Python sqlite3值查找

Question

I have a problem that I've created for myself as a challenge and I need your help.

I have two tables. [students] - whose columns include name, course, score [gradingscheme] - whose columns include letter, upper, lower

The [gradingscheme] table basically exists to serve as a lookup table so that if my score from [students] is between the upper and lower boundaries from [gradingscheme], it takes on the text value of the letter column. (I am aware that you can simply do a CASE, WHEN, THEN to assign a letter grade, but I'm trying to expand my knowledge base)

import sqlite3
conn = sqlite3.connect('example3.db')
c = conn.cursor()

c.execute('''                                   
UPDATE students                                     
SET LetterGrade = CASE
    WHEN score between 90 and 100 then 'A'
    WHEN score between 80 and 90 then 'B'
    ELSE 'C'
END
''')                                        

c.execute('''
SELECT s.name,s.courseid,c.name,c.classroom,s.score,s.LetterGrade
FROM students AS s
LEFT OUTER JOIN courses AS c
WHERE s.courseid=c.courseid
''')                                        

import pprint
pp=pprint.PrettyPrinter()
pp.pprint(c.fetchall())          

conn.commit()
conn.close()

This is what I have so far, and it's the UPDATE/SET part that I'm trying to replace in this challenge. I have no idea how to execute this. Any help is appreciated!

Answer 1

1. Your grading scheme is ambiguous. Eg SELECT 90 BETWEEN 90 AND 100; evaluates to true just as SELECT 90 BETWEEN 80 AND 90; does. You should probably rather use ranges like 90-100, 80 - 89, 70 - 79, ...
2. Do you really need to add the grading letter to the students table? Usually, one would simply join the letter from the gradingscheme table whenever such a select is performed. Eg SELECT s.name, s.course, g.letter FROM students s, gradingscheme g WHERE s.score BETWEEN g.lower AND g.upper; . You could even create a view to have this done automatically. This avoids redundancy in your data.
3. If you really want to add the letter grading explicitly to the students table, you could try something like UPDATE students SET letterGrade = (SELECT letter FROM gradingscheme WHERE score BETWEEN lower AND upper); .