sname rating 8 .forward Relational Algebra Operator Precedence Sample Query 0 Example Schema bid 103 sname( ( Sailors) Relational Algebra Queries

Transcription

1 .forward Please put your preferred address in.forward file of your login directory at cs.umb.edu, for example: Relational Algebra Queries cat >.forward <control-d> Then to will be forwarded to allowing us to use the class list In particular, I will use this list to announce new homework, corrections to it, etc. <control-d> is a single character produced on the keyboard by depressing the control key and then the D key. C430/630 Lecture 3 lides based on Database Management ystems 3 rd ed, Ramakrishnan and Gehrke Relational Algebra Basic operations: election elects a subset of rows from relation Projection Deletes unwanted columns from relation Cross-product Allows us to combine several relations Join Combines several relations using conditions Division A bit more complex, will cover later on et-difference Union Intersection Renaming Helper operator, does not derive new result, just renames relations and fields ( R( F), E) F contains oldname newname pairs Operator Precedence In decreasing order of priority: 1. election Projection 2. Cross-product Join 3. et-difference Intersection 4. Union Example: means not bid 103 ( ) bid 103 ( ) bid 103 Example chema ample Query 0 Find names of sailors who have ratings at least 8. Detail of sailor sid Another sailor detail sname( ( ) rating 8 sname rating8

2 Join sname rating age /10/9 dustin /12/9 6 rusty 10 Each sid in is filled out with ailor attributes ample Query 1 bid only, no Boat deta Find names of sailors who ve reserved boat #103 Detail of sailor sid sid, bid in reserves table sname(( ) ) bid 103 sname ( ( bid 103 Example chema ample Query 2 Find names of sailors who ve reserved a red boat sname( (( ( B R) ) color ' sname (( serves color ' red ' ) Re ) ample Query 2 Find names of sailors who ve reserved a red boat One way that s right: sname (( serves color ' red ' ) Re ) but this next is Wrong!: Watch out for precedence! sname color' ( sname( color' empty! ample Query 3 Find names of sailors who ve reserved a red or a green boat ( Tempboats, ( color ' red ' color ' green' sname( Tempboats ) sname ( B R ) color ' red color ' green'

3 ample Query 4 Find names of sailors who ve reserved a red and a green boat ( Tempred, (( ) Re serves sid color ' red ' ( Tempgreen, (( ) Re serves sid color ' green' sname (( Tempred Tempgreen) ) ample Query 5 Find names of sailors who ve reserved only red boats ( Tempred, (( ) Re serves sid color ' red ' ( Tempothers, (( ) sid color' sname(( Tempred Tempothers) ) Time to try it yourself Try the exercises on the handed-out sheet You can confer with neighbors this is not graded Turn in completed paper for the 3 points Lab sheet (olution) (olution posted later) Note: you need to attend class to get credit for this work it is a form of class participation. An Example of elf-joins Find sailors with maximum age No max operator in RA (QL has this) Need a trick: self join with left age smaller than right age This will list rows for all ages for left side but the max age row(s) Then use all-sailors this list An Example of elf-join: cross-product with rows eliminated by condition LeftHalf sid1 sname1 rating1 age1 sid2 sname2 rating2 age2 Join condition: left age smaller than right age An Example of elf-join: Max ages ( 1, ) ( 2, ) ( f1,2 f 2,3 f 3,4 f 4), 1 age 2. age ( LeftHalf, f TempJoin) 1, f 2, f 3, f 4 Finally, subtract the resulting left hand side from the initial relation, and you get sailors with maximum ages Final result is LeftHalf

4 More on Natural Joins Natural Joins match all same-named columns Consider two tables T1 and T2: T1(id1, attr1, city) T2(id2, id1, attr2, city) Probably want to join on id1, a key for T1 showing up in both tables But T1 and T2 have id1 and city in common, so a natural join T1 T2 matches both If we don t want non-key columns matched like this We can use a theta join with an explicit condition: T1 t1.id1=t2.id1 T2 Or project out city before one of the joins Consider the Example chema, modified to have a name attribute for two entities sid name rating age ample Query 2 on modified schema sid name rating age Find names of sailors who ve reserved a red boat Old solution: sname (( color ' red ' ) Re serves ) Returns an empty relation! It s looking for matches on name as well as bid, sid ample Query 2 on modified schema sid name rating age Find names of sailors who ve reserved a red boat Old solution for unmodified schema: returns empty table here sname (( serves color ' red ' ) Re ) Here we can project out boat names before join to name ((( B) R) ) bid color ' Another self join: Close competitors Find pairs of sailors (sids) with ratings that differ by no more than one. ( 1, ) ( 2, ) We don t want a lot of these results 28 yuppy guppy rusty 5 10 ( sid1,5 sid, 1 rating2. rating1^ rating2. rating1 TempJoin sid1, sid2 sid1 sid Another self join Find pairs of different sailors (sids) with ratings that differ by no more than one, listing each unordered pair once. ( 1, ) ( 2, ) ( sid1,5 sid, 1 TempJoin sid1, sid2 That s better! 28 yuppy guppy rusty 5 10 rating 2. rating 1^ rating 2. rating 1^ sid 2. sid sid1 sid

5 Like Query 0 Find colors of boats with names starting with C Detail of Boat Another Boat detail (assume lowercase names) (( ) color name' c'^name' d' color name' c'^name' d' Like Query 3 Find names of sailors who ve reserved a red or a green boat. List names and the boat color (two rows if the sailor rented both color boats) ( Tempboats, ( color ' red ' color ' green', ( Tempboats ) sname color