1. 6001 structure & interpretation of computer programs recitation 8/ october 10, 1997

2. overview today’s ideas relational databases
relational operators and queries
simple query optimization
10/31/2019 daniel jackson

3. history origins 1960’s: network and hierarchical databases
1970: Codd invents the relational model; revolutionizes the field
early 1980’s: IBM starts DB2, Oracle founded
growth
late 1980’s: RDBs become big business: Oracle, Sybase, Ingres, Informix, etc
1997: Oracle’s revenue is $5B, total RDB market about $10-15B
future
object-oriented databases unlikely to take off
but hybrid ‘object-relational databases’ are starting to get popular
10/31/2019 daniel jackson

4. what’s a database? what does a database do?
stores large volumes of data
allows rapid access for many different kinds of query
databases includes mechanisms for handling
secondary storage management: indexing and storage of data that doesn’t fit in memory
persistence: data shouldn’t go away!
concurrency control: many users at once
distribution: users at different sites
data protection: making sure updates are consistent
recovery: when system fails, eg
the difference between DB systems and file systems
programmer doesn’t have to worry about how data is laid out on disk (sometimes called the ‘data independence principle’)
10/31/2019 daniel jackson

5. the big idea
what was wrong with early databases (network and hierarchical)
the user’s operations were coded directly in terms of data structures
these data structures closely mirrored the way the data was actually stored
when developer changed the data structures for performance, the queries had to be rewritten
the solution
have an abstract data model
queries and insertions all expressed in terms of abstract model
hide the details of the actual data structures from the user
in other words…
data abstraction!
10/31/2019 daniel jackson

6. tables what’s in a relational database
an RDB consists of a collection of tables or relations
each table has a name
columns have names too (called attributes)
each row is a sequence of values one for each column
these may be numbers, booleans, strings, etc
rows are also called tuples or records
example
EVENTS table
EVENT CAFEMEISTER HOURS
java-jump ben 10
tour-de-café alice 15
seattle-sleepless carol 8
ATHLETES table
NAME SPONSOR
alice coffee-connection
ben starbucks
carol green-mountain
10/31/2019 daniel jackson

7. queries to extract information from the RDB present it with a query
examples
who won the java-jump event?
what company is sponsoring ben?
which company sponsored the winner of the java-jump event?
kinds of query
some queries can be answered by examining only one table
who won the java-jump event? –> look at EVENTS table
what company is sponsoring ben? –> look at ATHLETES table
some need more than one table
–> look up winner in EVENTS and then find sponsor in ATHLETES
query processing
queries are declarative: they say what you want, not how to get it
database system translates query into operation on internal data structures
may optimize the query before applying, so that it goes faster
10/31/2019 daniel jackson

8. queries on a single table
for uniformity
think of result of a query as a table itself
two ways to make a table smaller
eliminate some columns
eliminate some rows
eliminating columns
query
select <column names> from <table>
example
select CAFEMEISTER, HOURS from EVENTS
CAFEMEISTER HOURS
ben 10
alice 15
carol 8
10/31/2019 daniel jackson

9. simple queries, continued
eliminating rows
query
filter <condition> from <table>
example
filter NAME = ben from ATHLETES
NAME SPONSOR
ben starbucks
operators
filter and select are called relational operators
they are like the operations of an abstract data type
take a table and produce another table
10/31/2019 daniel jackson

10. combining queries example who won the java-jump event?
want a table containing only the name of the winner
so only column is CAFEMEISTER
want to restrict rows so that EVENT is java-jump
query:
select CAFEMEISTER from
filter EVENT = java-jump from
EVENTS
could do it the other way round too
other examples
who sponsors alice?
which athletes did more than 9 hours?
whom does starbucks sponsor?
10/31/2019 daniel jackson

11. relational join relational join
given two tables T1 and T2 with columns a1, b1, c1, … and a2, b2, c2, …
make a new table with columns a1, b1, c1, … a2, b2, c2, …
for each row R1 in T1 and each row R2 in T2, make a new row R1 R2
how many rows if each table has k rows? k^2
assume that column names of T1 and T2 are disjoint
10/31/2019 daniel jackson

12. example of a join join EVENTS and ATHLETES
EVENT CAFEMEISTER HOURS NAME SPONSOR
java-jump ben alice coffee- connection
tour-de-café alice alice coffee- connection
seattle-sleepless carol 8 alice coffee- connection
java-jump ben ben starbucks
tour-de-café alice ben starbucks
seattle-sleepless carol 8 ben starbucks
java-jump ben carol green-mountain
tour-de-café alice carol green-mountain
seattle-sleepless carol 8 carol green-mountain
10/31/2019 daniel jackson

13. queries involving two tables
now we can formulate a query on two tables by
joining the tables together into a single table
applying filter and select to the new table
example
which company sponsored the winner of the java-cup event?
query is
select SPONSOR from
filter EVENT = java-jump from
filter CAFEMEISTER = NAME from
join EVENTS and ATHLETES
before filter, we have
EVENT CAFEMEISTER HOURS NAME SPONSOR
java-jump ben ben starbucks
final table (result of query) is
SPONSOR
starbucks
10/31/2019 daniel jackson

14. more example queries exercises
which events did coffee-connection sponsor the winner of?
who sponsored an athlete who did more than 12 hours?
10/31/2019 daniel jackson

15. query optimization note join is expensive!
want to apply it to small tables whereever possible
could have done one of the selects before the join first
instead of
select SPONSOR from
filter EVENT = java-jump
filter CAFEMEISTER = NAME from
join EVENTS and ATHLETES
we could write
join ATHLETES and
filter EVENT = java-jump from EVENTS
now join gives table with only 3 rows instead of 9
we’d like
to write the query either way, and have the database figure out a better way to write it
this is called query optimization: can be expressed with simple rules
10/31/2019 daniel jackson

16. join of tables with shared column names
convenient to
use same names for related columns
for example, CAFEMEISTER in EVENTS and NAME in ATHLETES could both be NAME
refine the meaning of join
can’t have two columns with same name so
form a table with both columns
eliminate rows with different values in the two columns
merge the two columns
example
assume CAFEMEISTER column in EVENTS is now called NAME
join of EVENTS and ATHLETES:
EVENT NAME HOURS SPONSOR
tour-de-café alice 15 coffee- connection
java-jump ben 10 starbucks
seattle-sleepless carol 8 green-mountain
10/31/2019 daniel jackson

17. query optimizations which of these rules are valid?
filter C1 from filter C2 from T = filter C1 and C2 from T
select COLS1 from select COLS2 from T = select COLS1 U COLS2 from T
under what conditions are these valid?
select COLS from (join of T1 and T2) = join (select COLS from T1) and (select COLS from T2)
filter C from (join of T1 and T2) = join of (select C from T1) and T2
10/31/2019 daniel jackson

18. puzzles for student presentations
procedures with variable length argument lists
find out from the R4RS manual how to write a procedure that can take a variable number of arguments
make up an entertaining procedure that exploits this feature
powerlists
define a procedure that takes a list and returns a list of all the sublists
for example, (p (list 1 2 3)) should return a list containing () (1) (2) (3) (1 2) (1 3) (2 3) (1 2 3) in some order
use map!
10/31/2019 daniel jackson