OVER clause with ORDER BY in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to calculate the running total of sales ordered by date.

SQL

SELECT sales_date, amount, SUM(amount) OVER (ORDER BY [1]) AS running_total FROM sales;

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

Aamount

Bsales_date

Ccustomer_id

Dregion

Attempts:

3 left

💡 Hint

Common Mistakes

Ordering by amount instead of date changes the running total order.

Using a column unrelated to ordering like customer_id causes wrong results.

The OVER clause with ORDER BY sales_date calculates the running total in date order.

2fill in blank

medium

Complete the code to rank employees by their salary within each department.

SQL

SELECT employee_id, department_id, salary, RANK() OVER (PARTITION BY department_id ORDER BY [1] DESC) AS dept_rank FROM employees;

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

Asalary

Bhire_date

Cdepartment_id

Demployee_id

Attempts:

3 left

💡 Hint

Common Mistakes

Ordering by employee_id or department_id instead of salary.

Not using DESC to rank highest salary first.

The RANK() function ranks employees by salary descending within each department.

3fill in blank

hard

Fix the error in the code to calculate the cumulative average salary ordered by hire date.

SQL

SELECT employee_id, salary, AVG(salary) OVER (ORDER BY [1] ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS cum_avg_salary FROM employees;

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

Ahire_date

Bsalary

Cemployee_id

Ddepartment_id

Attempts:

3 left

💡 Hint

Common Mistakes

Ordering by salary causes incorrect cumulative average.

Using employee_id or department_id for ordering is incorrect here.

The cumulative average should be ordered by hire_date to calculate average salary up to each employee's hire date.

4fill in blank

hard

Fill both blanks to calculate the difference between each sale amount and the previous sale amount ordered by sales_date.

SQL

SELECT sales_date, amount, amount - LAG(amount) OVER (ORDER BY [1] [2]) AS diff FROM sales;

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

Asales_date

BASC

CDESC

Damount

Attempts:

3 left

💡 Hint

Common Mistakes

Using DESC reverses the order and changes the meaning of previous row.

Ordering by amount instead of sales_date gives wrong previous row.

The LAG() function looks at the previous row ordered by sales_date ASC to find the difference.

5fill in blank

hard

Fill all three blanks to calculate the cumulative sum of sales amount partitioned by region and ordered by sales_date.

SQL

SELECT region, sales_date, amount, SUM(amount) OVER (PARTITION BY [1] ORDER BY [2] [3]) AS cum_sum FROM sales;

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

Aregion

Bsales_date

CASC

Damount

Attempts:

3 left

💡 Hint

Common Mistakes

Partitioning by sales_date or amount is incorrect.

Ordering by amount or descending order changes the cumulative sum meaning.

The cumulative sum is partitioned by region and ordered by sales_date ASC to sum sales in date order per region.

Practice

1. What does the OVER clause with ORDER BY do in SQL?

easy

A. It creates a new table sorted by the specified column.

B. It performs calculations on rows in a specific order without grouping them.

C. It filters rows based on order conditions.

D. It groups rows and then orders the groups.

Solution

1. Step 1: Understand the OVER clause

   The OVER clause allows window functions to work on a set of rows related to the current row without grouping them.

2. Step 2: Role of ORDER BY inside OVER

   ORDER BY inside OVER defines the order in which rows are processed for functions like ROW_NUMBER() or SUM() running totals.

3. Final Answer:

   It performs calculations on rows in a specific order without grouping them. -> Option B

4. Quick Check:

   OVER with ORDER BY = ordered calculations without grouping [OK]

Hint: OVER with ORDER BY orders rows for calculations, no grouping needed [OK]

Common Mistakes:

• Thinking OVER groups rows like GROUP BY
• Confusing ORDER BY inside OVER with filtering
• Assuming it creates a new sorted table

2. Which of the following is the correct syntax to assign row numbers ordered by salary using the OVER clause?

easy

A. SELECT ROW_NUMBER(ORDER BY salary) AS rank FROM employees;

B. SELECT ROW_NUMBER() AS rank OVER ORDER BY salary FROM employees;

C. SELECT ROW_NUMBER() ORDER BY salary FROM employees;

D. SELECT ROW_NUMBER() OVER (ORDER BY salary) AS rank FROM employees;

Solution

1. Step 1: Identify correct ROW_NUMBER() syntax

   ROW_NUMBER() requires OVER() with ORDER BY inside parentheses to define ordering.

2. Step 2: Check each option

   SELECT ROW_NUMBER() OVER (ORDER BY salary) AS rank FROM employees; correctly uses ROW_NUMBER() OVER (ORDER BY salary). Others miss parentheses or misplace clauses.

3. Final Answer:

   SELECT ROW_NUMBER() OVER (ORDER BY salary) AS rank FROM employees; -> Option D

4. Quick Check:

   ROW_NUMBER() needs OVER(ORDER BY ...) [OK]

Hint: ROW_NUMBER() must be followed by OVER(ORDER BY column) [OK]

Common Mistakes:

• Omitting OVER() parentheses
• Placing ORDER BY outside OVER()
• Using ROW_NUMBER() without OVER

3. Given the table sales with columns id, amount, what is the output of this query?

SELECT id, amount, SUM(amount) OVER (ORDER BY id) AS running_total FROM sales ORDER BY id;

medium

A. The query returns an error due to missing GROUP BY.

B. Each row shows id, amount, and total sum of all amounts repeated.

C. Each row shows id, amount, and a running total of amounts ordered by id.

D. Rows are grouped by id with sum of amounts per group.

Solution

1. Step 1: Understand SUM() OVER (ORDER BY id)

   This calculates a running total of amount, adding amounts in order of id for each row.

2. Step 2: Analyze output per row

   Each row shows its id, amount, and the sum of amounts from first id up to current id.

3. Final Answer:

   Each row shows id, amount, and a running total of amounts ordered by id. -> Option C

4. Quick Check:

   SUM() OVER with ORDER BY = running total [OK]

Hint: SUM() OVER(ORDER BY) gives running totals, not grouped sums [OK]

Common Mistakes:

• Thinking it sums all rows for each row
• Expecting GROUP BY to be required
• Confusing running total with grouped sum

4. Identify the error in this query:

SELECT id, RANK() OVER ORDER BY score FROM players;

medium

A. Missing parentheses after OVER and ORDER BY inside them.

B. RANK() cannot be used with OVER clause.

C. ORDER BY should be outside the OVER clause.

D. The query is correct and will run without errors.

Solution

1. Step 1: Check syntax of RANK() with OVER

   RANK() requires OVER() with parentheses enclosing ORDER BY clause.

2. Step 2: Identify missing parentheses

   The query misses parentheses after OVER and does not put ORDER BY inside them, causing syntax error.

3. Final Answer:

   Missing parentheses after OVER and ORDER BY inside them. -> Option A

4. Quick Check:

   OVER() must have parentheses with ORDER BY inside [OK]

Hint: Always use OVER(...) with parentheses enclosing ORDER BY [OK]

Common Mistakes:

• Writing OVER without parentheses
• Placing ORDER BY outside OVER()
• Assuming RANK() works without OVER

5. You want to assign a rank to employees based on their salary, but if two employees have the same salary, they should get the same rank and the next rank should skip accordingly. Which query correctly uses the OVER clause with ORDER BY to achieve this?

hard

A. SELECT employee_id, salary, RANK() OVER (ORDER BY salary DESC) AS salary_rank FROM employees;

B. SELECT employee_id, salary, ROW_NUMBER() OVER (ORDER BY salary DESC) AS salary_rank FROM employees;

C. SELECT employee_id, salary, DENSE_RANK() OVER (ORDER BY salary DESC) AS salary_rank FROM employees;

D. SELECT employee_id, salary, RANK() OVER ORDER BY salary DESC AS salary_rank FROM employees;

Solution

1. Step 1: Understand ranking functions

   RANK() assigns same rank to ties and skips ranks after ties; DENSE_RANK() assigns same rank but no skips; ROW_NUMBER() assigns unique numbers.

2. Step 2: Check syntax and behavior

   SELECT employee_id, salary, RANK() OVER (ORDER BY salary DESC) AS salary_rank FROM employees; uses RANK() with correct OVER(ORDER BY salary DESC) syntax, matching requirement. SELECT employee_id, salary, RANK() OVER ORDER BY salary DESC AS salary_rank FROM employees; has syntax error. ROW_NUMBER() and DENSE_RANK() do not skip ranks after ties.

3. Final Answer:

   SELECT employee_id, salary, RANK() OVER (ORDER BY salary DESC) AS salary_rank FROM employees; -> Option A

4. Quick Check:

   RANK() with OVER(ORDER BY) skips ranks after ties [OK]

Hint: Use RANK() OVER(ORDER BY) for ranks with gaps on ties [OK]

Common Mistakes:

• Using ROW_NUMBER() which never skips ranks
• Using DENSE_RANK() which does not skip ranks
• Missing parentheses in OVER clause