UNION and UNION ALL
You want a query that returns all cities listed in the Earthquakes database. It should be an easy query on the Cities table. However, to be sure you get all cities in the database you will append the query to the Nations table to include capital cities as well. You will use UNION to remove any duplicate rows.
Out of curiosity, you want to know if there were any duplicate rows. If you do the same query but append with UNION ALL instead, and compare the number of rows returned in each query, UNION ALL will return more rows if there are duplicates.
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Improving Query Performance in SQL Server
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SELECT ___ AS NearCityName, -- City name column
CountryCode
FROM Cities
___ -- Append queries
SELECT ___ AS NearCityName, -- Nation capital column
Code2 AS CountryCode
FROM Nations;