Please clear this confusion regarding relational algebra/tuple relational calculus -

-

1: explanation given query "the set of tuples t such there exists tuple s in relation borrow values of t , s cname attribute equal, , value of s amount attribute greater 1200". never mentioned relation t belongs. relation it, , why?

enter image description here

2: in underlying image, "and"ing 2 projections (containing attribute "customerid" "customer" relation & attribute "orderid" "order" relation) give cartesian product of these 2 one-column relations? enter image description here

1:

tuple t, appearing on left hand side, tuple of the answer. { t | ...t... } means "the set of every value, t say, such ...t... true". each tuple value refer t , if satisfies condition goes in answer.

2:

in part 1 "∧" used connective , in predicate logic expression in relational tuple calculus query. here in 2 using between 2 relation expressions relational algebra operator. "⋈" used natural join , "⨯" used cartesian product (aka cross join). there's natural join operator , if used no common attributes use/value called cartesian product.

those 2 different kinds of expression. "∧" used in both, although different things. isn't problem because between 2 predicate logic expressions it's , and between 2 relational algebra expressions it's relational algebra operator. expect natural join. haven't seen used cartesian product, remember natural join no common attributes is/returns "cartesian product". example "give cartesian product of these two".

some relational algebra variants have tuples unordered unique attribute names. typically cartesian product takes 2 relations don't have attributes in common , returns natural join. since attribute names of tuple t in calculus example aren't given order, maybe it's kind of relation.

some relational algebra variants have ordered tuples attribute names can appear multiple times. typically cartesian product returns tuples common attribute names appearing multiple times. seems case in the slides textbook silberschatz, korth & sudarshan. (which uses "∧" and.) natural join has 1 copy of common attributes cartesian join has two, can differ. in example 2 projection results have disjoint attribute sets. natural join , cartesian product return same relation. (assuming attribute order same.)

[the reason same symbol used , and natural join (and used cartesian product) if relational algebra expression r holds tuples r(...) , relational algebra expression s holds tuples s(...) r natural join s holds tuples r(...) , s(...). in fact every relational algebra expression value of corresponding relational calculus expression relation names correspond given predicates (meanings) , logic operators correspond relation operators.]

use relations, operators , symbols have been told use instructor/textbook. if in doubt define symbols or use words.


Comments

Post a Comment

Popular posts from this blog

javascript - Chart.js (Radar Chart) different scaleLineColor for each scaleLine -

-
Image
i try , create radar chart using chart.js has various colours each scaleline, or coloured between scalelines. wondering if possible? from: to: i have working graph, though there doesn't seem method change individual scale lines. kind regards leigh you can extend radar chart type this, so chart.types.radar.extend({ name: "radaralt", initialize: function (data) { chart.types.radar.prototype.initialize.apply(this, arguments); var originalscaledraw = this.scale.draw; var ctx = this.chart.ctx; this.scale.draw = function () { var linewidth = this.linewidth; // bypasses line drawing in originalscaledraw this.linewidth = linewidth; originalscaledraw.apply(this, arguments); ctx.linewidth = this.linewidth; var scale = this; // draw chart.helpers.each(scale.ylabels, function (label, index) { // color of ...
Read more

apache - Error with PHP mail(): Multiple or malformed newlines found in additional_header -

-
suddenly have started receiving above error without changes having been made script. host 1and1 (i know...) the script still works fine on different server, , suspicion there must have been server config change has lead this, although hosts plead ignorance. there's no information on above error @ in google can find - have ideas? server running apache if helps. had similar problem. came out of blue. no php code changed. what changed: php upgraded 5.5.25-1 5.5.26. a security risk in php mail() function has been fixed , newlines in additional_headers allowed no more. because newlines mean: starts email message (and surely don't want inject newlines through headers followed evil message). what have worked fine, e.g. having newlines after headers or passing whole message additional_headers , function no more. solution : sanitize headers. no multiple newlines in additional_headers argument. these count "multiple or malformed newlines":...
Read more

java - Android – MapFragment overlay button shadow, just like MyLocation button -

-
Image
i'm struggling add shadow custom overlay button on google map. goal make custom button google's mylocation button. here's how app looks now: layout.xml <relativelayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" xmlns:map="http://schemas.android.com/apk/res-auto" android:layout_height="match_parent" android:layout_width="match_parent" style="@style/apptheme"> <fragment android:layout_width="match_parent" android:layout_height="match_parent" android:id="@+id/map" tools:context=".mapsactivity" android:name="com.google.android.gms.maps.supportmapfragment" android:layout_alignparentend="true" android:layout_alignparentstart="true" android:layout_alignparentbottom="true" ...
Read more