Solution:Step 1: Understand the requirementWe want people in employees or contractors but not in both (symmetric difference).Step 2: Use EXCEPT and UNION to find symmetric differenceEXCEPT finds those in one table but not the other; UNION combines both exclusive sets.Step 3: Analyze optionsSELECT id

[Solved] You have two tables:employees with columns id, namecontractors with columns id, nameYou want to find people who are either employees or contractors but not both.... | PostgreSQL

PostgreSQL - Set Operations and Advanced Queries

You have two tables:
employees with columns id, name
contractors with columns id, name
You want to find people who are either employees or contractors but not both.
Which query correctly returns this list?

ASELECT id, name FROM employees EXCEPT SELECT id, name FROM contractors UNION SELECT id, name FROM contractors EXCEPT SELECT id, name FROM employees;

BSELECT id, name FROM employees INTERSECT SELECT id, name FROM contractors;

CSELECT id, name FROM employees UNION SELECT id, name FROM contractors;

DSELECT id, name FROM employees EXCEPT SELECT id, name FROM contractors;

Step-by-Step Solution

Solution:

Step 1: Understand the requirement
We want people in employees or contractors but not in both (symmetric difference).
Step 2: Use EXCEPT and UNION to find symmetric difference
EXCEPT finds those in one table but not the other; UNION combines both exclusive sets.
Step 3: Analyze options
SELECT id, name FROM employees EXCEPT SELECT id, name FROM contractors UNION SELECT id, name FROM contractors EXCEPT SELECT id, name FROM employees; correctly combines EXCEPT in both directions with UNION to get symmetric difference.
Final Answer:
SELECT id, name FROM employees EXCEPT SELECT id, name FROM contractors UNION SELECT id, name FROM contractors EXCEPT SELECT id, name FROM employees; -> Option A
Quick Check:
Symmetric difference = EXCEPT + UNION [OK]

Quick Trick: Symmetric difference = (A EXCEPT B) UNION (B EXCEPT A) [OK]

Common Mistakes:

Using only INTERSECT or UNION
Using EXCEPT one way only
Confusing symmetric difference with intersection

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You have two tables:

Step 1: Understand the requirement

Step 2: Use EXCEPT and UNION to find symmetric difference

Step 3: Analyze options

Final Answer:

Quick Check:

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