<stránka

	xmlns="https://trac.frantovo.cz/xml-web-generator/wiki/xmlns/strana"

	xmlns:m="https://trac.frantovo.cz/xml-web-generator/wiki/xmlns/makro">

	<nadpis>Doing projection and restriction using cut and grep</nadpis>

	<perex>SELECT mount_point FROM fstab WHERE type IN ('btrfs', 'xfs')</perex>

	<m:pořadí-příkladu>01000</m:pořadí-příkladu>

	<text xmlns="http://www.w3.org/1999/xhtml">

		<p>

			While reading classic pipelines involving <code>grep</code> and <code>cut</code> commands

			we must notice that there is some similarity with simple SQL queries looking like:

		</p>

		<m:pre jazyk="SQL">SELECT "some", "cut", "fields" FROM stdin WHERE grep_matches(whole_line);</m:pre>

		<p>

			And that is true: <code>grep</code> does restriction<m:podČarou>

				<a href="https://en.wikipedia.org/wiki/Selection_(relational_algebra)">selecting</a> only certain records from the original relation according to their match with given conditions</m:podČarou>

			and <code>cut</code> does projection<m:podČarou>limited subset of what <a href="https://en.wikipedia.org/wiki/Projection_(relational_algebra)">projection</a> means</m:podČarou>.

			Now we can do these relational operations using our relational tools called <code>relpipe-tr-grep</code> and <code>relpipe-tr-cut</code>.

		</p>

		<p>

			Assume that we need only <code>mount_point</code> fields from our <code>fstab</code> where <code>type</code> is <code>btrfs</code> or <code>xfs</code>

			and we want to do something (a shell script block) with these directory paths.

		</p>

		<m:pre jazyk="bash"><![CDATA[relpipe-in-fstab \

	| relpipe-tr-grep 'fstab' 'type' '^btrfs|xfs$' \

	| relpipe-tr-cut 'fstab' 'mount_point' \

	| relpipe-out-nullbyte \

	| while read -r -d '' m; do

		echo "$m";

	done]]></m:pre>

		<p>

			The <code>relpipe-tr-cut</code> tool has similar syntax to its <em>grep</em> and <em>sed</em> siblings and also uses the power of regular expressions.

			In this case it modifies on-the-fly the <code>fstab</code> relation and drops all its attributes except the <code>mount_point</code> one.

		</p>

		<p>

			Then we pass the data to the Bash <code>while</code> cycle.

			In such simple scenario (just <code>echo</code>), we could use <code>xargs</code> as in examples above,

			but in this syntax, we can write whole block of shell commands for each record/value and do more complex actions with them.

		</p>

		<h2>More projections with relpipe-tr-cut</h2>

		<p>

			Assume that we have a simple relation containing numbers:

		</p>

		<m:pre jazyk="bash"><![CDATA[seq 0 8 \

	| tr \\n \\0 \

	| relpipe-in-cli generate-from-stdin numbers 3 a integer b integer c integer \

	> numbers.rp]]></m:pre>

		<p>and second one containing letters:</p>

		<m:pre jazyk="bash"><![CDATA[relpipe-in-cli generate letters 2 a string b string A B C D > letters.rp]]></m:pre>

		<p>We saved them into two files and then combined them into a single file. We will work with them as they are a single stream of relations:</p>

		<m:pre jazyk="bash"><![CDATA[cat numbers.rp letters.rp > both.rp;

cat both.rp | relpipe-out-tabular]]></m:pre>

		<p>Will print:</p>

		<pre><![CDATA[numbers:

 ╭─────────────┬─────────────┬─────────────╮

 │ a (integer) │ b (integer) │ c (integer) │

 ├─────────────┼─────────────┼─────────────┤

 │           0 │           1 │           2 │

 │           3 │           4 │           5 │

 │           6 │           7 │           8 │

 ╰─────────────┴─────────────┴─────────────╯

Record count: 3

letters:

 ╭─────────────┬─────────────╮

 │ a  (string) │ b  (string) │

 ├─────────────┼─────────────┤

 │ A           │ B           │

 │ C           │ D           │

 ╰─────────────┴─────────────╯

Record count: 2]]></pre>

		<p>We can put away the <code>a</code> attribute from the <code>numbers</code> relation:</p>

		<m:pre jazyk="bash">cat both.rp | relpipe-tr-cut 'numbers' 'b|c' | relpipe-out-tabular</m:pre>

		<p>and leave the <code>letters</code> relation unaffected:</p>

		<pre><![CDATA[numbers:

 ╭─────────────┬─────────────╮

 │ b (integer) │ c (integer) │

 ├─────────────┼─────────────┤

 │           1 │           2 │

 │           4 │           5 │

 │           7 │           8 │

 ╰─────────────┴─────────────╯

Record count: 3

letters:

 ╭─────────────┬─────────────╮

 │ a  (string) │ b  (string) │

 ├─────────────┼─────────────┤

 │ A           │ B           │

 │ C           │ D           │

 ╰─────────────┴─────────────╯

Record count: 2]]></pre>

		<p>Or we can remove <code>a</code> from both relations resp. keep there only attributes whose names match <code>'b|c'</code> regex:</p>

		<m:pre jazyk="bash">cat both.rp | relpipe-tr-cut '.*' 'b|c' | relpipe-out-tabular</m:pre>

		<p>Instead of <code>'.*'</code> we could use <code>'numbers|letters'</code> and in this case it will give the same result:</p>

		<pre><![CDATA[numbers:

 ╭─────────────┬─────────────╮

 │ b (integer) │ c (integer) │

 ├─────────────┼─────────────┤

 │           1 │           2 │

 │           4 │           5 │

 │           7 │           8 │

 ╰─────────────┴─────────────╯

Record count: 3

letters:

 ╭─────────────╮

 │ b  (string) │

 ├─────────────┤

 │ B           │

 │ D           │

 ╰─────────────╯

Record count: 2]]></pre>

		<p>All the time, we are reducing the attributes. But we can also multiply them or change their order:</p>

		<m:pre jazyk="bash">cat both.rp | relpipe-tr-cut 'numbers' 'b|a|c' 'b' 'a' 'a' | relpipe-out-tabular</m:pre>

		<p>

			n.b. the order in <code>'b|a|c'</code> does not matter and if such regex matches, it preserves the original order of the attributes;

			but if we use multiple regexes to specify attributes, their order and count matters:

		</p>

		<pre><![CDATA[numbers:

 ╭─────────────┬─────────────┬─────────────┬─────────────┬─────────────┬─────────────╮

 │ a (integer) │ b (integer) │ c (integer) │ b (integer) │ a (integer) │ a (integer) │

 ├─────────────┼─────────────┼─────────────┼─────────────┼─────────────┼─────────────┤

 │           0 │           1 │           2 │           1 │           0 │           0 │

 │           3 │           4 │           5 │           4 │           3 │           3 │

 │           6 │           7 │           8 │           7 │           6 │           6 │

 ╰─────────────┴─────────────┴─────────────┴─────────────┴─────────────┴─────────────╯

Record count: 3

letters:

 ╭─────────────┬─────────────╮

 │ a  (string) │ b  (string) │

 ├─────────────┼─────────────┤

 │ A           │ B           │

 │ C           │ D           │

 ╰─────────────┴─────────────╯

Record count: 2]]></pre>

		<p>

			The <code>letters</code> relation stays rock steady and <code>relpipe-tr-cut 'numbers'</code> does not affect it in any way.

		</p>

	</text>

</stránka>