DAC DAC Database & SQL Analytics

Question 1

Which SQL clause is used to filter rows after aggregation has been performed?

Accepted Answer

HAVING

Answer

The HAVING clause filters groups after aggregation (e.g., after GROUP BY), while WHERE filters individual rows before aggregation.

Question 2

What is the difference between an INNER JOIN and a LEFT JOIN in SQL?

Accepted Answer

INNER JOIN returns only matching rows; LEFT JOIN returns all rows from the left table plus matching rows from the right

Answer

INNER JOIN returns only rows with matching values in both tables, while LEFT JOIN returns all rows from the left table and matching rows from the right (NULLs for non-matches).

Question 3

Which SQL aggregate function returns the number of non-NULL values in a column?

Accepted Answer

COUNT()

Answer

COUNT() counts non-NULL values when given a column name, or all rows (including NULLs) when used as COUNT(*).

Question 4

What does a database index primarily improve?

Accepted Answer

Data query and retrieval speed

Answer

Database indexes speed up SELECT queries by allowing the database engine to find rows quickly without scanning the entire table.

Question 5

In SQL, what does the DISTINCT keyword do?

Accepted Answer

Removes duplicate values from the result set

Answer

DISTINCT eliminates duplicate rows from a query result, returning only unique values for the selected columns.

Data Analytics Certifications (DAC) Practice Test

Data Analytics Certifications (DAC) Practice Test

DAC DAC Database & SQL Analytics