Combining multiple aggregates in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to calculate the total number of orders.

SQL

SELECT COUNT([1]) FROM orders;

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

A*

Border_id

Ccustomer_id

Dorder_date

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(*) counts all rows but here we want to specify the column explicitly.

Using a column that can have null values may give incorrect counts.

The COUNT function counts non-null values in the specified column. Using order_id counts all orders.

2fill in blank

medium

Complete the code to find the average order amount.

SQL

SELECT AVG([1]) FROM orders;

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

Aorder_amount

Border_date

Ccustomer_id

Dorder_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-numeric column causes errors.

Choosing the wrong column leads to meaningless results.

The AVG function calculates the average of numeric values in the specified column. Here, order_amount holds the amounts.

3fill in blank

hard

Fix the error in the code to get the maximum and minimum order amounts.

SQL

SELECT MAX(order_amount) AS max_amount, MIN([1]) AS min_amount FROM orders;

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

Acustomer_id

Border_id

Corder_date

Dorder_amount

Attempts:

3 left

💡 Hint

Common Mistakes

Using different columns for MAX and MIN causes inconsistent results.

Using non-numeric columns causes errors.

Both MAX and MIN should be applied to the same numeric column order_amount to get correct results.

4fill in blank

hard

Fill both blanks to calculate the total and average order amounts.

SQL

SELECT [1](order_amount) AS total, [2](order_amount) AS average FROM orders;

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

ASUM

BAVG

CCOUNT

DMAX

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT instead of SUM for total.

Mixing up AVG and MAX functions.

SUM adds all order amounts to get the total. AVG calculates the average order amount.

5fill in blank

hard

Fill all three blanks to calculate the total orders, average amount, and maximum amount.

SQL

SELECT [1](order_id) AS total_orders, [2](order_amount) AS avg_amount, [3](order_amount) AS max_amount FROM orders;

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

ACOUNT

BAVG

CMAX

DMIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using SUM instead of COUNT for total orders.

Using MIN instead of MAX for maximum amount.

COUNT counts total orders by order_id, AVG finds average order amount, and MAX finds the highest order amount.

Practice

1. What does combining multiple aggregate functions in a single SQL query allow you to do?

easy

A. Get several summary values like totals and averages in one result

B. Run multiple queries at the same time

C. Create new tables automatically

D. Sort data without using ORDER BY

Solution

1. Step 1: Understand aggregate functions

   Aggregate functions like SUM() and AVG() calculate summary values from data.

2. Step 2: Combining aggregates in one query

   Using commas, you can list multiple aggregates in SELECT to get many summaries at once.

3. Final Answer:

   Get several summary values like totals and averages in one result -> Option A

4. Quick Check:

   Multiple aggregates = multiple summaries [OK]

Hint: Use commas to separate aggregates in SELECT [OK]

Common Mistakes:

• Thinking multiple queries run simultaneously
• Confusing aggregates with table creation
• Assuming aggregates sort data automatically

2. Which of the following is the correct syntax to combine multiple aggregates in one SQL SELECT statement?

easy

A. SELECT SUM(price) AND AVG(price) FROM sales;

B. SELECT SUM(price), AVG(price) FROM sales;

C. SELECT SUM(price) AVG(price) FROM sales;

D. SELECT SUM(price) OR AVG(price) FROM sales;

Solution

1. Step 1: Check aggregate separation

   Aggregates must be separated by commas in SELECT clause.

2. Step 2: Identify correct syntax

   SELECT SUM(price), AVG(price) FROM sales; uses commas correctly; others use AND, OR, or no separator which is invalid.

3. Final Answer:

   SELECT SUM(price), AVG(price) FROM sales; -> Option B

4. Quick Check:

   Aggregates separated by commas [OK]

Hint: Separate aggregates with commas, not AND/OR [OK]

Common Mistakes:

• Using AND or OR instead of commas
• Omitting commas between aggregates
• Writing aggregates without any separator

3. Given the table orders with column amount, what will this query return?

SELECT COUNT(*), MAX(amount), MIN(amount) FROM orders;

medium

A. Number of rows, highest amount, lowest amount

B. Sum of amounts, average amount, number of rows

C. Only the highest amount

D. Syntax error due to multiple aggregates

Solution

1. Step 1: Understand each aggregate

   COUNT(*) counts rows, MAX(amount) finds highest value, MIN(amount) finds lowest value.

2. Step 2: Combine results

   All three aggregates return one row with three summary values: count, max, and min.

3. Final Answer:

   Number of rows, highest amount, lowest amount -> Option A

4. Quick Check:

   COUNT, MAX, MIN = count, max, min [OK]

Hint: Each aggregate returns one summary value in the result [OK]

Common Mistakes:

• Confusing COUNT(*) with SUM(amount)
• Expecting multiple rows instead of one
• Thinking multiple aggregates cause syntax error

4. Identify the error in this SQL query:

SELECT SUM(price), AVG(price) FROM sales GROUP BY category;

medium

A. No error, query is correct

B. Cannot use SUM and AVG together

C. Missing GROUP BY column in SELECT clause

D. GROUP BY should be after WHERE clause

Solution

1. Step 1: Check GROUP BY usage

   When using GROUP BY category, category must appear in SELECT to show groups.

2. Step 2: Identify missing column

   Query selects only aggregates but misses category column, causing error or unexpected output.

3. Final Answer:

   Missing GROUP BY column in SELECT clause -> Option C

4. Quick Check:

   GROUP BY columns must appear in SELECT [OK]

Hint: Include GROUP BY columns in SELECT list [OK]

Common Mistakes:

• Omitting GROUP BY column in SELECT
• Thinking SUM and AVG can't be combined
• Misplacing GROUP BY clause

5. You want to find the total sales, average sales, and number of sales for each product category in a sales table with columns category and amount. Which query correctly combines these aggregates?

hard

A. SELECT category, SUM(amount) AND AVG(amount) AND COUNT(*) FROM sales GROUP BY category;

B. SELECT SUM(amount), AVG(amount), COUNT(*) FROM sales;

C. SELECT category, SUM(amount), AVG(amount), COUNT(*) FROM sales;

D. SELECT category, SUM(amount), AVG(amount), COUNT(*) FROM sales GROUP BY category;

Solution

1. Step 1: Include category in SELECT and GROUP BY

   To get aggregates per category, category must be in SELECT and GROUP BY.

2. Step 2: Combine aggregates with commas

   SUM(amount), AVG(amount), COUNT(*) are combined with commas to get totals, averages, and counts.

3. Final Answer:

   SELECT category, SUM(amount), AVG(amount), COUNT(*) FROM sales GROUP BY category; -> Option D

4. Quick Check:

   GROUP BY category with aggregates = SELECT category, SUM(amount), AVG(amount), COUNT(*) FROM sales GROUP BY category; [OK]

Hint: Group by category and list aggregates with commas [OK]

Common Mistakes:

• Missing GROUP BY clause
• Using AND instead of commas between aggregates
• Not including category in SELECT when grouping