GROUP BY single and multiple columns in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to group the sales by the product_id.

PostgreSQL

SELECT product_id, SUM(quantity) FROM sales GROUP BY [1];

Drag options to blanks, or click blank then click option'

Asales

Bquantity

Cproduct_id

DSUM(quantity)

Attempts:

3 left

💡 Hint

Common Mistakes

Grouping by quantity instead of product_id.

Using aggregate functions in GROUP BY.

We group by product_id to aggregate sales per product.

2fill in blank

medium

Complete the code to group sales by both product_id and store_id.

PostgreSQL

SELECT product_id, store_id, SUM(quantity) FROM sales GROUP BY [1], store_id;

Drag options to blanks, or click blank then click option'

Aquantity

Bproduct_id

CSUM(quantity)

Dstore_id

Attempts:

3 left

💡 Hint

Common Mistakes

Grouping by quantity or aggregate functions.

Forgetting to include all selected non-aggregated columns in GROUP BY.

We group by both product_id and store_id to get totals per product per store.

3fill in blank

hard

Fix the error in the GROUP BY clause to correctly group by customer_id.

PostgreSQL

SELECT customer_id, COUNT(*) FROM orders GROUP BY [1];

Drag options to blanks, or click blank then click option'

Acustomer_id

BCOUNT(*)

Corders

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Grouping by COUNT(*) which is an aggregate function.

Using table names or * in GROUP BY.

The GROUP BY clause must use the column name customer_id, not an aggregate or table name.

4fill in blank

hard

Fill both blanks to group sales by year and month.

PostgreSQL

SELECT EXTRACT([1] FROM sale_date) AS year, EXTRACT([2] FROM sale_date) AS month, SUM(amount) FROM sales GROUP BY year, month;

Drag options to blanks, or click blank then click option'

Ayear

Bmonth

Cday

Dquarter

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'day' or 'quarter' instead of 'month'.

Not matching the extracted parts with GROUP BY aliases.

We extract year and month from sale_date to group sales by these periods.

5fill in blank

hard

Fill all three blanks to group employees by department and job title, showing average salary.

PostgreSQL

SELECT [1], [2], AVG(salary) FROM employees GROUP BY [3], job_title;

Drag options to blanks, or click blank then click option'

Adepartment

Bjob_title

Dsalary

Attempts:

3 left

💡 Hint

Common Mistakes

Grouping by salary which is aggregated.

Mismatch between SELECT columns and GROUP BY columns.

We select and group by department and job_title to get average salary per group.

Practice

1. What does the GROUP BY clause do in a PostgreSQL query?

easy

A. It updates values in the specified columns.

B. It sorts the rows in ascending order.

C. It deletes duplicate rows from the table.

D. It groups rows that have the same values in specified columns.

Solution

1. Step 1: Understand the purpose of GROUP BY

   The GROUP BY clause collects rows that share the same value in one or more columns into groups.

2. Step 2: Differentiate from other clauses

   Unlike ORDER BY which sorts rows, GROUP BY organizes rows into groups for aggregation.

3. Final Answer:

   It groups rows that have the same values in specified columns. -> Option D

4. Quick Check:

   GROUP BY = groups rows by column values [OK]

Hint: GROUP BY collects rows sharing column values into groups [OK]

Common Mistakes:

• Confusing GROUP BY with ORDER BY
• Thinking GROUP BY deletes duplicates
• Assuming GROUP BY updates data

2. Which of the following is the correct syntax to group data by two columns, category and region?

easy

A. SELECT category, region, COUNT(*) FROM sales GROUP BY category, region;

B. SELECT category, region, COUNT(*) FROM sales GROUP BY category region;

C. SELECT category, region, COUNT(*) FROM sales GROUP BY (category, region);

D. SELECT category, region, COUNT(*) FROM sales GROUP category, region;

Solution

1. Step 1: Check correct GROUP BY syntax

   In PostgreSQL, multiple columns are separated by commas in GROUP BY without parentheses.

2. Step 2: Validate each option

   SELECT category, region, COUNT(*) FROM sales GROUP BY category, region; uses commas correctly; SELECT category, region, COUNT(*) FROM sales GROUP BY category region; misses comma; SELECT category, region, COUNT(*) FROM sales GROUP BY (category, region); uses parentheses incorrectly; SELECT category, region, COUNT(*) FROM sales GROUP category, region; misses BY keyword.

3. Final Answer:

   SELECT category, region, COUNT(*) FROM sales GROUP BY category, region; -> Option A

4. Quick Check:

   GROUP BY columns separated by commas [OK]

Hint: Separate multiple columns with commas in GROUP BY [OK]

Common Mistakes:

• Omitting commas between columns
• Using parentheses around columns
• Forgetting the BY keyword

3. Given the table orders with columns customer_id, product_id, and quantity, what will this query return?

SELECT customer_id, SUM(quantity) FROM orders GROUP BY customer_id;

medium

A. Total quantity ordered by each customer.

B. Total quantity ordered by each product.

C. Total quantity ordered by each customer and product.

D. Syntax error due to missing GROUP BY columns.

Solution

1. Step 1: Analyze the SELECT and GROUP BY columns

   The query groups rows by customer_id and sums quantity for each group.

2. Step 2: Understand the aggregation result

   SUM(quantity) calculates total quantity per customer_id, ignoring product_id.

3. Final Answer:

   Total quantity ordered by each customer. -> Option A

4. Quick Check:

   GROUP BY customer_id sums quantity per customer [OK]

Hint: GROUP BY column matches SELECT non-aggregates [OK]

Common Mistakes:

• Assuming product_id affects grouping
• Thinking query returns per product totals
• Confusing aggregation with filtering

4. Identify the error in this query:

SELECT product_id, region, COUNT(*) FROM sales GROUP BY product_id;

medium

A. COUNT(*) cannot be used with GROUP BY.

B. Missing region in GROUP BY causes error.

C. product_id cannot be grouped alone.

D. Syntax error due to missing FROM clause.

Solution

1. Step 1: Check SELECT columns vs GROUP BY columns

   SELECT includes product_id and region, but GROUP BY only has product_id.

2. Step 2: Understand PostgreSQL grouping rules

   All non-aggregated columns in SELECT must appear in GROUP BY; region is missing.

3. Final Answer:

   Missing region in GROUP BY causes error. -> Option B

4. Quick Check:

   All SELECT non-aggregates must be in GROUP BY [OK]

Hint: Include all non-aggregated SELECT columns in GROUP BY [OK]

Common Mistakes:

• Forgetting to add all columns to GROUP BY
• Thinking COUNT(*) disallows grouping
• Ignoring SELECT and GROUP BY mismatch

5. You want to find the total sales quantity for each product in each region from the sales table with columns product_id, region, and quantity. Which query correctly groups and sums the data?

hard

A. SELECT product_id, SUM(quantity) FROM sales GROUP BY product_id;

B. SELECT product_id, region, SUM(quantity) FROM sales GROUP BY product_id;

C. SELECT product_id, region, SUM(quantity) FROM sales GROUP BY product_id, region;

D. SELECT product_id, region, SUM(quantity) FROM sales;

Solution

1. Step 1: Identify grouping needs

   We want totals per product and per region, so both columns must be grouped.

2. Step 2: Check each option's GROUP BY clause

   SELECT product_id, region, SUM(quantity) FROM sales GROUP BY product_id, region; groups by product_id and region, matching SELECT columns; others miss region or GROUP BY entirely.

3. Final Answer:

   SELECT product_id, region, SUM(quantity) FROM sales GROUP BY product_id, region; -> Option C

4. Quick Check:

   Group by all non-aggregated columns for correct totals [OK]

Hint: Group by all selected non-aggregated columns [OK]

Common Mistakes:

• Grouping by only one column when selecting multiple
• Omitting GROUP BY causing aggregation errors
• Selecting columns without aggregation or grouping