Chapter 18, Problem 3PE

Explanation of Solution

Given:

The database schema R and relation r on the schema R:

SQL query to check the functional dependency:

• To check whether the functional dependency $\text{b}\to \text{c}$ holds on relation r,

The query is given below.

select b from r group by b having count(distinct c) > 1

• It is recommended to use the group clause on attribute b to get the count of distinct values of attribute c from relation r.
• Then usage of having clause to get the attribute b of those records whose count is more than 1.
• The above will result is non-empty if and only if $\text{b}\to \text{c}$ does not hold on r.

Thus, when the result of the query is empty it indicates that the functional dependency $\text{b}\to \text{c}$ will hold