Lesson 4 of 81970–today7 min
Joins
A marketing manager asks for two lists before Monday: every customer with the number of orders they have placed, and the customers who have never ordered at all. The data sits in two tables, customers and orders, linked by a customer id.
Both answers come from the same operation, the join, which Codd already listed among the operations on relations in 1970. Get the join type wrong and the second list comes back empty, without any error to warn you.
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Sources
- PostgreSQL tutorial: Joins Between Tables — PostgreSQL Global Development Group
- PostgreSQL documentation: Table Expressions (joined tables) — PostgreSQL Global Development Group
- A Relational Model of Data for Large Shared Data Banks (E. F. Codd, 1970) — ACM Digital Library