mysql - Do DBMSs store referencing attributes separately? -


suppose have following relations (tables) :
student (id, name , family) course (id, title, unit, ref) std-course (std-id, course-id) // std-id , course-id foreign keys referencing student , course relations respectively.

are std-id , course-id stored separately or stored pointers? (specially in mysql , ms sqlserver)

edit: let me explain more way :

if there such row (123456, john, smith) in student table, , (123456,db1) in std-course 123456 stored twice or in second table (as foreign key) it's link?

as per documentation:

mysql supports foreign keys, let cross-reference related data across tables, , foreign key constraints, keep spread-out data consistent.

meaning, use child , parent store data. instruction of foreign key ... references ... stored in system table table_constraints maintain defined relation among tables.

edit:

if there such row (123456, john, smith) in student table, , (123456,db1) in std-course, 123456 stored twice or in second table (as foreign key) it's link?

it stored twice. first in parent table , second in child table.

references not directly part of table data. directives database system , maintained in own schema called information_schema.

the table table_constraints part of schema , holds information on such constraints defined across various databases under current instance of mysql.

refer to:


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