Which of the following statement(s) is/are correct regarding COUNT functions…
2021
Which of the following statement(s) is/are correct regarding COUNT functions in Structured Query Language of Relational Database Management System? I. COUNT(* ) is used to count the number of values in a column. II. COUNT() is used to count the number of rows of the query result.
Answer: D. Neither I nor II — ConceptSQL aggregate functions reduce a set of input rows to a single result. COUNT(*) counts rows, whereas COUNT(expression) counts rows for which the…
- A.
Only I
- B.
Only II
- C.
Both I and II
- D.
Neither I nor II
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Correct answer: D
Concept
SQL aggregate functions reduce a set of input rows to a single result. COUNT(*) counts rows, whereas COUNT(expression) counts rows for which the expression is not NULL.
A function call must also follow valid SQL syntax: empty COUNT() is not a standard COUNT form; an argument such as * or an expression is required.
Application
For Statement I, COUNT(*) is row-oriented. It counts every row that survives the query filters, even when a particular column contains NULL, so it does not mean the count of non-NULL values in one named column.
For Statement II, COUNT() supplies no argument. The standard forms are COUNT(*) and COUNT(expression), so empty COUNT() is not the syntax used to count query-result rows.
Cross-check
SQL form | What it counts |
|---|---|
COUNT(*) | All rows retained by the query |
COUNT(MARKS) | Rows whose MARKS value is not NULL |
With rows (MARKS = 80), (MARKS = NULL), and (MARKS = 65), COUNT(*) returns 3 while COUNT(MARKS) returns 2.
This contrast confirms that Statement I assigns column-value behavior to COUNT(*), while Statement II uses an empty call instead of COUNT(*).
Contrast
Only I represents the truth pattern I = true and II = false.
Only II represents the truth pattern I = false and II = true.
Both I and II represents the truth pattern I = true and II = true.
Neither I nor II represents the truth pattern I = false and II = false.
Result
Therefore the answer is Neither I nor II.