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LESSON 20

SQL (Core) - Set Operations (UNION & INTERSECT)

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SQL Lesson 20: Set Operations

Set operations allow you to combine or subtract result sets from two or more SELECT queries into a single unified result.

1. UNION, INTERSECT, and EXCEPT Operators

  • UNION: Combines distinct rows from both queries (automatically eliminates duplicates).
  • UNION ALL: Combines all rows from both queries (retains duplicate rows for max speed).
  • INTERSECT: Returns only rows present in both query result sets.
  • EXCEPT (MINUS): Returns rows from the first query that do not exist in the second.
SELECT building_name FROM buildings
UNION
SELECT building FROM employees WHERE building IS NOT NULL;

2. Set Operation Compatibility Rules

To perform a set operation, both SELECT statements must return the exact same number of columns in the same order, and corresponding columns must have compatible data types.

🗄️ Schema Explorer SELECTABLE

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🎯 Exercises Checklist:

☐ 1. Find all unique building names from both buildings and employees tables using UNION
☐ 2. Find buildings listed in buildings table that have no employees assigned using EXCEPT
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