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CMSC424: Database Design

CMSC424: Database Design. Instructor: Amol Deshpande amol@cs.umd.edu. Today. Mapping relational algebra to SQL Complex SQL Queries See “movies database sql queries” on class webpage for the actual queries SQL Formal Semantics Advanced SQL Features. SQL Query Examples.

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CMSC424: Database Design

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  1. CMSC424: Database Design Instructor: Amol Deshpande amol@cs.umd.edu

  2. Today • Mapping relational algebra to SQL • Complex SQL Queries • See “movies database sql queries” on class webpage for the actual queries • SQL Formal Semantics • Advanced SQL Features

  3. SQL Query Examples • Movie(title, year, length, inColor, studioName, producerC#) • StarsIn(movieTitle, movieYear, starName) • MovieStar(name, address, gender, birthdate) • MovieExec(name, address, cert#, netWorth) • Studio(name, address, presC#) • Queries: • Producer with maximum average length of movies • Find producer of Star Wars. • All producers of movies in which harrison ford stars

  4. SQL Query Examples • Movie(title, year, length, inColor, studioName, producerC#) • StarsIn(movieTitle, movieYear, starName) • MovieStar(name, address, gender, birthdate) • MovieExec(name, address, cert#, netWorth) • Studio(name, address, presC#) • Queries: • Find movie titles that appear more than once • Find number of people 3 hops away from Kevin Bacon

  5. More SQL • Select * into temptable from X1, … • Having • WHERE is to FROM what HAVING is to GROUPBY

  6. Duplicates • By definition, relations are sets • So  No duplicates allowed • Problem: • Not practical to remove duplicates after every operation • Why ? • So… • SQL by default does not remove duplicates • SQL follows bag semantics, not set semantics • Implicitly we keep count of number of copies of each tuple

  7. Formal Semantics of SQL • RA can only express SELECT DISTINCT queries • To express SQL, must extend RA to a bag algebra • Bags (aka: multisets) like sets, but can have duplicates e.g: {5, 3, 3} e.g: homes = • Next: will define RA*: a bag version of RA

  8. Formal Semantics of SQL: RA* • *p (r): preserves copies in r e.g: *city = Brighton (homes) = • *A1, …, An (r): no duplicate elimination e.g:  *cname (homes) =

  9. Formal Semantics of SQL: RA* • r* s : additive union • * = r s • r-* s: bag difference e.g: r -* s = s -* r =

  10. Formal Semantics of SQL: RA* • r* s: cartesian product = *

  11. Formal Semantics of SQL • Query: SELECT a1, ….., an FROM r1, ….., rm WHERE p • Semantics: *A1, …, An (*p (r1* … * rm) ) (1) • Query: SELECT DISTINCT a1, ….., an FROM r1, ….., rm WHERE p What is the only operator to change in (1)? • Semantics: A1, …, An (*p (r1* … * rm) ) (2)

  12. Set/Bag Operations Revisited • Set Operations • UNION ≡ U • INTERSECT ≡ ∩ • EXCEPT ≡ - • Bag Operations • UNION ALL ≡ U* • INTERSECT ALL ≡ ∩* • EXCEPT ALL ≡ -* • Duplicate Counting: • Given m copies of t in r, n copies of t in s, how many copies of t in: r UNION ALL s? A: m + n r INTERSECT ALL s? A: min (m, n) r EXCEPT ALL s? A: max (0, m-n)

  13. SQL: Summary

  14. Next • Database updates

  15. Modification of the Database – Deletion • Delete all account records at the Perryridge branch • delete from account where branch-name = ‘Perryridge’ • Delete all accounts at every branch located in Needham city. • delete from accountwhere branch-name in (select branch-name from branch where branch-city = ‘Needham’)delete from depositorwhere account-number in (select account-number from branch, account where branch-city = ‘Needham’ and branch.branch-name = account.branch-name)

  16. Example Query • Delete the record of all accounts with balances below the average at the bank. delete from accountwhere balance < (select avg (balance)from account) Problem: as we delete tuples from deposit, the average balance changes Solution used in SQL: • First, compute avg balance and find all tuples to delete • Next, delete all tuples found above (without recomputing avg or retesting the tuples)

  17. Modification of the Database – Insertion • Add a new tuple to account • insert into account values (‘A-9732’, ‘Perryridge’,1200) • or equivalentlyinsert into account (branch-name, balance, account-number) values (‘Perryridge’, 1200, ‘A-9732’) • Add a new tuple to account with balance set to null • insert into account values (‘A-777’,‘Perryridge’, null)

  18. Modification of the Database – Updates • Increase all accounts with balances over $10,000 by 6%, all other accounts receive 5%. • Write two update statements: • update account set balance = balance  1.06 where balance > 10000 • update account set balance = balance  1.05 where balance  10000 • The order is important • Can be done better using the case statement

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