Recent questions tagged databases

Consider the following schedule. $T1$ $T2$ $T3$ $R(Y)$ $R(X)$ $R(X)$ $W(Y)$ $R(Z)$ $W(Z)$ $R(X)$ denotes the read operation on data item $X$ by transaction $T_{i}$. W(X) denotes the write operation on data item $X$ by transaction $T_{i}$.how many conflict serializable schedule are possible for the above schedule? 6 5 3
Consider the relation employee(name,sex,manager) with name as the key, manager gives the name of the supervisor of the employee under consideration. What does the following Tuple Relational Calculus query produce? A. Names of employees who have a female subordinate B. Names of ... one female manager How to interpret this TRC expression? The y.name = e.manager =>x.sex=female part is troublesome.
In translating from an entity-relationship (E-R) diagram to a relational schema, one piece of E-R logic that cannot be captured by primary keys, uniques, and foreign keys is (A) the weak entity. (B) any ternary relationship. (C) mandatory participation for one-time occurrence (that is, with the arrow). (D) mandatory participation for many-time occurrence (that is, without the arrow).
Consider the following entity-relationship (ER) model For the following group of statements, which one does not coincide (Or not true) with the model ? (A) Each person has at most one e-mail address. (B) Each e-mail address belongs to exactly one person. ... 1 passport number? eg I can assign 3 passport numbers to the same person Pinkman. Passport Number Name 101 Pinkman 102 Pinkman 103 Pinkman
Consider the following table Mystery A B 1 null 2 4 3 5 null 6 select count() from Mystery where A not in( select B from Mystery) What is the output of this query? (A) 1 (B) 0 (C) 4 (D) 3
im getting 5, but answer is not 5
Can someone explain me group by and having clause in a clear way.
1 vote
What is the minimum number of tables needed in the relational model? 2 3 4 5
Consider a relation R(A,B) where A is the primary key and B is a foreign key which references the same relation. Consider the following relation instance of R. A 2 3 5 7 4 3 B 4 4 2 2 5 4 Which of the following tuple to be deleted first to preserve the referential integrity constraint when the tuple (2,4) is deleted? (2,4) (5,2) (7,2) (4,5) p.s The obvious answer (5,2) is given wrong.
'A' is set of all possible schedules 'C' is set of all possible schedules that are guaranteed to produce a correct final result 'S' is the set of all serializable schedules 'P' is the set of all schedules possible under 2-phase locking protocol Which is FALSE? (A) P⊆C (B) S⊂P (C) S⊆P (D) P⊂C
Is there any difference between lossless decomposition and join dependency or both are same. both definition has same criteria. https://en.wikipedia.org/wiki/Join_dependency https://en.wikipedia.org/wiki/Lossless-Join_Decomposition Thanks
ENo SSN 1 15 2 20 3 15 4 null 5 null Relation R SELECT SSN,count(*) FROM R GROUP BY SSN Will the output be: SSN count 15 2 20 1 null 2 SSN count 15 2 20 1 null 1 null 1 SSN count 15 2 20 1
Consider the following ER diagram. Assume that there are no multivalued attributes present in any of E1,E2,R and each entity set has a key. What is the minimum number of tables required to convert this ER diagram into relations when: (i) E1 has total participation in R (ii) E2 has total participation in R (iii) Both E1 and E2 have total participation in R
What do you mean by Seek? For Natural Loop Join, we calculate No. of Seeks are (nr + br). Assume that, Relation r is called the Outer Relation and Relation s is called the Inner Relation. (ni,bj) – ni represents total no. of tuples and bj represents no. of blocks in that relation. Reference (Korth) – For Block Nested-Loop Join –
From this link How The last query is safe?
what does this statement means? “In TO the schedule is equivalent to the particular serial order corresponding to the order of the transaction timestamps”
I In the above question whether my solution is correct ? If not can anyone please explain with an example
Consider the following FDs : Sid → Sname Sname → Age The above set of FD is not - non trivial. I knw da definition of non - trivial in FD is that there should be no common element between X and Y attribute sets in the FD X → Y. The above set of FD is not non ... X and Y have something in common that is Sname. That is why the set of FDs is not non-trivial. Is my perception towards the FDs correct ?