Tag: relational algebra
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Distribution, commutation and association in database queries
Another reminder note. The RDBMS query engine relies on these laws to build efficient queries: Commutation: Numbers may travel (commute) or in relational algebra Association: Numbers may freely associate or in relational algebra Distribution: Operators can be distributed or, in relational algebra A more general statement of these laws: Commutative law: Associative law: Distributive law: […]
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Solving a relational query: part 5
Found: the relational algebra to tackle the problem outlined in part 3. This is based on a solution to a similar problem found in Database Management Systems, Third Edition, though, unfortunately, that book prints a solution that is not relational algebra at all, as it relies on the order in which results are returned (relational […]
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Solving a relational query: part 4
As Paul Rubin pointed out the SQL in the last post was broken – here’s some that, I think (please do not let it be wrong a second time!), by relying on SQL’s coercion of a tuple to a scalar, would work (Paul proposed an even more ambitions way of doing this in a single […]
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Solving a relational query: part 3
This is starting to depress me, now, because I thought I was full on top of all this material until I tried this question. So, going back to the database specified in part 1 – I now need to find the relational algebra (not the SQL, I have done that) for: “find the name and […]
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Solving a relational query: part 2
With thanks to Professor Paul A. Rubin, I think this may be the way to solve this: This joins LOT to a relation with one attribute (LOT_ID) made up from tuples where LOT_ID should only be returned where no bids have been made that are equal or greater than the reserve price. Very difficult part […]
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Solving a relational query: part 1
I have not found the answer to this, so thought I’d go through the steps. Assume we have a relational database with these tables: USER (USER_ID, NAME, EMAIL) LOT (LOT_ID, SLLER_ID, TYPE, DESCRIPTION, RESERVE_PRICE, START_DATE, END_DATE) BID (BID_ID, LOT_ID, BIDDER_ID, BID_DATE, AMOUNT) FEE (LOT_ID, AMOUNT) What is the relational algebra that finds “the description and […]
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Relational division again
Take the same relational database tables as before: RESTAURANT(RT_ID, NAME, TYPE, LOCATION) PROXIMITY(RT1_ID, RT2_ID, DISTANCE) USER(U_ID, NAME, EMAIL) REVIEW(U_ID, RT_ID, RT_DATE, RATING, COMMENT) Find the names of the reviewers who reviewed all the restaurants in Earlsfield. In relational algebra: But what about SQL? As I wrote of relational division when I started the blog: one […]
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Relational algebra problem
Have a relational database with the following tables RESTAURANT(RT_ID, NAME, TYPE, LOCATION) PROXIMITY(RT1_ID, RT2_ID, DISTANCE) USER(U_ID, NAME, EMAIL) REVIEW(U_ID, RT_ID, RT_DATE, RATING, COMMENT) What’s the relational algebra for “name of any user who has never given Chez Brian a rating lower than 5”? Is this right? Related Articles CoSQL: SQL and noSQL are really just […]
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Basic symbols of relational algebra
Apologies in advance but this blog might start to look like online revision notes (and a LaTeX scratchpad) for the next six weeks. Projection Selection Rename Conditional Join Natural Join (Page 100 onwards of Database Management Systems, Third Edition)
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Relational division in SQL
In 1970 EF Codd proposed the relational model for databases. Although other and older models exist and are in use (primarily in the legacy world), relational databases are the dominant form. Well, you know all that already – probably. You probably also know that most database management systems make the relational model available to end […]
cartesian product ... stuff about computing mostly 