Exercises

Q1

Q1. What does the SQL clause LIMIT do?

It removes duplicate rows from a table It limits the number of rows returned by a query It specifies the maximum number of rows to display in the output It sorts the rows in descending order

Q2

What is the purpose of SELECT DISTINCT?

It filters rows based on a condition It removes duplicate values from the selected column(s) It groups rows into categories It orders rows alphabetically

Q3

What does the SQL CASE WHEN structure allow you to do?

To rename a column To join two tables To assign values conditionally within a SELECT statement To remove missing values

Q4

Which statement about HAVING is correct?

WHERE filters groups after aggregation HAVING filters groups after aggregation WHERE and HAVING are interchangeable HAVING must appear before GROUP BY

Q5

Which join returns only rows that appear in both tables?

LEFT JOIN INNER JOIN FULL OUTER JOIN RIGHT JOIN

Q6

Why is the following SQL query invalid?

SELECT Education, AVG(Age) FROM nhanes GROUP BY Education WHERE Age > 30;

WHERE cannot appear after GROUP BY; it must appear before grouping. WHERE must always be used with DISTINCT. WHERE can only filter numeric columns. WHERE must be the last clause in a query.

Q7

Which statement correctly explains why the query SELECT DISTINCT Age, AVG(Age) FROM nhanes; is invalid?

DISTINCT can only be used with numeric columns. DISTINCT applies to the entire selected set of columns, so DISTINCT Age, Education is valid but DISTINCT Age, AVG(Age) is not. DISTINCT must always be paired with GROUP BY. DISTINCT removes NULL values.

Q8

Why is the following query invalid?

SELECT * FROM nhanes_demographics NATURAL JOIN nhanes_health ON nhanes_demographics.SEQN = nhanes_health.SEQN;

NATURAL JOIN requires specifying the join key explicitly. NATURAL JOIN automatically joins on all shared column names, so using ON is not allowed. NATURAL JOIN can only be used with numeric keys. NATURAL JOIN always produces duplicate rows.

Q9

A student claims this query is invalid:

SELECT Education, AVG(Age) AS avg_age

FROM nhanes

GROUP BY Education

ORDER BY avg_age DESC;

Which explanation is correct?

Aliases cannot be used in SELECT statements. The alias avg_age is defined in SELECT, so it can be used in ORDER BY, the error must be elsewhere. ORDER BY must always reference the original column name. ORDER BY cannot be used with aggregated values.

Q10

Why is the following query invalid?

SELECT SEQN, Age FROM nhanes_set1

UNION

SELECT SEQN FROM nhanes_set2;

UNION can only combine tables with the same number of rows. UNION requires both queries to return the same number of columns in the same order. UNION cannot be used with SELECT *. UNION always preserves duplicates.

Q11

Why is this query invalid?

SELECT * FROM nhanes WHERE Ethnicity LIKE %Hispanic%;

LIKE cannot be used with text columns. LIKE only works with numeric values. The pattern must be a string literal, so %Hispanic% must be quoted. LIKE must always be paired with DISTINCT.

Q12

Why is this query invalid?

SELECT Education, Age, AVG(Triglyceride) FROM nhanes GROUP BY Education;

AVG() cannot be used with numeric columns. All non‑aggregated columns in SELECT must appear in GROUP BY. GROUP BY must always include all columns in the table. Aggregations must appear before SELECT.

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