

Be careful ： When an alias is specified for a table , Elsewhere in the query statement , Aliases should be used wherever table names are used , Instead of using the original table name .

4. About self connection query

• Special inner connection
• Interconnected tables are physically the same table
• You must alias two tables , Make it logically two tables .
  Such as ：
  FROM surface 1 AS T1 – Generate... In memory “T1”
  JOIN surface 1 AS T2 – Generate... In memory “T2”

Two 、 Internal connection query （inner）join

(1) Inquire the course number taught by Mr. Liu Wei , List the teacher number , Teacher's name , Course no. （3 Methods ）

① select Tno,TN,Cno
from T,TC
where T.Tno = TC.Tno and TN=' Liu Wei '

②select Tno,TN,Cno
form T join TC on T.Tno=TC.Tno
where TN=' Liu Wei '

③select R1.Tno,R1.TN,R2.Cno
from (select Tno,Cno from TC) as R1
inner join (select Tno,TN from T where TN = ' Liu Wei ') as R2
on R1.Tno=R2.Tno

(2) Inquire about ‘ Information management department ’ Repaired ‘ Fundamentals of Computer Culture ’ Names and grades of students

select S.Sn,scorce
from S
join SC on S.Sno = SC.Sno
join C on C.Cno 
where dept = ' Information management department ' and CN =' Fundamentals of Computer Culture ' 

(3) Count the number of students taking elective courses in each age group

select age,count(distinct sno)  #  duplicate removal 
from S,SC
where S.Sno = SC.Sno
group by age

3、 ... and 、 External connection query

(1) Check the student numbers of all the students , full name , Course name and grade （ The course selection information of students without course selection is empty ）

select Sno,Sn,Cn,Scorce
from S 
left join SC on S.Sno = SC.Sno
left join C on C.Cno=SC.Cno

(2) Query the course name of unattended courses

①select CN
from C left  join SC on C.Cno = SC.Cno
where SC.Cno is null # where SC.Sno is null  It's fine too 

②select CN 
from C where
Cno not in （select Cno from SC )

Four 、 Cross query （ No special requirements , Any table can be cross queried ）

Cross check the student schedule and curriculum

select * from S cross join C

5、 ... and 、 Self connect query

(1) Look up all of them “ Liu Wei ” The name of the teacher who works high 、 Wages 、 And Liu Wei's salary （3 Kind of ）

①select X.TN,X.Sal as Sal_a,Y.Sal as Sal_b 
from T as X ,T as Y
where X.sal >Y.sal and Y.TN = ' Liu Wei '

②select X.TN,X.Sal,Y.Sal
from T as X inner join T as Y
on X.Sal >Y.Sal and Y.TN = ’ Liu Wei 

③select R1.TN,R1.Sal,R2.Sal 
from (select TN,Sal from T) as R1
inner join
(select Sal from T where TN =‘ Liu Wei ') as R2 on R1.Sal <font color = 'red'> > R2.Sal  # Greater than connection 

(2) Retrieve all student names , Age and course name （2 Kind of ）

①select SN,Age,CN
from S,SC ,C
where S.Sno = SC.Sno and SC.Cno = C,.Cno

②select 
from 
(select Sno,SN,Age from S) as R3 # from S Student number of watch handle , full name , Choose your age , Put in R3
inner join 
(select R2.Sno,R1.CN               # from C,SC Find out the student number of the student in the class , Name of the selected course , Put it in R4
   from 
     (select Cno,Cn from C ) as R1
     inner join 
     (select Sno,Cno from SC) as R2
     on R1.Cno = R2.Co) as R4 
on R3.Sno = R4.Sno #R3,R4 Connect , Retrieve all student names , Age and course name 

(3) And Liu Chen are students in the same department , Department

select S2.Sn,S2.dept
from S as S1
inner join S as S2
on S1.dept = S2.dept 
where S1.SN = ' Liu Chen '
and S2.SN <font color='red'>< ></font> ' Liu Chen '

Be careful ： Alias must be used here