Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>"How do I represent my query in this standard form of RA?"</p> <p>It's not so much a question of "type of algebra" as it is of "type of notation".</p> <p>Notation using greek symbols typically uses sigma, the restrict condition in subscript appended to the sigma character, and then the subject of the restriction (the relational expression that is subjected to the restrict condition).</p> <p>Date avoid that notation, because typesetting and/or creating text using such notations is usually a lot harder than it is using just the western alphabet (a math teacher of mine once told us that math textbooks contain the most errors of all).</p> <p>σ &lt;cond&gt; (&lt;rel exp&gt;) thus denotes the very same algebra expression as (Date's syntax) "&lt;rel exp&gt; WHERE &lt;cond&gt;".</p> <p>Similarly, with greek symbols, projection is typically denoted using the letter Pi, with the list of retained attributes in subscript appended to the Pi, and the expression that is the subject of the projection following that.</p> <p>Π &lt;attr list&gt; (&lt;rel exp&gt;) thus denotes the very same algebra expression as (Date's syntax) "&lt;rel exp&gt; { &lt;attr list&gt; }".</p> <p>The join family of operators is usually denoted, in "greek" symbols, using (variations of) the Unicode BOWTIE character, or that character consisting of a lowercase letter 'x' surrounded by a full circle (usually used to denote full cartesian product, cross-product, ... whatever your algebra course happens to name it).</p> <p>Some courses provide a "greek-symbol" notation for rename, using the greek letter Rho. Appended in subscript is the rename list, in the form a1->b1,a2->b2,... Appended after that comes the relational expression that is subjected to the rename. Likewise, Date has a non-greek-symbol equivalent syntax : &lt;rel exp&gt; RENAME a1 AS b1, a2 AS b2 , ...</p> <p>The important thing is to see that these differences are merely differences <em>in syntactical notation</em>, not "different algebrae".</p> <p>EDIT</p> <p>One could imagine that the greek symbols notation would be the way to program relational algebra into an APL engine, Date's syntax would be the way to program relational algebra into a cobol-like or PL/1-like engine (there effectively exists such an engine called Rel), and the way to program relational algebra into an OO-like engine, could look something like relation.NaturalJoin(otherRelation).Matching(yetOtherRelation.Restrict(condition).project(attributesList)).</p>
    singulars
    1. This table or related slice is empty.
    1. POConvert SQL Query to Relational Algebra
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USErwin Smout
    1. USErwin Smout
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COOk, thanks for the answer - very informative. Could you show me how to rewrite the query that you gave in Tutorial D using the Greek characters? The crux of my problem is how to represent the MATCHING part of the query. Thanks for all the help.
      singulars
      1. PO
      1. USuser1022788
    2. COMATCHING is just the SEMIJOIN operator. See e.g. http://en.wikipedia.org/wiki/Relational_algebra#Semijoin_.28.E2.8B.89.29.28.E2.8B.8A.29 . I do try to live up to a policy not to spoonfeed complete solutions. (BTW the Tutorial D query was given not by me, but by @onedaywhen.)
      singulars
      1. PO
      1. USErwin Smout
    3. COI admit this is an area I am not clear on. Consider this quote from Darwin: "Sometimes that term, relational algebra, is used with the definite article: *the* relational algebra, even though several minor variations exist in the literature. Indeed, the term relational completeness is sometimes defined with reference to “the” relational algebra -- a language deemed relationally complete if it supports, directly or indirectly, all of the operators of "that" algebra."
      singulars
      1. PO
      1. USonedaywhen
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload