#notes# database chapter 2

1.terminologies:

attributes(column in table)　　table(relation)　　domain(permitted value)

tuple(row)　　　　　　　　

database shema && database instance    --------------->>>>　　type definition  && variable

2.key

superkey (can uniquely mark a tuple in a relation)  ---->>>>  candidate key (its subset cannot be superkey) 

---->>>>primary key(designer chosen)

 

3.relational-Algebra operation

a. select 　　b.project　　

c.union: here are two conditions to hold for validaty 

i. the same number of attributes(the same arity);　　ii.corresponding attribute is the same

d. - (con)　　e. ×　　f.rename

g.∩　　h.natural join(theta)　i.÷(condition) 　　j.assignment <- 　

 

~Extended~　　Generalized projection　　　　aggregate function(max, min, count, count-distinct, avg, sum...)

　　　　　　　　 outer join(left, right, full)　　  null values　　

 

~Database modification~

　　　　　　　　deletion(set difference)　　updating(select, projection)　　　　insertion(union)

 

just a daily review or extraction  : P