LEAD function for next row access in SQL - Step-by-Step Execution

Concept Flow - LEAD function for next row access

Start with ordered rows

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For each row

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Look at next row in order

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Return value from next row

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If no next row, return NULL or default

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Output current row + next row value

The LEAD function looks at the next row in a defined order and returns a value from that row for the current row, or NULL if there is no next row.

Execution Sample

SQL

SELECT id, sales, LEAD(sales) OVER (ORDER BY id) AS next_sales
FROM sales_data;

This query shows each row's sales and the sales value from the next row ordered by id.

Execution Table

Row	id	sales	LEAD(sales) OVER (ORDER BY id)	Explanation
1	101	200	250	Next row id=102 sales=250 returned
2	102	250	300	Next row id=103 sales=300 returned
3	103	300	NULL	No next row, so NULL returned

💡 No next row after id=103, so LEAD returns NULL

Variable Tracker

Variable	Start	After Row 1	After Row 2	After Row 3
id	N/A	101	102	103
sales	N/A	200	250	300
LEAD(sales)	N/A	250	300	NULL

Key Moments - 2 Insights

Why does LEAD return NULL for the last row?

Because there is no next row after the last one, LEAD returns NULL as shown in execution_table row 3.

Does LEAD change the order of rows?

No, LEAD uses the order to look ahead but the output rows stay in the original order, as seen in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the LEAD(sales) value for row with id=102?

A 200

B 250

C 300

D NULL

💡 Hint

Check the second row in execution_table under LEAD(sales) column

At which row does LEAD return NULL?

A Row 1

B Row 3

C Row 2

D None

💡 Hint

Look at the last row in execution_table for LEAD(sales) value

If we order by sales descending, how would LEAD(sales) for id=101 change?

A It would be 300

B It would be 250

C It would be NULL

D It would be 200

💡 Hint

Ordering changes next row; check how LEAD depends on ORDER BY clause

Concept Snapshot

LEAD(column) OVER (ORDER BY col) returns the next row's column value
in the specified order. If no next row, returns NULL by default.
Useful for comparing current row with next row data.
Does not change row order in output.
Can specify default value instead of NULL.

Full Transcript

The LEAD function in SQL lets you look at the next row's value in a result set ordered by a column. For each row, LEAD returns the value from the next row in that order. If there is no next row, it returns NULL by default. This is useful to compare current row data with the next row, like seeing next sales amount. The rows stay in the original order; LEAD just peeks ahead. For example, with rows ordered by id, LEAD(sales) returns the sales value from the next id. The last row has no next row, so LEAD returns NULL there. Changing the ORDER BY changes which row is considered next. LEAD helps analyze sequences or trends in data.