LAG and LEAD for row comparison in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the LAG() function do in SQL?

LAG() lets you look at a value from a previous row in the same result set without using a self-join. It's like looking back one step in a list.

beginner

How does the LEAD() function differ from LAG()?

LEAD() looks forward to the next row's value, while LAG() looks backward to the previous row's value. Both help compare rows easily.

intermediate

What is a common use case for LAG and LEAD functions?

They are often used to compare current row values with previous or next rows, like finding changes over time or calculating differences between rows.

intermediate

Write a simple SQL snippet using LAG() to get the previous month's sales for each row.

SELECT month, sales, LAG(sales) OVER (ORDER BY month) AS prev_month_sales FROM sales_data;

beginner

What happens if there is no previous row when using LAG()?

LAG() returns NULL if there is no previous row to look at, so you can handle missing data gracefully.

What does LEAD() function return?

A Value from the previous row

B Current row value

C Sum of all rows

D Value from the next row

LEAD() returns the value from the next row in the result set.

Which clause is required to use LAG() or LEAD() functions?

A ORDER BY inside OVER()

B WHERE

C GROUP BY

D HAVING

LAG() and LEAD() require an ORDER BY clause inside the OVER() to define row order.

If you want to compare each row with the row two steps before, which function and parameter do you use?

A LEAD(column, 1)

B LEAD(column, 2)

C LAG(column, 2)

D LAG(column, 1)

LAG(column, 2) looks two rows back from the current row.

What value does LAG() return if there is no previous row?

A 0

B NULL

C Empty string

D Error

LAG() returns NULL when no previous row exists.

Which of these is NOT a use case for LAG and LEAD?

A Looking up values in unrelated tables

B Calculating running totals

C Finding differences between rows

D Comparing current row with previous or next row

LAG and LEAD work within the same result set, not for unrelated table lookups.

Explain how LAG and LEAD functions help compare rows in a table.

Think about looking backward and forward in a list.

Write a SQL query using LEAD() to find the next month's sales for each month in a sales table.

Use LEAD() with ORDER BY month inside OVER()

Practice

1. What is the main purpose of the LAG function in PostgreSQL?

easy

A. To sort rows in ascending order

B. To access data from the next row in the same result set

C. To delete duplicate rows

D. To access data from a previous row in the same result set

Solution

1. Step 1: Understand LAG function behavior

   The LAG function returns the value from a previous row based on the current row's position.

2. Step 2: Compare with other options

   LEAD accesses the next row, sorting and deleting duplicates are unrelated to LAG.

3. Final Answer:

   To access data from a previous row in the same result set -> Option D

4. Quick Check:

   LAG = previous row data access [OK]

Hint: LAG looks backward, LEAD looks forward in rows [OK]

Common Mistakes:

• Confusing LAG with LEAD
• Thinking LAG sorts rows
• Assuming LAG deletes duplicates

2. Which of the following is the correct syntax to use LEAD 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(ORDER BY salary) OVER (employee_id)

Solution

1. Step 1: Recall correct LEAD syntax

   The correct syntax uses LEAD(column) OVER (ORDER BY column) to specify the column and ordering.

2. Step 2: Check each option

   LEAD(salary) OVER (ORDER BY employee_id) matches the correct syntax; others misuse parentheses or clause order.

3. Final Answer:

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

4. Quick Check:

   LEAD(column) OVER (ORDER BY column) [OK]

Hint: LEAD/LAG need OVER with ORDER BY inside parentheses [OK]

Common Mistakes:

• Placing ORDER BY outside OVER clause
• Missing parentheses around ORDER BY
• Putting ORDER BY inside function parameters

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

SELECT month, revenue, LAG(revenue) OVER (ORDER BY month) AS prev_revenue FROM sales;

medium

A. Each row shows only current month revenue

B. Each row shows current month revenue and next month's revenue

C. Each row shows current month revenue and previous month's revenue

D. Query will cause a syntax error

Solution

1. Step 1: Understand LAG with ORDER BY month

   The LAG(revenue) returns the revenue from the previous month ordered by month.

2. Step 2: Interpret query output

   Each row shows current month revenue and previous month's revenue in prev_revenue. The first row's prev_revenue will be NULL.

3. Final Answer:

   Each row shows current month revenue and previous month's revenue -> Option C

4. Quick Check:

   LAG = previous row value [OK]

Hint: LAG returns previous row's value based on ORDER BY [OK]

Common Mistakes:

• Confusing LAG with LEAD
• Expecting next month's revenue instead
• Thinking query causes error

4. Identify the error in this query:

SELECT employee_id, salary, LEAD(salary) ORDER BY employee_id FROM employees;

medium

A. ORDER BY should be inside LEAD() function

B. Missing OVER() clause for LEAD function

C. LEAD cannot be used with salary column

D. employee_id must be in GROUP BY clause

Solution

1. Step 1: Check LEAD function syntax

   LEAD requires an OVER() clause to specify ordering and partitioning.

2. Step 2: Analyze query structure

   The query uses LEAD(salary) without OVER(), causing a syntax error.

3. Final Answer:

   Missing OVER() clause for LEAD function -> Option B

4. Quick Check:

   Window functions need OVER() [OK]

Hint: Always use OVER() with LEAD/LAG [OK]

Common Mistakes:

• Omitting OVER() clause
• Placing ORDER BY outside OVER()
• Confusing window functions with aggregates

5. You want to find all employees whose salary increased compared to the previous employee ordered by hire_date. Which query correctly uses LAG to achieve this?

hard

A. SELECT employee_id, salary FROM (SELECT employee_id, salary, LAG(salary) OVER (ORDER BY hire_date) AS prev_salary FROM employees) t WHERE salary > prev_salary;

B. SELECT employee_id, salary FROM employees WHERE salary > (SELECT LAG(salary) FROM employees ORDER BY hire_date);

C. SELECT employee_id, salary, LAG(salary) OVER (ORDER BY hire_date) AS prev_salary FROM employees WHERE salary > prev_salary;

D. SELECT employee_id, salary, LAG(salary) OVER (ORDER BY hire_date) AS prev_salary FROM employees HAVING salary > prev_salary;

Solution

1. Step 1: Understand LAG usage for filtering

   Window functions like LAG cannot be used directly in WHERE clause; use subquery or CTE to compute then filter.

2. Step 2: Analyze options

   SELECT employee_id, salary FROM (SELECT employee_id, salary, LAG(salary) OVER (ORDER BY hire_date) AS prev_salary FROM employees) t WHERE salary > prev_salary; correctly uses subquery. The second option's subquery LAG lacks OVER clause (syntax error). The third uses alias in WHERE (invalid). The fourth uses HAVING without GROUP BY (invalid).

3. Final Answer:

   SELECT employee_id, salary FROM (SELECT employee_id, salary, LAG(salary) OVER (ORDER BY hire_date) AS prev_salary FROM employees) t WHERE salary > prev_salary; -> Option A

4. Quick Check:

   Subquery to filter on window functions [OK]

Hint: Use subquery to filter on window function results [OK]

Common Mistakes:

• Using alias in WHERE clause
• Misusing HAVING without GROUP BY
• Incorrect subquery for LAG