FILTER clause for conditional aggregation in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to count only the orders with status 'shipped'.

PostgreSQL

SELECT COUNT(*) FILTER (WHERE status = [1]) AS shipped_orders FROM orders;

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

A'pending'

B'shipped'

C'cancelled'

D'returned'

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(status) instead of COUNT(*) with FILTER.

Forgetting to put quotes around the status value.

The FILTER clause counts only rows where the condition is true. Here, it counts orders with status 'shipped'.

2fill in blank

medium

Complete the code to sum the amount only for orders with status 'completed'.

PostgreSQL

SELECT SUM(amount) FILTER (WHERE status = [1]) AS completed_total FROM orders;

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

A'completed'

B'pending'

C'shipped'

D'cancelled'

Attempts:

3 left

💡 Hint

Common Mistakes

Summing without FILTER, which sums all amounts.

Using wrong status value in the condition.

The FILTER clause here sums amounts only for orders where status is 'completed'.

3fill in blank

hard

Fix the error in the code to count orders with status 'returned' using FILTER.

PostgreSQL

SELECT COUNT(*) FILTER [1] AS returned_orders FROM orders;

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

A(status = 'returned')

BWHERE status = 'returned'

C(WHERE status = 'returned')

DWHERE (status = 'returned')

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting parentheses after FILTER.

Writing WHERE without parentheses.

The FILTER clause requires parentheses around the WHERE condition.

4fill in blank

hard

Fill both blanks to count orders with status 'pending' and sum amounts for 'completed' orders.

PostgreSQL

SELECT
  COUNT(*) FILTER (WHERE status = [1]) AS pending_count,
  SUM(amount) FILTER (WHERE status = [2]) AS completed_sum
FROM orders;

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

A'pending'

B'completed'

C'shipped'

D'cancelled'

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the status values between count and sum.

Forgetting quotes around status values.

The first FILTER counts 'pending' orders, the second sums amounts for 'completed' orders.

5fill in blank

hard

Fill all three blanks to count 'shipped' orders, sum 'returned' amounts, and average 'completed' amounts.

PostgreSQL

SELECT
  COUNT(*) FILTER (WHERE status = [1]) AS shipped_count,
  SUM(amount) FILTER (WHERE status = [2]) AS returned_sum,
  AVG(amount) FILTER (WHERE status = [3]) AS completed_avg
FROM orders;

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

A'pending'

B'returned'

C'completed'

D'shipped'

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up status values between aggregates.

Missing quotes around status strings.

Each FILTER clause applies to the correct status: count for 'shipped', sum for 'returned', average for 'completed'.

Practice

1. What does the FILTER clause do in a PostgreSQL aggregate function?

easy

A. It filters the entire table before aggregation starts.

B. It applies a condition to include only certain rows in the aggregation.

C. It changes the grouping columns in the query.

D. It sorts the results after aggregation.

Solution

1. Step 1: Understand the purpose of FILTER in aggregation

   The FILTER clause allows you to specify a condition inside an aggregate function to include only rows that meet that condition.

2. Step 2: Compare with other options

   FILTER does not filter the whole table before aggregation, nor does it change grouping or sort results. It only limits rows counted or summed inside the aggregate.

3. Final Answer:

   It applies a condition to include only certain rows in the aggregation. -> Option B

4. Quick Check:

   FILTER = condition inside aggregate [OK]

Hint: FILTER applies conditions inside aggregates, not on whole table [OK]

Common Mistakes:

• Thinking FILTER filters rows before aggregation
• Confusing FILTER with WHERE clause
• Assuming FILTER changes GROUP BY columns

2. Which of the following is the correct syntax to count only rows where status = 'active' using FILTER in PostgreSQL?

easy

A. SELECT COUNT(*) FILTER (WHERE status = 'active') FROM users;

B. SELECT COUNT(*) FROM users FILTER (WHERE status = 'active');

C. SELECT COUNT(*) WHERE FILTER status = 'active' FROM users;

D. SELECT COUNT(*) FILTER WHERE status = 'active' FROM users;

Solution

1. Step 1: Recall FILTER syntax in aggregate functions

   The correct syntax places FILTER with parentheses and a WHERE clause inside the aggregate function: COUNT(*) FILTER (WHERE condition).

2. Step 2: Check each option

   SELECT COUNT(*) FILTER (WHERE status = 'active') FROM users; matches the correct syntax. The other options misuse FILTER placement or syntax.

3. Final Answer:

   SELECT COUNT(*) FILTER (WHERE status = 'active') FROM users; -> Option A

4. Quick Check:

   FILTER syntax = FILTER (WHERE condition) [OK]

Hint: Use FILTER (WHERE condition) inside aggregate parentheses [OK]

Common Mistakes:

• Omitting parentheses after FILTER
• Placing FILTER outside aggregate function
• Using FILTER without WHERE keyword

3. Given the table sales with columns region and amount, what will this query return?

SELECT region, SUM(amount) FILTER (WHERE amount > 100) AS big_sales FROM sales GROUP BY region;

medium

A. Sum of all sales amounts per region.

B. Count of sales with amount greater than 100 per region.

C. Sum of sales amounts less than or equal to 100 per region.

D. Sum of sales amounts greater than 100 per region.

Solution

1. Step 1: Understand the FILTER condition in SUM

   The FILTER clause limits the SUM to only rows where amount > 100.

2. Step 2: Analyze the GROUP BY effect

   The query groups rows by region, then sums only amounts above 100 in each group.

3. Final Answer:

   Sum of sales amounts greater than 100 per region. -> Option D

4. Quick Check:

   SUM with FILTER (WHERE amount > 100) = sum of big sales [OK]

Hint: FILTER limits rows inside SUM to amount > 100 [OK]

Common Mistakes:

• Thinking it sums all amounts, ignoring FILTER
• Confusing SUM with COUNT
• Ignoring GROUP BY effect

4. Identify the error in this query:

SELECT department, COUNT(*) FILTER (status = 'active') FROM employees GROUP BY department;

medium

A. Missing WHERE keyword inside FILTER clause.

B. GROUP BY should include status column.

C. COUNT(*) cannot be used with FILTER.

D. FILTER clause must be outside aggregate function.

Solution

1. Step 1: Check FILTER clause syntax

   FILTER requires a WHERE keyword before the condition: FILTER (WHERE condition).

2. Step 2: Verify other parts

   GROUP BY on department is correct. COUNT(*) works with FILTER. FILTER must be inside aggregate function.

3. Final Answer:

   Missing WHERE keyword inside FILTER clause. -> Option A

4. Quick Check:

   FILTER syntax needs WHERE [OK]

Hint: Always include WHERE inside FILTER parentheses [OK]

Common Mistakes:

• Omitting WHERE in FILTER
• Placing FILTER outside aggregate
• Adding unnecessary columns to GROUP BY

5. You want to find the total sales and the total sales above 500 for each store in one query. Which query correctly uses FILTER to achieve this?

hard

A. SELECT store, SUM(amount) FILTER (WHERE amount > 500) AS total_sales, SUM(amount) AS big_sales FROM sales GROUP BY store;

B. SELECT store, SUM(amount) AS total_sales, COUNT(amount) FILTER (WHERE amount > 500) AS big_sales FROM sales GROUP BY store;

C. SELECT store, SUM(amount) AS total_sales, SUM(amount) FILTER (WHERE amount > 500) AS big_sales FROM sales GROUP BY store;

D. SELECT store, COUNT(amount) AS total_sales, SUM(amount) FILTER (WHERE amount > 500) AS big_sales FROM sales GROUP BY store;

Solution

1. Step 1: Identify total sales calculation

   Total sales is the sum of all amounts per store, so SUM(amount) without filter is correct.

2. Step 2: Identify big sales calculation

   Big sales are sums of amounts greater than 500, so SUM(amount) FILTER (WHERE amount > 500) is correct.

3. Step 3: Check options

   SELECT store, SUM(amount) AS total_sales, SUM(amount) FILTER (WHERE amount > 500) AS big_sales FROM sales GROUP BY store; correctly assigns total_sales and big_sales. The other options mix sums and counts incorrectly or swap columns.

4. Final Answer:

   SELECT store, SUM(amount) AS total_sales, SUM(amount) FILTER (WHERE amount > 500) AS big_sales FROM sales GROUP BY store; -> Option C

5. Quick Check:

   SUM with and without FILTER for totals [OK]

Hint: Use SUM(amount) and SUM(amount) FILTER (WHERE ...) together [OK]

Common Mistakes:

• Swapping filtered and unfiltered sums
• Using COUNT instead of SUM for amounts
• Misplacing FILTER clause