• Joins only really make sense once data is normalized — normalization splits facts across tables, and joins bring them back together 1:31.
  • A join combines two or more tables into one result 1:31.

Types covered

  • Inner join — returns only the matching records between the tables. Example: users Alice and Bob appear in both users and orders, so their rows get bundled into the result 1:31 2:09.
  • Left join — returns all rows from the left table; where there is no match on the right, the result comes back as NULL 2:09 2:41.
  • You can use a WHERE clause to filter the joined result (e.g., keep only one side) 2:41.