SQL Server › Joins

What common mistakes do developers make when writing multi table joins that can lead to unexpectedly large or incorrect result sets?

Intermediate

Answer

Common mistakes include forgetting a join condition entirely, which accidentally creates a CROSS JOIN, joining on the wrong columns which produces duplicate or missing matches, and not accounting for one to many relationships properly, which can cause aggregate calculations to be inflated due to unexpected row duplication from the join itself.

-- Missing join condition accidentally creates a CROSS JOIN
SELECT * FROM Orders, Customers; -- likely a mistake

-- Correct version with proper join condition
SELECT * FROM Orders o JOIN Customers c ON o.CustomerId = c.CustomerId;

Real-world example A developer notices a sales total report showing suspiciously inflated numbers, and discovers a join to an order items table was duplicating each order's total once for every item in that order, requiring a fix to the aggregation logic.

Common follow-ups: How do you verify a join is producing the expected number of rows before trusting a report's numbers?;What techniques help catch these kinds of join mistakes during development?

Aggregate Functions & GROUP BY;Data Types & Schema Design