SQL
Select all columns from a table called employees.
SELECT * FROM employees;
Read a CSV file called sales.csv into a DataFrame.
import pandas as pd
df = pd.read_csv("sales.csv")
What does mean represent in a dataset?
The mean is the average value (sum of all values ÷ number of values).
What does DISTINCT do in a query?
Removes duplicate rows for the selected columns, returning unique combinations (e.g., SELECT DISTINCT customer_id gives unique customers).
Count how many rows are in the transactions table.
SELECT COUNT(*) FROM transactions;
Show the first 10 rows of a DataFrame.
df.head(10)
What does median tell you about a dataset?
The median is the middle value when data is ordered; it shows the center without being skewed by outliers.
Why might you use a WHERE clause vs. a HAVING clause?
WHERE vs HAVING
WHERE filters rows before grouping/aggregation.
HAVING filters groups after aggregation
Find the total sales per product_id in the sales table.
SELECT product_id, SUM(amount) AS total_sales
FROM sales
GROUP BY product_id;
Get the average of the column order_amount.
df["order_amount"].mean()
What is standard deviation measuring?
Standard deviation measures how spread out the data is from the mean.
What is the largest desert in the world?
Antarctica
Return the 5 most recent orders by order_date.
SELECT *
FROM orders
ORDER BY order_date DESC
LIMIT 5;
Filter rows where column region equals "West".
df[df["region"] == "West"]
What does a correlation coefficient close to 1 mean?
A correlation close to 1 means a strong positive relationship between two variables.
What is the only state without a natural lake
Maryland
Show the earliest order date for each customer_id in the orders table.
SELECT customer_id, MIN(order_date) AS first_order
FROM orders
GROUP BY customer_id;
Group by customer_id and count how many orders they made.
df.groupby("customer_id")["order_id"].count()
What does a correlation coefficient near 0 mean about two variables?
no linear relationship — though variables may still have a nonlinear relationship.
What is the loudest animal on Earth?
Sperm Whale