LEAD function for next row access in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the next salary value using LEAD function.

SQL

SELECT employee_id, salary, LEAD(salary) OVER (ORDER BY employee_id) AS next_salary FROM employees WHERE employee_id = [1];

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

A10

B5

C'10'

D'5'

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around numeric values in WHERE clause.

Confusing LEAD with LAG function.

The LEAD function is used to access the next row's salary ordered by employee_id. The WHERE clause filters for employee_id = 5 (without quotes because employee_id is numeric).

2fill in blank

medium

Complete the code to get the next order date for each customer ordered by order_date.

SQL

SELECT customer_id, order_date, LEAD(order_date) OVER (PARTITION BY customer_id ORDER BY [1]) AS next_order_date FROM orders;

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

Aorder_id

Bcustomer_id

Corder_date

Dorder_amount

Attempts:

3 left

💡 Hint

Common Mistakes

Ordering by customer_id instead of order_date.

Using a column unrelated to date for ordering.

The LEAD function needs to order rows by order_date within each customer partition to get the next order date.

3fill in blank

hard

Fix the error in the LEAD function usage to get the next product price ordered by product_id.

SQL

SELECT product_id, price, LEAD(price [1]) OVER (ORDER BY product_id) AS next_price FROM products;

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

A, 1

B(1)

C, 2

D(2)

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of comma for offset argument.

Omitting the offset argument.

The LEAD function takes the offset as a second argument separated by a comma, so the correct syntax is LEAD(price, 1).

4fill in blank

hard

Fill both blanks to get the next and previous salaries for each employee ordered by employee_id.

SQL

SELECT employee_id, salary, LEAD(salary, [1]) OVER (ORDER BY employee_id) AS next_salary, LAG(salary, [2]) OVER (ORDER BY employee_id) AS previous_salary FROM employees;

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

A1

B2

C0

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as offset which returns the current row value.

Using NULL which is invalid as offset.

Both LEAD and LAG functions use 1 as the offset to get the next and previous row values respectively.

5fill in blank

hard

Fill all three blanks to get the next salary, defaulting to 0 if none, ordered by employee_id.

SQL

SELECT employee_id, salary, LEAD(salary, [1], [2]) OVER (ORDER BY [3]) AS next_salary FROM employees;

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

A1

B0

Cemployee_id

Dsalary

Attempts:

3 left

💡 Hint

Common Mistakes

Using salary for ORDER BY instead of employee_id.

Omitting default value causing NULL results.

The LEAD function uses 1 as offset, 0 as default value if no next row, and orders by employee_id.

Practice

1. What does the SQL LEAD function do in a query?

easy

A. Returns the value from the previous row in the result set

B. Returns the value from the next row in the result set

C. Calculates the sum of values in a column

D. Filters rows based on a condition

Solution

1. Step 1: Understand LEAD function purpose

   The LEAD function is used to access data from the next row relative to the current row without changing the current row position.

2. Step 2: Compare options with LEAD behavior

   Returns the value from the next row in the result set correctly describes this behavior. Options B, C, and D describe other functions or operations.

3. Final Answer:

   Returns the value from the next row in the result set -> Option B

4. Quick Check:

   LEAD = next row value [OK]

Hint: LEAD looks forward one row by default [OK]

Common Mistakes:

• Confusing LEAD with LAG (previous row)
• Thinking LEAD filters rows
• Assuming LEAD sums values

2. Which of the following is the correct syntax to use the LEAD function to get the next row's salary ordered by employee_id?

easy

A. LEAD(salary) OVER (ORDER BY employee_id)

B. LEAD(salary ORDER BY employee_id)

C. LEAD(salary) ORDER BY employee_id

D. LEAD(salary) BY employee_id

Solution

1. Step 1: Recall LEAD function syntax

   The LEAD function requires an OVER clause with an ORDER BY to define the row order for accessing the next row.

2. Step 2: Match syntax with options

   LEAD(salary) OVER (ORDER BY employee_id) correctly uses LEAD(salary) OVER (ORDER BY employee_id). Options A, B, and D have incorrect syntax.

3. Final Answer:

   LEAD(salary) OVER (ORDER BY employee_id) -> Option A

4. Quick Check:

   LEAD + OVER + ORDER BY = correct syntax [OK]

Hint: LEAD needs OVER(ORDER BY ...) syntax [OK]

Common Mistakes:

• Omitting OVER clause
• Placing ORDER BY inside LEAD parentheses
• Using BY instead of ORDER BY

3. Given the table Employees with columns id and salary sorted by id, what will be the output of this query?

SELECT id, salary, LEAD(salary) OVER (ORDER BY id) AS next_salary FROM Employees;

Assuming data:
id | salary
1 | 5000
2 | 6000
3 | 5500

medium

A. [{id:1, salary:5000, next_salary:6000}, {id:2, salary:6000, next_salary:null}, {id:3, salary:5500, next_salary:5500}]

B. [{id:1, salary:5000, next_salary:5500}, {id:2, salary:6000, next_salary:5000}, {id:3, salary:5500, next_salary:6000}]

C. [{id:1, salary:5000, next_salary:null}, {id:2, salary:6000, next_salary:null}, {id:3, salary:5500, next_salary:null}]

D. [{id:1, salary:5000, next_salary:6000}, {id:2, salary:6000, next_salary:5500}, {id:3, salary:5500, next_salary:null}]

Solution

1. Step 1: Understand LEAD output for each row

   LEAD(salary) returns the salary from the next row ordered by id. For id=1, next_salary is salary of id=2 (6000). For id=2, next_salary is salary of id=3 (5500). For id=3, no next row exists, so next_salary is NULL.

2. Step 2: Match output with options

   [{id:1, salary:5000, next_salary:6000}, {id:2, salary:6000, next_salary:5500}, {id:3, salary:5500, next_salary:null}] matches this output exactly. Other options have incorrect next_salary values.

3. Final Answer:

   [{id:1, salary:5000, next_salary:6000}, {id:2, salary:6000, next_salary:5500}, {id:3, salary:5500, next_salary:null}] -> Option D

4. Quick Check:

   LEAD returns next row's salary or NULL [OK]

Hint: Next row salary follows order, last row gets NULL [OK]

Common Mistakes:

• Mixing up next and previous rows
• Expecting last row to have a value
• Ignoring ORDER BY in LEAD

4. Identify the error in this SQL query using LEAD:

SELECT id, salary, LEAD(salary, 2) ORDER BY id FROM Employees;

medium

A. salary column does not exist

B. LEAD cannot take a second argument

C. Missing OVER clause with ORDER BY inside LEAD function

D. ORDER BY cannot be used in SELECT clause

Solution

1. Step 1: Check LEAD function syntax

   LEAD requires an OVER clause with ORDER BY to specify row order. The query misses the OVER(...) part.

2. Step 2: Analyze the query error

   ORDER BY is incorrectly placed outside OVER clause and LEAD function lacks OVER clause, causing syntax error.

3. Final Answer:

   Missing OVER clause with ORDER BY inside LEAD function -> Option C

4. Quick Check:

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

Hint: LEAD always needs OVER with ORDER BY [OK]

Common Mistakes:

• Omitting OVER clause
• Placing ORDER BY outside OVER
• Assuming LEAD can't take offset argument

5. You want to find each employee's salary and the salary two rows ahead ordered by hire_date. If no such row exists, show 0 instead of NULL. Which query correctly uses LEAD to achieve this?

hard

A. SELECT id, salary, LEAD(salary, 2, 0) OVER (ORDER BY hire_date) AS salary_after_two FROM Employees;

B. SELECT id, salary, LEAD(salary, 0, 2) OVER (ORDER BY hire_date) AS salary_after_two FROM Employees;

C. SELECT id, salary, LEAD(salary, 2) OVER (ORDER BY hire_date) AS salary_after_two FROM Employees WHERE salary_after_two IS NOT NULL;

D. SELECT id, salary, LEAD(salary, 2, NULL) OVER (ORDER BY hire_date) AS salary_after_two FROM Employees;

Solution

1. Step 1: Understand LEAD parameters

   LEAD(column, offset, default) lets you specify how many rows ahead to look and what to return if no row exists.

2. Step 2: Match query to requirements

   SELECT id, salary, LEAD(salary, 2, 0) OVER (ORDER BY hire_date) AS salary_after_two FROM Employees; uses offset 2 and default 0, ordered by hire_date, matching the requirement. SELECT id, salary, LEAD(salary, 0, 2) OVER (ORDER BY hire_date) AS salary_after_two FROM Employees; has wrong offset and default order. SELECT id, salary, LEAD(salary, 2) OVER (ORDER BY hire_date) AS salary_after_two FROM Employees WHERE salary_after_two IS NOT NULL; filters out NULLs instead of replacing. SELECT id, salary, LEAD(salary, 2, NULL) OVER (ORDER BY hire_date) AS salary_after_two FROM Employees; returns NULL instead of 0.

3. Final Answer:

   SELECT id, salary, LEAD(salary, 2, 0) OVER (ORDER BY hire_date) AS salary_after_two FROM Employees; -> Option A

4. Quick Check:

   LEAD with offset 2 and default 0 [OK]

Hint: Use LEAD(column, offset, default) with OVER(ORDER BY ...) [OK]

Common Mistakes:

• Swapping offset and default arguments
• Filtering NULL instead of setting default
• Omitting OVER clause