DBMS & SQL Previous Year Questions (page 3)

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The hierarchical model (e.g. IBM's IMS, 1960s) is the oldest of these. The relational model came in 1970, and the object-oriented and deductive models later.

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DDLC is the Database Development Life Cycle: the stages of planning, designing, building and maintaining a database.

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In X → Y, X is the determinant and Y is the dependent, because X determines the value of Y.

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Connection traps (fan traps and chasm traps) are problems that can arise in an ER model, where the relationships do not give the intended paths between entities.

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A multivalued attribute is drawn as a double oval. A double diamond is an identifying relationship and a double rectangle is a weak entity.

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A checkpoint is a point where all log records and modified buffers are written to disk, which synchronises the database with the log and limits how much must be redone during recovery.

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Using the forward difference approximation, f'(3) ≈ [f(4) − f(3)]/h = (0.602 − 0.477)/1 = 0.125.

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In a DFD, a rectangle is an external entity, two parallel lines are a data store, the document shape is output, and a circle (bubble) is a process.

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The File Allocation Table (FAT) keeps entries for the files on a disk, recording which clusters each file occupies.

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The output is 0 only when both inputs are 0 and 1 otherwise, which is the OR function (A + B).

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m select lines can choose one of 2^m inputs, so a multiplexer with n inputs needs 2^m = n (e.g. 8 inputs need 3 select lines).

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Shadow paging is a recovery technique that needs no log: it keeps a shadow page table unchanged and a current page table for updates, and commits by switching tables.

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In an ER diagram an ellipse is an attribute and a diamond is a relationship. In crow's foot notation a circle with a bar means optional one, and a double bar means mandatory one.

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A timestamp is a unique value the DBMS gives each transaction when it starts, used to order transactions in timestamp-based concurrency control.

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The timestamp-ordering variants are total, partial and multiversion timestamp ordering. Multi-level timestamp ordering is not one of them.

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Data allocation is the process of deciding which sites of a distributed database will store each fragment or copy of the data.

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Data fragmentation splits a database (horizontally, vertically or mixed) into logical units called fragments, which can be stored at different sites.

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>= (greater than or equal to) is a comparison operator. + and / are arithmetic operators. = is also used for comparison in SQL, but the key takes >=.

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Mobile security is a major barrier to m-commerce, m-commerce lowers transaction costs through streamlined processes, and mobile coupons and tickets are m-commerce services. All three are correct.

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An MIS draws on internal and external databases for its reports. Knowledge engines belong to expert systems, and data entry and inquiry processing are transaction processing tasks.

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By linear interpolation, y rises by (12 − 8)/(6 − 2) = 1 per unit of x, so y(5) = 8 + 3 × 1 = 11.

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A DBMS is software for creating, managing, maintaining and retrieving data stored in a database (e.g. MySQL, Oracle).

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Key attributes of good information include accuracy, timeliness, relevance, completeness. Isolation is not an attribute of information.

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DASD stands for Direct Access Storage Device, such as a magnetic disk, where any record can be accessed directly.

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An entity is described by its attribute values. For example, a student entity is described by values of roll number, name and class.

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REAL, INTERVAL and TIMESTAMP are SQL data types. BASE is not an SQL data type.

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A truth table lists the output for every combination of binary inputs, giving the input-output relation of a gate in tabular form.

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A Transaction Processing System is a type of information system that records routine business transactions. The other options are modes of data processing.

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Indices i, i+1, ..., j contain j − i + 1 elements. For example, indices 0 to 9 give 10 elements.

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The output is 1 only when both inputs are 1, which is the AND function.

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A demultiplexer with m select lines routes the input to one of 2^m outputs, so n = 2^m (e.g. 1-to-8 DEMUX has 3 select lines).

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ISAM is Indexed Sequential Access Method: records are stored sequentially by key and an index allows direct access.

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Note from the source book: a primary index on a sequentially ordered file is in fact a sparse index (option d), but the official key (as per the Board) is (b) Clustering index.

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A deductive database model derives new facts from stored facts using rules (logical inference), so it is also called the inference model.

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The data catalog (data dictionary) is a system database holding metadata, i.e. the description of tables, columns, constraints and other database objects.

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A 2NF relation must be in 1NF (I is correct). Statement II says non-key attributes are "independent" of the primary key, but 2NF requires them to be fully functionally dependent on it, so II is wrong.

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The OS keeps a Thread Control Block (TCB) per thread, storing its thread ID, register set, stack pointer, state and priority. A PCB stores information for a whole process.

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In batch processing, transactions are collected and the database is updated later (I false), it is relatively cheap (II false), and reports are produced at scheduled intervals (III true).

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A webmaster manages and maintains web servers and websites. A web designer creates layout and look, not the server.

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UPDATE ... SET ... WHERE changes only the rows that match. Dividing by 1.1 gives the discounted bill, and the WHERE clause limits it to 'High privilege' accounts. MODIFY is not DML, and B has no WHERE.

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If the subquery returns no rows, the IN list is empty and the condition is false for every row, so no rows are deleted from Product and no error occurs.

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TRUNCATE removes all rows at once with little logging and no row-by-row triggers, so it is faster than DELETE. It keeps the table structure and is a DDL command.

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DROP TABLE removes the table's structure and all its data from the database permanently. TRUNCATE is the command that empties a table but keeps it.

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ENUM is a MySQL-specific type and is not in the SQL:1999 standard, which has DATE, BOOLEAN, CHAR and VARCHAR. Strictly, STRING is not a standard SQL type either.

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For this task the key's order is abstraction first: keep only the important features of an entry. Then decompose the task, recognise duplicate patterns, and finally design the algorithm.

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A NAND gate gives 0 only when all inputs are 1, so P and Q are true and R is false. AB + BC = ((AB)'·(BC)')' needs three NAND gates, so S is true.

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Debits: cash 54,750 + purchases 62,000 + sales return 1,600 + Y 21,500 + furniture 3,000 + wages 5,200 = 1,48,050. The credits also total 1,48,050.

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A FULL (OUTER) JOIN keeps every row from both tables and puts NULL where no match exists. LEFT or RIGHT JOIN keep unmatched rows from only one side, and INNER JOIN keeps only matching rows.

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Paging divides logical memory into pages, and the page table maps each page number to a physical frame number, which translates logical addresses into physical addresses.