LEAD function for next row access in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the LEAD function do in SQL?

The LEAD function lets you look at the value in the next row of a result set without moving the cursor. It helps compare current row data with the next row's data.

beginner

Write the basic syntax of the LEAD function.

LEAD(column_name, offset, default_value) OVER (PARTITION BY partition_column ORDER BY order_column)

beginner

What happens if the LEAD function tries to access a row beyond the last row?

If there is no next row, LEAD returns NULL or the default value you provide.

intermediate

Why do we use the OVER clause with LEAD?

The OVER clause defines how to group (partition) and order rows so LEAD knows which row is next.

beginner

Give a real-life example where LEAD can be useful.

LEAD can help find the next day's sales for each product to compare if sales increased or decreased.

What does LEAD(column, 1) return?

A Value from the current row in the column

B Value from the previous row in the column

C Value from the next row in the column

D Sum of all values in the column

LEAD with offset 1 returns the value from the next row in the specified column.

What will LEAD(column, 2, 0) return if there is no second next row?

A 0

B Error

C Value from the current row

D NULL

The third parameter is the default value returned if the next row does not exist. Here it is 0.

Which clause is mandatory to use LEAD function properly?

A WHERE

B GROUP BY

C HAVING

D ORDER BY inside OVER()

LEAD requires an ORDER BY clause inside the OVER() to define the row order.

Can LEAD be used without PARTITION BY?

A No, PARTITION BY is mandatory

B Yes, it works on the entire result set

C Only with GROUP BY

D Only with HAVING

PARTITION BY is optional. Without it, LEAD works on the entire ordered result set.

What type of SQL function is LEAD?

A Window function

B Aggregate function

C Scalar function

D String function

LEAD is a window function that accesses data from another row in the result set.

Explain how the LEAD function works and why it is useful.

Think about looking ahead in a list of rows.

Describe a scenario where you would use LEAD in a sales report.

Imagine checking if sales went up or down the next day.

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