Computer Awareness Quiz: Databases and DBMS

The hierarchical model is the first data model used in commercial database software; IBM built its Information Management System on it in the 1960s for the Apollo programme.

Codd, a British computer scientist working at IBM, set out the relational model in a paper published in 1970, proposing that all data be held in simple two-dimensional tables and joined by matching values rather than by fixed pointers.

In the vocabulary of the relational model a table is a relation, and one complete horizontal entry of that table, such as all the details of a single student, is a tuple; the ordinary words for it are row and record.

The Data Definition Language deals with the structure of the database rather than with the data inside it, and CREATE is its basic command, used to build a table, a view or the database itself; ALTER, DROP, TRUNCATE and RENAME belong to the same group.

TRUNCATE empties a table in one operation, leaving the columns, data types and constraints in place so that the table can be filled again at once; because it changes the object rather than selected rows it is grouped with the Data Definition Language and it is much faster than deleting…

The designer picks one candidate key as the primary key of a table, and the database then enforces two rules on it: its value must be different in every row, and it can never be left null, since a missing value would make a row impossible to identify.

A foreign key is the column that stores values belonging to the primary key of another table, so a marks table can carry the roll number of a student table and the two tables can then be joined; the database uses it to enforce referential integrity, refusing a value that…

Isolation means that transactions running at the same time do not see one another's incomplete work, so two clerks booking the last seat cannot both succeed; the database behaves as though the transactions ran one after the other.

The degree of a relation counts its attributes, that is its columns, so a student table holding roll number, name, class and marks has degree four whatever the number of students in it.

A table is in second normal form when it is already in first normal form and no non-key attribute depends on only a part of a composite primary key; such a partial dependency is removed by moving that attribute into a table of its own.

MongoDB stores data as documents in a flexible, JSON-like form instead of as rows in tables with a fixed set of columns, and it is queried through its own interface rather than mainly through SQL, which makes it the standard example of a NoSQL database in exam papers.

The Data Control Language governs who may do what inside the database: GRANT gives a user a privilege such as the right to read or to update a table, and REVOKE withdraws a privilege already given.