Database hyperphone (III)

One 、 Use TOP Limit the number of rows in the result set

• TOP n [ percent ] [WITH TIES ]
• TOP n： Get the first... Of the query results n Row data .
• TOP n percent： Get the first... Of the query results n% That's ok .
• WITH TIES： Including parallel results .
• TOP The predicate is written in SELECT Behind the word

(1) Check the names of the three oldest students 、 Age and department ,

Including juxtaposition .

SELECT TOP 3 WITH TIES Sn, age, dept` 
 FROM S
 ORDER BY age DESC

notes ： If you use WITH TIES Options , Must be used ORDER BY Clause , Otherwise, there will be grammatical errors

(2) Inquire about VB The names of the top three students with the highest test scores 、 Department and
VB Test results , Including juxtaposition .

SELECT TOP 3 WITH TIES Sn, dept, Score
  FROM S JOIN SC on S.Sno = SC.Sno 
  JOIN C ON C.Cno = SC.Cno
  WHERE Cn = 'VB'
  ORDER BY Score DESC

(3) Query the two courses with the least number of students （ It does not include courses without candidates ）,
List the course number and the number of people selected .

select top 2 with ties Cno count(Cno)  The number of elective courses 
from SC
group by Cno
order by count(Sno) 

(4) Inquire about VB The top one with the highest score in the exam 3 Names of students , Department and VB Examination results include juxtaposition

select top 3 with ties Sn,dept scorce
from S,SC,C
where S.Sno = SC.Sno and SC.Cno=C.Cno and CN = 'VB' 
order by scorce desc

(5) The number of courses selected by the computer department exceeds 2 Among the students of the school , The top with the highest average score in the exam 2 name （ Including juxtaposition ） Student's student number , Number of courses selected and average grade

select top 2 with ties S.Sno,count(Cno)  The number of elective courses  AVG(Scorce)
from S,SC
where  S.Sno = SC.Sno and dept =' Department of Computer Science ' 
group by S. Sno
having count(Cno)
order by AVG(Scorce)

CASE expression

• A multi branch expression , One of several possible results can be returned according to the value of the condition list Can be used wherever expressions are allowed .
• Not a complete T-SQL sentence , You can't do it alone .

CASE Test expression
WHEN Simple expressions 1 THEN The result expression 1
WHEN Simple expressions 2 THEN The result expression 2
…
WHEN Simple expressions n THEN The result expression n
[ ELSE The result expression n+1 ]
END

• Evaluate the test expression , Then match the value of the test expression with each WHEN Clause .

• If it is equal to the value of the test expression , Returns the first matching WHEN The value of the result expression corresponding to clause .

• If it does not match the values of all test expressions ：

• If you have any ELSE Clause , Then return to ELSE Clause

• If there is no clause , Then return to NULL

(1) The query selected VB Student id of the course 、 full name 、 Department and grades ,
And deal with the Department as follows ：
“ Department of Computer Science ”： Show “CS”;
“ Information management department ”： Show “IM”;
“ Department of communication engineering ”： Show “COM”.

SELECT S.Sno  Student number ,Sn  full name ,
  CASE dept
     WHEN ' Department of Computer Science ' THEN 'CS'
     WHEN ' Information management department ' THEN 'IM'
     WHEN ' Department of communication engineering ' THEN 'COM'
  END AS  Department ,Score  achievement 
  FROM S join SC ON S.Sno=SC.Sno
  JOIN C ON C.Cno = SC.Cno
  WHERE Cn = 'VB'  

CASE
WHEN Boolean expression 1 THEN The result expression 1
WHEN Boolean expression 2 THEN The result expression 2
…
WHEN Boolean expression n THEN The result expression n
[ ELSE The result expression n+1 ]
END

• Calculate each... In writing order from top to bottom WHEN Boolean expression of clause .
• Returns the first value of TRUE The result expression value corresponding to the Boolean expression of .
• If not, the value is TRUE The Boolean expression of ：
• If you have any ELSE Clause , Then return to ELSE Clause
• If there is no clause , Then return to NULL.

(2) Count the number of courses selected by each student in the computer department , Including students who don't choose courses .
List the student number 、 The number of courses selected and the situation of course selection , The treatment of course selection is ：
If the number of courses selected exceeds 4 door , Then for “ many ”;
If the number of courses selected is 2~4 Within the scope of , Then for “ commonly ”;
If the number of courses selected is less than 2 door , Then for “ Less ”;
If the student doesn't choose a course , be “ Unselected ”.
Sort the query results in descending order according to the number of courses selected .

SELECT S.Sno, COUNT(SC.Cno)  The number of elective courses ,CASE 
  WHEN COUNT(SC.Cno) > 4 THEN ' many '
  WHEN COUNT(SC.Cno) BETWEEN 2 AND 4 THEN ' commonly '
  WHEN COUNT(SC.Cno) BETWEEN 1 AND 2 THEN ' Less '
  WHEN COUNT(SC.Cno) = 0 THEN ' Unselected '
END AS  Course selection 
FROM S LEFT JOIN SC ON S.Sno = SC.Sno
WHERE dept = ' Department of Computer Science '
GROUP BY S.Sno
ORDER BY COUNT(SC.Cno) DESC