What is the difference among ...

The SQL set operators UNION, MINUS, and INTERSECT are used to combine the results of two or more SELECT statements. Each operator has a distinct purpose and behavior:

UNION

• Purpose: Combines the results of two or more SELECT statements into a single result set.
• Behavior:
  • UNION removes duplicate rows from the result set.
  • UNION ALL includes all rows from the combined SELECT statements, including duplicates.
• Usage: Useful when you need to merge results from different queries and ensure that the final result set contains unique rows.
• Example:
  sql Copy

  SELECT column_name FROM table1
  UNION
  SELECT column_name FROM table2;

MINUS (or EXCEPT)

• Purpose: Returns the rows from the first SELECT statement that are not present in the second SELECT statement.
• Behavior:
  • Removes duplicates from the result set.
  • The result set includes only the rows that are unique to the first SELECT statement.
• Usage: Useful when you need to find differences between two result sets.
• Example:
  sql Copy

  SELECT column_name FROM table1
  MINUS
  SELECT column_name FROM table2;

INTERSECT

• Purpose: Returns only the rows that are present in both SELECT statements.
• Behavior:
  • Removes duplicates from the result set.
  • The result set includes only the rows that are common to both SELECT statements.
• Usage: Useful when you need to find common elements between two result sets.
• Example:
  sql Copy

  SELECT column_name FROM table1
  INTERSECT
  SELECT column_name FROM table2;

Key Points

• Column Compatibility: For all these operators, the number of columns and their data types in the SELECT statements must match.
• Performance Considerations: ...