Let R(D,E,F) be a relational schema. Determine for each of the following equalit
ID: 3677828 • Letter: L
Question
Let R(D,E,F) be a relational schema. Determine for each of the following equalities whether both sides have the same result. Justify the reply. i. _D,E(SIGMA_(D) 10 AND F=7)(_D,E,F (SIGMA_D > 15 (R)))) = _D,E(sigma_(D> 15 AND F=7)(R)) ii. _D,E(sigma_(D> 10 OR F=7)(_D,E,F (sigma_D> 15 (R)))) = _D,E(sigma_(D> 15 OR F=7) (R)) 2. Consider the following relational schema; the schema describes a database containing data about theses carried out by students of a master degree. Theses have supervisors and co-supervisors. The underlined attributes denote the primary keys of the relations. Students(S#, FName, LName, Street, City, Phone#) Theses(T#, Title,Topic,Profit, S#,Evaluation) Professors(Prof#, LName, Rank,Phone#, Dept#) Theses-Co-Supervisors(T#,Prof#) a. Determine for each relation, the foreign keys. b. Based on the primary and foreign keys of the relations, answer the following questions (justify your answers): Can a professor supervise more than one thesis? Can a thesis have multiple supervisors? Can a thesis have multiple co-supervisors? 3. Consider the relational schema in question (2) above. Discuss if and when _titleTheses _title(Theses Theses-Co-Supervisors) 4. Express the following queries in relational algebra a. Determine the topic of the thesis by student Bob Rossi. b. Determine the name of the co-supervisor(s) of the thesis by student Bob Rossi. c. Determine if there is any professor that has supervised no thesis.Explanation / Answer
1)
SELECT
The SELECT opeartor is (sigma) symbol and used as an expression to choose tuples that meet the selection condition…
<Selection condition>(R)
PROJECT
The PROJECT operator in RA is (pi) symbol used to choose attributes from a relation.
<attribute list>(R )
i. In the given relation, there are two parts. Right Hand Side Expression and Left Hand Side expression.
In the L.H.S first select the values of D, E, F where D>15 and it is the input to the remaining part of the relation. The R.H.S is the relation in a single expression i.e D>15 AND F=7. Both L.H.S and R.H.S produce the same results.
ii. In the L.H.S first select the values of D, E, F where D>15 and it is the input to the remaining part of the relation. The R.H.S is the relation in a single expression i.e D>15 OR F=7. Both L.H.S and R.H.S produce the same results.
2)
i.
S# is the Primary Key in Student table and Foreign key in These table.
T# is the Primary key in These table and Foreign key in Theses-Co-Supervisors table.
Prof# is the Primary key in Professor table and Foreign key in Theses-Co-Supervisors table.
ii.
Prof# is the Primary key in Professor table and Foreign key in Theses-Co-Supervisors table.
T# is the Primary key in These table and Foreign key in Theses-Co-Supervisors table.
Therefore it may be possible to handle multiple theses by professors.
Every theses have professor but not multiple professors.
Theses does not have multiple Co-supervisors.
3)
Joins are used to join the tables depends on the given condition.
The given relation is true when the Thesis handles multiple Supervisors. Other wise both relations are equal.