2.1.18. Queries and Joins

  • Relational database allows inter-related data
  • SQL select FROM gives Cartesian product
  • WHERE clause defines join condition
    • SELECT proj.pnum, mgr.ssn
    • FROM project AS proj, employee AS mgr
    • WHERE proj.mgrssn = mgr.ssn;
  • Alternatively, explicitly define join (note type)
    • SELECT project.pnum, employee.ssn
    • FROM project INNER JOIN employee
    • ON project.mgrssn = employee.ssn;