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29 lines
2.0 KiB
TeX
29 lines
2.0 KiB
TeX
\subsection{Query Language}
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The basic structure of a \sql\ statement is \texttt{SELECT I FROM T WHERE C},
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which corresponds to $\Pi_{\texttt{I}}(\sigma_{\texttt{C}} \texttt{T})$.
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We can set \texttt{I = *} if we want to return all columns for a table.
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To rename, we can set \texttt{I = Column as Name, Column2 as Name2}, etc
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\inlinetheorem Every SPJR \acrshort{ra} expression can be written in \texttt{SELECT ... FROM ... WHERE ...} form:
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\begin{tables}{lll}{Operation & Notation & SQL}
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Selection & $\sigma_c(R)$ & \texttt{SELECT * FROM R WHERE c;} \\
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Projection & $\Pi_{A_1, \ldots, A_n} R$ & \texttt{SELECT A1, \ldots, An FROM R;} \\
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Cartesian Product & $R_1 \times R_2$ & \texttt{SELECT * FROM (R1, R2);} \\
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Rename & $\rho_{a, b, c} R$ & \texttt{SELECT A as a, \ldots, B as c FROM R;} \\
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Union & $R_1 \cup R_2$ & \texttt{R1 UNION R2;} \\
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Difference & $R_1 - R_2$ & \texttt{R1 EXCEPT R2;} \\
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Intersection & $R_1 - R_2$ & \texttt{R1 INTERSECT R2;} \\
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\end{tables}
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Since \sql\ implements \gls{bag} and not set semantics, there is a \texttt{DISTINCT} keyword, which is used to remove duplicates.
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It is to be applied in the \texttt{SELECT} clause (\texttt{I} above), e.g.
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\mint{sql}|SELECT DISTINCT name FROM Data;|
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For the last three operations, \texttt{R1} and \texttt{R2} typically are other \sql\ statements, example:
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\mint{sql}|(SELECT name FROM FirstTable) UNION (SELECT name FROM SecondTable)|
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In addition, to apply \textit{bag semantics}, as opposed to \textit{set semantics} append \texttt{ALL} to the expression (e.g. \texttt{UNION ALL})
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We can also name the tables, e.g. $\texttt{T} = \texttt{One o, Two t}$, and then access columns from a specific table using
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$\texttt{I} = \texttt{o.Col, b.Col}$, or the like.
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