The best replacement for foreign key is primary key.

The best alternatives to foreign key are rated as following:
★★★★★  primary key
★★★  constraint
★★  composite key
★★  references
★★  unique key
★★  surrogate key
★★  secondary key
★★  index mysql
★  one to one
★  candidate key
★  many to many
★  check constraint
  relation

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Speaking about foreign key, one probably shouldn't forget about composite primary key.

Other search terms related to foreign key and their ratings are:
★★★★★ composite primary key
★★★★★ navigation property
★★★★ cascade
★★★★ check constraint
alter table
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foreign key
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