Primary Keys and Foreign Keys

• This kind of cross-reference is so important that SQL provides syntax to allow automatic “integrity checks” and allow optimizations.

• We can change the abbrev column to be a primary key of the departments table by the following SQL command:

  ALTER TABLE departments ADD PRIMARY KEY (abbrev)
  

• We add a constraint that any allowed value in the dept column of students must be a valid primary key in the departments table by the SQL command:

  ALTER TABLE students ADD FOREIGN KEY (dept)
  
  REFERENCES departments (abbrev)

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