Group by and aggregations
You decide to take another look at countries with missing population data. You join the World Bank demographics data to the Olympic oregions data.
You want to look at trends so group countries regionally to see how many countries, by region, are missing population data. You know you want to view the results by Olympic region and count the countries with population data using the country code olympic_cc.
Select Run Code to view the results. Did you get an error?
Read the red error message then follow the instructions below to correct the query.
Deze oefening maakt deel uit van de cursus
Improving Query Performance in PostgreSQL
Oefeninstructies
- Read the red error output.
- Change the non-aggregated column in the
SELECTclause to match theGROUP BYcolumn .
Praktische interactieve oefening
Probeer deze oefening eens door deze voorbeeldcode in te vullen.
SELECT reg.country, COUNT(DISTINCT dem.olympic_cc)
FROM regions reg -- Olympics region data
LEFT JOIN demographics dem -- World Bank population data
ON dem.olympic_cc = reg.olympic_cc
GROUP BY reg.region;