Row count when Bob appears twice

Question

Accepted Answer

UNION ALL concatenates the two result sets without removing duplicates. table_a has 2 rows, table_b has 2 rows - total 4 rows. Bob appears twice because he is in both tables. Prefer UNION ALL over UNION when you know there are no duplicates or you want them - it avoids the sorting/hashing overhead that UNION uses to deduplicate.