SQL Fundamentals - Programming

WHERE filters individual rows before grouping, so it cannot reference aggregate functions. HAVING filters groups after GROUP BY has aggregated them, so it can use functions like COUNT or SUM. For example, WHERE narrows which orders to include, while HAVING keeps only groups whose total exceeds a threshold.

An INNER JOIN returns only rows that have a matching value in both tables based on the join condition. Rows without a match in either table are excluded. For example, joining customers and orders on a customer id returns only customers who have at least one order, pairing each order with its customer.

GROUP BY collapses rows that share the same values in the listed columns into a single group, so you can apply aggregate functions per group. For instance, grouping orders by customer lets COUNT return the number of orders each customer placed. Non-aggregated columns in the SELECT must appear in the GROUP BY.

DISTINCT removes duplicate rows from the result so each combination of the selected columns appears only once. For example, SELECT DISTINCT country from a customers table returns the unique list of countries. It applies across all selected columns together, not just the first one.

Scores are based on the number of correct answers divided by total questions, with a breakdown by topic category.

Yes, questions are randomly selected and ordered from our question bank to ensure each attempt is unique.

No account is required. You can take the test immediately. Optionally provide an email to save your results.

There is no pass/fail threshold. The test measures your knowledge level and provides detailed feedback for improvement.

For knowledge tests, we recommend answering without external help to get an accurate assessment. Practice exercises are designed for learning, so references are acceptable.

Our questions are written for structured educational practice and can give a useful snapshot of your current knowledge in the tested topics.