PostgreSQL_Notes

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Union and Intersections with Sets

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Handling Sets with Union

• UNION will append two or more queries together

  • The queries are wrapped in parenthesis with the UNION keyword in the middle.
    

  • By default, UNION will not show duplicates

    • To show duplicate values use UNION ALL

A Few Notes on Union

• Parenthesis are not strictly necessary, but are good practice.
• We are only allowed to use the UNION keyword where the results of the queries will have the same columns.

Commonalities with Intersect

• INTERSECT - Will give us the common results between two queries.
  

• INTERSECT ALL - Will give us the common results between two queries and show any duplicates as long as those duplicates exist in both results prior to intersection.

Removing Commonalities with Except

• EXCEPT - Finds the rows that exist in the first result set that DO NOT exist in the second result set.
  • Order matters for EXCEPT
    • We only care about the rows in the FIRST result set. Any rows from the second set that do not exist in the first result set do not matter.
      
• EXCEPT ALL - EXCEPT but does not remove duplicates.

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