[DMDB] relational model, relational algebra, SQL very basics

2026-07-27 21:29:09 +02:00 · 2026-06-22 15:28:27 +02:00
parent cef70242ce
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+\subsection{Operations}
+\begin{tables}{ll}{Keyword                    & Description}
+              \texttt{SELECT val, val2}       & returns the columns \texttt{val} and \texttt{val2}                      \\
+              \texttt{FROM table d, table2 c} & selects the target tables. Can use range variables (\texttt{d, c} here) \\
+              \texttt{WHERE}                  & Filter results. Can also target non-acquired columns                    \\
+              \texttt{DISTINCT}               & Returns elements without duplicates                                     \\
+              \texttt{IN}                     & Checks if the element is present in subquery                            \\
+              \texttt{UNION}                  & Set union (also removes duplicates)                                     \\
+              \texttt{INTERSECT}              & Set intersection                                                        \\
+              \texttt{EXCEPT}                 & Set difference                                                          \\
+              \texttt{JOIN}                   & Join tables, always requires a ON clause                                \\
+              \texttt{ON}                     & Specify on which column with what condition to join the tables          \\
+              \texttt{DATE}                   & Used to do comparisons against fixed Dates (followed by Datestring)     \\
+              \texttt{AS}                     & Rename a column                                                         \\
+              \texttt{AVG(column)}            & Computes the average of the specified column                            \\
+              \texttt{SUM(column)}            & Computes the sum of the specified column                                \\
+\end{tables}
+
+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;}
+
+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}
+
+We can compute the number of all passed grades for a person using something like
+\mint{sql}+SUM(CASE Grade >= 1 THEN 1 ELSE 0 END) AS PassedSubjectsCount+