DBMS - Interview Questions and Answers


  1. Write an SQL SELECT statement to display all the columns of the STUDENT table but only those rows where the Grade column is greater than or equal to 90.
    SELECT * FROM STUDENT WHERE Grade >= 90;
  2. Name and briefly describe the five SQL built-in functions.
    COUNT: computes the number of rows in a table. SUM: totals numeric columns. AVG: computes the average value. MAX: obtains the maximum value of a column in a table. MIN: obtains the minimum value of a column in a table.
  3. Write an SQL SELECT statement to count the number of rows in STUDENT table and display the result with the label NumStudents.
    SELECT COUNT(*) AS NumStudents FROM STUDENT;
  4. What is an SQL subquery?
    An SQL subquery is a means of querying two or more tables at the same time. The subquery itself is an SQL SELECT statement contained within the WHERE clause of another SQL SELECT statement, and separated by being enclosed in parenthesis. Some subqueries have equivalent SQL join structures, but correlated subqueries cannot be duplicated by a join..
  5. Discuss the alternative terminology that is used in the relational model.
    Relations are also called tables, and sometimes by the older data processing term files. A row is known as a tuple in the relational model, but may also be referred to as a record. Finally, relational model attributes are known as table columns and sometimes as fields.
  6. Why are functional dependencies not equations?
    Equations deal with numerical relationships. A functional dependency deals with the existence of a determinant relationship between attributes, regardless of whether or not there is a numerical relationship between them. Thus, if we know that there is no hot water every Wednesday, No-Hot-Water is functionally dependent on Wednesday. So, if we know it is Wednesday, then we know we will have No-Hot-Water. This is a functional dependency, but not an equation.

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