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Many-to-many example
# For http://stackoverflow.com/a/7296873/396458
student: student_id, first_name, last_name
classes: class_id, name, teacher_id
student_classes: class_id, student_id # the junction table
students:
id | first | last
=====================
1 | John | Lee
2 | Jane | Wilson
3 | Daniel | Gomez
classes:
id | name | teacher_id
==========================
1 | Biology | 2
2 | Physics | 4
3 | English | 77
student_classes
s_id | c_id
======================
1 | 2 # John is taking Physics
1 | 3 # John is taking English
2 | 2 # Jane is taking Physics
3 | 1 # Daniel is taking Biology
@vinod023
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vinod023 commented Mar 7, 2019

Relation ships means table creation or writing the queries combination of tables

@nfangueiroLGC
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nfangueiroLGC commented Jan 25, 2022

Can Daniel take Biology twice in student_classes table?

@Zer0SkiLL
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Zer0SkiLL commented Mar 28, 2022

Can Daniel take Biology twice in student_classes table?

not if PK are being used. Fiddle here you can fiddle around and replace the last line with values (1, 1), (2, 2), (1, 1), (1,2);
it won't let you build the schema, since that would be a PK constraint violation.

@bingjiexiong
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bingjiexiong commented May 23, 2022

for the student_classes case, can we create a schema without the PK? Cause I know PK must be unique.

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