mysql - Trouble querying results from three tables -


i'm trying query data practice , these 3 tables:

course    assignment    assignment_submissions id        id            id           course        assignment

as can see relationships pretty basic, i'm rusty plain sql, that's why i'm practicing.

given course 'id', how can find assignment_submission rows belong it?

i need assignments, each assignment each submissions. here's attempt, fails , returns larger dataset expected.

select sub.id, sub.grade, uv_assignment_submissions sub, uv_assignment ass ass.course = 1245

you need join 3 tables, course, assignment , assignment_submissions. have used join construct assuming need display course details when records exists assignments , assignment_submissions.

the reason why query fails because, getting cartesian product of records 2 tables, need condition specify rows valid current assignment specifying join condition

something should work

select   c.id,   asub.id   course c   join assignment on ( c.id = a.course )   join assignment_submissions asub on ( asub.id = a.assignment )   c.id = 10 --your course id goes here or c.id in (10, 20, 30) -- multiple ids

more on sql joins: http://dev.mysql.com/doc/refman/5.0/en/join.html


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