4.3: Projection Query

Last updated
Save as PDF

Page ID: 15512

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

Next we build a query that displays a subset of the columns of a table. Suppose we need to produce a listing of call numbers and titles. Proceed as in the previous example so that the Book table is shown in the Relationships area. Now, double-click the callNo and title fields:

Figure \(\PageIndex{1}\): Query showing specific fields.

The definition of the query is now complete. The grid area indicates the fields involved, and both fields will be displayed because each has a check mark on the Show line. Only fields checked on the Show line are displayed in the results. Running this query yields:

Table \(\PageIndex{1}\): Query results.
callNo	Title
CB 351 M293 1983	Atlas of medieval Europe
HQ 1143 P68 1975	Medieval women
PC 14 V48 1965	Medieval miscellany
QA 76.73 S67C435 2004	Joe Celko's Trees and hierarchies in SQL for smarties
QA 76.73 S67C46 1997	Joe Celko's SQL puzzles & answers
QA 76.76 A65P76 2011	Programming Android
QA 76.9 D26H355 2008	Information modeling and relational databases
QA 76.9 D26H39 1996	Data model patterns : conventions of thought
QA 76.9 D35C45 1999	Joe Celko's data & databases : concepts in practice
R 141 E45 2006	Medieval medicine and the plague
R 487 T35 1967	Medicine in medieval England.

Save the query. A projection query, because it displays a subset of the fields in the table, is said to produce a vertical slice of the table.