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[DMDB] Notes for this week

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Janis Hutz
2026-03-07 11:37:16 +01:00
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semester4/dmdb/parts/00_sql/01_operations.tex
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@@ -17,3 +17,13 @@
We can directly add new columns to our result using statements like (also note that we do not actually return salary and employment columns) We can directly add new columns to our result using statements like (also note that we do not actually return salary and employment columns)
\mint{sql}{SELECT name, salary * employment / 100 AS RealSalary FROM employees;} \mint{sql}{SELECT name, salary * employment / 100 AS RealSalary FROM employees;}
When applying \texttt{GROUP BY}, COUNT and the like operate on each group, not entire dataset.
\texttt{NATURAL JOIN} doesn't require a ON clause (it looks for columns with same name)
After \texttt{GROUP BY} we can't use \texttt{WHERE}, instead we can use \texttt{HAVING}, which operates on groups instead of on rows.
We can compare against a query with a single result column and row using a \texttt{WHERE} clause.
The \texttt{CROSS JOIN} is the both-sided extension to \texttt{LEFT JOIN} and \texttt{RIGHT JOIN}