In an ER diagram the ___shape specifies the Attibute and a ____ shape specifies the primary key attribute.
Answer: A
Correct answer: Oval; An oval with an underlined attribute
In an E-R diagram an attribute is drawn as an oval, and the attribute that serves as the primary key is shown by underlining its name. Rectangles are entities and diamonds relationships.
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Change Staff table by removing default of Assistant for position column
Answer: A
Correct answer: ALTER TABLE Staff ALTER position DROP DEFAULT;
Removing a default is a change to the column, so the statement is ALTER TABLE, then ALTER on the named column, then DROP DEFAULT. Omitting the column name leaves the clause with nothing to act on, and MODIFY is not the keyword that begins the statement.
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The _____ category includes storage media that can be operated on directly by the computers central processing unit.
Answer: A
Correct answer: Primary storage
Primary storage - main memory and cache - is the storage the CPU can address directly. Secondary storage such as disk must be read into primary storage before the processor can work on it.
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Which of the following is not a consequence of concurrent operations?
Answer: B
Correct answer: Update anomaly.
Lost updates, unrepeatable reads and dirty reads all arise from transactions running at the same time. An update anomaly is a consequence of poor normalisation - the same fact stored twice - and would occur even with one user.
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For using a specific database, the ______ command is used.
Answer: A
Correct answer: use database
The USE statement, followed by the database name, makes that database the current one for subsequent statements.
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The data in the database at a particular moment of time is called the _______
Answer: A
Correct answer: Snapshot
The state of the database at one instant is a snapshot, or an instance. The schema, by contrast, is the description that holds across all such instants.
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Drop Table cannot be used to drop a table referenced by a ______ constraint.
Answer: C
Correct answer: Foreign Key
If another table's foreign key refers to this one, dropping it would leave those references pointing at nothing, so the DBMS refuses. The referencing constraint or table has to go first.
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If two relations R and S are joined, then the non matching tuples of both R and S are ignored in
Answer: D
Correct answer: inner join
An inner join returns only the pairs of rows that match, discarding unmatched rows from both sides. The outer joins exist precisely to keep them: left keeps the unmatched rows of R, right those of S, and full both.
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If an entity can belong to only one lower level entity then the constraint is
Answer: B
Correct answer: disjoint
If an entity may belong to at most one of the lower-level sets, the specialisation is disjoint; if it may belong to several, it is overlapping. Partial and total answer a different question - whether every higher-level entity must belong to some lower-level set at all.
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The cost of reading and writing temporary files while evaluating a query can be reduced by
Answer: B
Correct answer: pipelining
Without pipelining each operation writes its whole result to a temporary file for the next to read. Pipelining passes each tuple straight from one operation to the next as it is produced, so the intermediate results are never written to disk at all.
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