mysql左外连接

Question

i have some sql table

table classes(id_classes,specialty,level)
table Teaching unit(id_unit,name,...,#id_classes)
table courses(#id_course,....,#id_unit)
table groupes(id_groupe,...,#id_classes)
table assignment(#id_groupe,#id_course)

i want to find courses which not in assignment for a given groupe so this courses are depending to unit that depend to the groupe's classe

i wrote this code

select * from courses c
left join assignment a
on a.idcourse=c.idcourse and a.id_groupe ='given  id'
where a.id_courses is null

but this return the courses depend to all classes that not in assignment

Additional Info (copied from comment)

for exempl i have a classe with id 1 and classe with id 2 groupe with id 1 courses 1 ,2 and 3 depend to unit 1 thats depent to classe 1 courses 4,5 and 6 depend to unit 2 that depend to classe 2 i have a groupe 1 depend to classe 1 in the table asignement i have 2 rows (groupe1,courses1),(groupe1,courses2) i want to make a query that return the courses 3 that still not assigned to the groupe 1 the query return courses 3,4,5,and 6 but i want to get only the courses 3

select c.* from courses c ,unit u where  (courses not in asignment where id_groupe=given id) and u.idunit=c.idunit and u.classes is the same classes form the given groupe

Answer 1

Base on your description your left join is not incorrect is that you didn't have any code that expresses your extra contraints "u.idunit=c.idunit and u.classes is the same classes form the given groupe"

This should be the code you want:

SELECT *
FROM courses c
LEFT JOIN assignment a ON c.id_course = a.id_course
INNER JOIN teachingUnit u ON u.id_unit = c.id_unit 
                            AND u.id_classes IN (SELECT id_classes
                            FROM groupes
                            WHERE id_groupe =  ‘givenID’)
WHERE a.id_course IS NULL AND a.id_groupe = ‘givenID’   

this sub query

 (SELECT id_classes
   FROM groupes
   WHERE id_groupe =  ‘givenID’)

handles the constraint of u.classes is the same classes form the given groupe. The INNER JOIN on teachingUnit u ON u.id_unit = c.id_unit handles the constraint of "and u.idunit=c.idunit"

I hope this helps. Because in your example your table name and columns are inconsistent and your objective is hard to understand. This solution is the best I can come up with minimal assumptions that were made out of neccesity.