MEPX
Chapter 7 of 10All chapters
  1. 1What SQL is
  2. 2SELECT and WHERE
  3. 3NULL
  4. 4Sorting and limiting
  5. 5Aggregates and GROUP BY
  6. 6Joins
  7. 7Keys and relationships
  8. 8Subqueries and CTEs
  9. 9Indexes and performance
  10. 10Changing data

Chapter 7 of 10

Keys and relationships

How tables point at each other.

Primary and foreign

A primary key identifies each row uniquely and never repeats. A foreign key is a column holding another table's primary key, which is what makes a join meaningful.

  • Foreign key constraints stop a row pointing at something that does not exist.
  • One to many is the common shape; many to many needs a join table between them.

Choosing a key

A generated id is usually safer than a natural key such as an email address, because natural values change and people expect to be able to change them.