Test Your PostgreSQL DISTINCT Command Knowledge with This Quiz

Progress

True / False

In PostgreSQL, the DISTINCT keyword is used to remove duplicate rows from the result set.

True False

The DISTINCT keyword eliminates duplicate rows in the result set, returning only unique rows.

True / False

In PostgreSQL, using DISTINCT in a SELECT statement will still return duplicate rows if used incorrectly.

True False

When used correctly, DISTINCT ensures that all returned rows are unique, with no duplicates.

True / False

In PostgreSQL, the DISTINCT keyword can be used with a single column or multiple columns in a query.

True False

DISTINCT can be applied to one or more columns, removing duplicate rows based on the combination of specified columns.

True / False

In PostgreSQL, the DISTINCT keyword can be used with aggregate functions like COUNT.

True False

DISTINCT can be used with aggregate functions, such as COUNT(DISTINCT column_name), to count unique values in a column.

True / False

In PostgreSQL, using DISTINCT always guarantees faster query performance.

True False

While DISTINCT removes duplicates, it may increase query processing time due to the additional work needed to identify and eliminate duplicates.

True / False

In PostgreSQL, DISTINCT can be used within a subquery to ensure unique results before combining them with the outer query.

True False

DISTINCT can be applied in subqueries to ensure unique rows are returned before combining the results with the outer query.

True / False

In PostgreSQL, using DISTINCT ON (column_name) allows you to specify which duplicates to keep based on ordering.

True False

DISTINCT ON (column_name) retains only the first row for each unique value of the specified column(s), based on the order defined by the ORDER BY clause.

True / False

In PostgreSQL, the DISTINCT keyword can be combined with the GROUP BY clause without any issues.

True False

DISTINCT and GROUP BY can be used together, but they serve different purposes: DISTINCT removes duplicates, while GROUP BY groups rows based on column

True / False

In PostgreSQL, DISTINCT can be used in window functions.

True False

DISTINCT is not typically used in window functions. Window functions operate on a set of rows and return a value for each row in the query result, while DISTINCT modifies the result set to remove duplicates.

Q10

True / False

In PostgreSQL, using DISTINCT in a SELECT statement without specifying any columns will result in a syntax error.

True False

DISTINCT must be followed by one or more column names in a SELECT statement. Without specifying columns, the query is incomplete and will result in a syntax error.

Q11

Single Choice

Which of the following SQL statements will return a list of unique city names from a customers table in PostgreSQL?

SELECT DISTINCT city FROM customers; SELECT UNIQUE city FROM customers; SELECT city FROM customers;SELECT ALL city FROM customers;

The DISTINCT keyword is used in PostgreSQL to return only unique values from a column. In this case, it retrieves a list of unique city names.

Q12

Single Choice

What will the following SQL statement return in PostgreSQL?

SQL Code

SELECT DISTINCT country FROM customers;

All country names including duplicates All unique country names All records from the customers tableAll country names in alphabetical order

The DISTINCT keyword ensures that only unique values are returned, so this query will return each country name only once.

Q13

Single Choice

Which PostgreSQL keyword is used to remove duplicate values in a query result set?

UNIQUE DISTINCT DISTINCTSINGLE

The DISTINCT keyword in PostgreSQL is used to remove duplicates from the result set of a query.

Q14

Single Choice

Given the following table orders with columns order_id, customer_id, and order_date, which SQL statement will retrieve a list of unique customer_ids?

SELECT DISTINCT customer_id FROM orders; SELECT UNIQUE customer_id FROM orders; SELECT DISTINCT customer_id FROM orders GROUP BY customer_id;SELECT DISTINCT customer_id FROM orders GROUP BY customer_id;

The DISTINCT keyword removes duplicates and returns a unique list of customer_ids from the orders table.

Q15

Single Choice

What will the following PostgreSQL query return?

SQL Code

SELECT DISTINCT ON (department) department, salary FROM employees ORDER BY department, salary DESC;

All unique department names The highest salary in each department The lowest salary in each departmentThe total salary per department

The DISTINCT ON (department) clause returns the first row of each unique department, and with ORDER BY salary DESC, it returns the highest salary per department.

Q16

Single Choice

In PostgreSQL, what is the outcome of the following SQL query?

SQL Code

SELECT DISTINCT city, state FROM addresses;

It returns all unique combinations of city and state. It returns unique cities across all states. It returns unique states across all cities.It returns all records from the addresses table.

The DISTINCT keyword applied to multiple columns returns unique combinations of the specified columns—in this case, unique combinations of city and state.

Q17

Single Choice

Consider the products table with the columns product_name, category, and price. How would you write a query to return unique categories where the average price of products is above $50?

SELECT DISTINCT category FROM products WHERE AVG(price) > 50; SELECT DISTINCT category FROM products GROUP BY category HAVING AVG(price) > 50; SELECT DISTINCT category FROM products WHERE price > 50;SELECT category FROM products GROUP BY category WHERE AVG(price) > 50;

The DISTINCT keyword combined with GROUP BY and HAVING filters the categories where the average price is greater than $50.

Q18

Single Choice

What will the following PostgreSQL query return?

SQL Code

SELECT DISTINCT customer_id, COUNT(order_id) FROM orders GROUP BY customer_id;

Unique customer IDs with a count of orders All customer IDs with a count of orders Unique order counts per customerThe total number of orders per customer

The DISTINCT keyword ensures unique customer IDs, and the COUNT(order_id) aggregates the number of orders per customer, grouped by customer_id.

Q19

Single Choice

Which SQL query will return a distinct list of customer names along with their corresponding order dates from the orders table?

SELECT DISTINCT customer_name FROM orders; SELECT DISTINCT customer_name, order_date FROM orders; SELECT customer_name, DISTINCT order_date FROM orders;SELECT DISTINCT ON (customer_name) customer_name, order_date FROM orders ORDER BY customer_name, order_date;

The query will return a unique combination of customer_name and order_date from the orders table.

Q20

Single Choice

Consider the employees table with the columns employee_id, department, and salary. Which query will return the distinct list of departments and the average salary of employees in each department?

SELECT DISTINCT department, AVG(salary) FROM employees; SELECT DISTINCT department, AVG(salary) FROM employees GROUP BY department; SELECT DISTINCT department FROM employees GROUP BY salary;SELECT department, AVG(salary) FROM employees GROUP BY department;

The correct query groups the employees by department and calculates the average salary for each department. The DISTINCT keyword is unnecessary here because the GROUP BY clause already ensures unique departments.

Q21

Multiple Choice

Identify the SQL scripts that correctly perform a basic DISTINCT operation in PostgreSQL.

SELECT DISTINCT department_id FROM employees; SELECT department_id FROM employees WHERE DISTINCT; SELECT UNIQUE department_id FROM employees;SELECT department_id FROM employees GROUP BY department_id;

The DISTINCT keyword is used to return only distinct (different) values in a query. The correct scripts will demonstrate how to retrieve distinct values using the DISTINCT keyword in PostgreSQL.

Q22

Multiple Choice

Select the SQL scripts that correctly perform a DISTINCT operation on multiple columns in PostgreSQL.

SELECT DISTINCT department_id, job_id FROM employees; SELECT department_id, job_id FROM employees WHERE DISTINCT; SELECT DISTINCT ON (department_id, job_id) * FROM employees;SELECT department_id, job_id FROM employees GROUP BY department_id, job_id;

DISTINCT can be applied to multiple columns to retrieve unique combinations of those columns. The correct scripts will demonstrate how to use DISTINCT on multiple columns in PostgreSQL.

Q23

Multiple Choice

Determine the SQL scripts that correctly perform a DISTINCT operation with ORDER BY in PostgreSQL.

SELECT DISTINCT product_id FROM orders ORDER BY order_date DESC; SELECT DISTINCT ON (product_id) * FROM orders ORDER BY order_date DESC; SELECT product_id FROM orders GROUP BY product_id ORDER BY order_date DESC;SELECT product_id FROM orders WHERE DISTINCT ORDER BY order_date DESC;

ORDER BY can be used with DISTINCT to sort the distinct values. The correct scripts will demonstrate how to use ORDER BY with DISTINCT in PostgreSQL.

Q24

Multiple Choice

Identify the SQL scripts that correctly perform a DISTINCT operation with WHERE clause in PostgreSQL.

SELECT DISTINCT customer_id FROM sales WHERE amount > 1000; SELECT customer_id FROM sales WHERE DISTINCT amount > 1000; SELECT DISTINCT customer_id FROM sales GROUP BY amount HAVING amount > 1000;SELECT DISTINCT ON (customer_id) * FROM sales WHERE amount > 1000;

The WHERE clause can be combined with DISTINCT to filter the distinct results. The correct scripts will demonstrate how to use WHERE with DISTINCT in PostgreSQL.

Q25

Multiple Choice

Select the SQL scripts that correctly perform a DISTINCT operation with JOINs in PostgreSQL.

SELECT DISTINCT o.order_id, c.customer_name FROM orders o JOIN customers c ON o.customer_id = c.customer_id; SELECT DISTINCT ON (o.order_id) o.order_id, c.customer_name FROM orders o JOIN customers c ON o.customer_id = c.customer_id; SELECT o.order_id, c.customer_name FROM orders o JOIN customers c ON o.customer_id = c.customer_id GROUP BY o.order_id, c.customer_name;SELECT DISTINCT o.order_id, c.customer_name FROM orders o, customers c WHERE o.customer_id = c.customer_id;

JOINs can be used with DISTINCT to retrieve unique rows from multiple tables. The correct scripts will demonstrate how to use DISTINCT with JOINs in PostgreSQL.

Q26

Multiple Choice

Determine the SQL scripts that correctly perform a DISTINCT operation with aggregate functions in PostgreSQL.

SELECT COUNT(DISTINCT employee_id) FROM employees; SELECT DISTINCT COUNT(employee_id) FROM employees; SELECT COUNT(*) FROM (SELECT DISTINCT employee_id FROM employees) AS sub;SELECT COUNT(DISTINCT employee_id) FROM employees GROUP BY department_id;

Aggregate functions like COUNT() can be used with DISTINCT to perform calculations on unique values. The correct scripts will demonstrate how to use aggregate functions with DISTINCT in PostgreSQL.

Q27

Multiple Choice

Identify the SQL scripts that correctly perform a DISTINCT operation with window functions in PostgreSQL.

SELECT DISTINCT customer_id, ROW_NUMBER() OVER (ORDER BY customer_id) AS row_num FROM orders; SELECT customer_id, ROW_NUMBER() OVER (ORDER BY customer_id) AS row_num FROM (SELECT DISTINCT customer_id FROM orders) AS sub; SELECT DISTINCT ON (customer_id) customer_id, ROW_NUMBER() OVER (ORDER BY customer_id) AS row_num FROM orders;SELECT customer_id, DISTINCT ROW_NUMBER() OVER (ORDER BY customer_id) AS row_num FROM orders;

Window functions can be used with DISTINCT to perform calculations across a subset of rows. The correct scripts will demonstrate how to use window functions with DISTINCT in PostgreSQL.

Q28

Multiple Choice

Select the SQL scripts that correctly perform a DISTINCT operation with UNION in PostgreSQL.

SELECT DISTINCT product_id FROM orders UNION SELECT DISTINCT product_id FROM returns; SELECT product_id FROM orders UNION SELECT product_id FROM returns; SELECT DISTINCT product_id FROM (SELECT product_id FROM orders UNION SELECT product_id FROM returns) AS combined;SELECT DISTINCT product_id FROM orders UNION DISTINCT SELECT DISTINCT product_id FROM returns;

The UNION operator can be used to combine result sets, and when combined with DISTINCT, it can retrieve unique values from multiple result sets. The correct scripts will demonstrate how to use UNION with DISTINCT in PostgreSQL.

Q29

Multiple Choice

Determine the SQL scripts that correctly perform a DISTINCT operation with LIMIT in PostgreSQL.

SELECT DISTINCT employee_id FROM employees LIMIT 5; SELECT employee_id FROM employees WHERE DISTINCT LIMIT 5; SELECT DISTINCT employee_id FROM employees ORDER BY employee_id LIMIT 5;SELECT DISTINCT employee_id FROM employees LIMIT 5 OFFSET 1;

LIMIT can be combined with DISTINCT to retrieve a specific number of unique values. The correct scripts will demonstrate how to use LIMIT with DISTINCT in PostgreSQL.

Q30

Multiple Choice

Identify the SQL scripts that correctly perform a DISTINCT operation with GROUP BY in PostgreSQL.

SELECT department_id, COUNT(*) FROM employees GROUP BY department_id; SELECT DISTINCT department_id, COUNT(*) FROM employees GROUP BY department_id; SELECT department_id FROM employees GROUP BY department_id HAVING COUNT(*) > 1;SELECT DISTINCT ON (department_id) department_id, COUNT(*) FROM employees GROUP BY department_id;

GROUP BY is used to group data by one or more columns, and when combined with DISTINCT, it can help retrieve unique grouped data. The correct scripts will demonstrate how to use GROUP BY with DISTINCT in PostgreSQL.

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