SQL · Question 28 · Hard

SQL question 28

Table Orders:
order_id | customer | city | amount
101 | Meera | Chennai | 1200
102 | Arjun | Bengaluru | 600
103 | Meera | Chennai | NULL
104 | Sana | Delhi | 1100
105 | Arjun | Bengaluru | 600
106 | Dev | Bhopal | 300
107 | Sana | Delhi | 400

Table Customers:
name | tier
Meera | Gold
Arjun | Silver
Sana | Gold
Kabir | Silver

What does this query return?

SELECT COUNT(*)
FROM (SELECT customer FROM Orders
      UNION
      SELECT name FROM Customers) t;
  1. 5
  2. 11
  3. 4
  4. 7
Answer and explanation

Answer: A. 5

Explanation: UNION removes duplicates. The distinct names are Meera, Arjun, Sana and Dev from Orders, plus Kabir from Customers, which gives 5. UNION ALL would keep all 7 + 4 = 11 rows.

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