MySQL LEAD Function

MySQL LEAD Function

The LEAD() function in MySQL is a window function introduced in MySQL 8.0. It allows you to access data from a subsequent row in the same result set without requiring a self-join. This is particularly useful for comparing values in consecutive rows or performing calculations based on subsequent rows.

Syntax

LEAD(expression, offset, default) OVER (
    [PARTITION BY column_name]
    [ORDER BY column_name]
)

• expression: The column or calculation whose value is returned from a subsequent row.
• offset: The number of rows ahead from the current row (default is 1).
• default: The value returned if no subsequent row exists (default is NULL).
• PARTITION BY: Divides the result set into partitions; the function is applied to each partition independently.
• ORDER BY: Specifies the order of rows in which the function is applied.

Key Points

1. Default Values:

   • If no subsequent row exists, the function returns NULL unless a default value is specified.

2. Offset:

   • The offset determines how far ahead the function looks for the value.
   • By default, offset is 1.

3. Partitioning:

   • The result set can be partitioned into groups using the PARTITION BY clause, and the function will operate within each group.

4. Ordering:

   • The ORDER BY clause ensures rows are processed in a specific order.

Example 1: Simple Use of LEAD()

Suppose you have a table sales:

sale_id	region	amount
1	North	100
2	North	200
3	North	150
4	South	300
5	South	250

To fetch the current amount and the next amount:

SELECT sale_id, region, amount,
       LEAD(amount) OVER (PARTITION BY region ORDER BY amount) AS next_amount
FROM sales;

Output:

sale_id	region	amount	next_amount
1	North	100	150
3	North	150	200
2	North	200	NULL
5	South	250	300
4	South	300	NULL

Example 2: Using LEAD() with an Offset

To fetch the amount two rows ahead:

SELECT sale_id, region, amount,
       LEAD(amount, 2) OVER (PARTITION BY region ORDER BY amount) AS two_rows_ahead
FROM sales;

Output:

sale_id	region	amount	two_rows_ahead
1	North	100	200
3	North	150	NULL
2	North	200	NULL
5	South	250	NULL
4	South	300	NULL

Example 3: Using LEAD() with a Default Value

To provide a default value when no subsequent row exists:

SELECT sale_id, region, amount,
       LEAD(amount, 1, 0) OVER (PARTITION BY region ORDER BY amount) AS next_amount_with_default
FROM sales;

Output:

sale_id	region	amount	next_amount_with_default
1	North	100	150
3	North	150	200
2	North	200	0
5	South	250	300
4	South	300	0

Example 4: Calculate Change Between Rows

Using the LEAD() function to calculate the difference between the current amount and the next amount:

SELECT sale_id, region, amount,
       LEAD(amount) OVER (PARTITION BY region ORDER BY amount) AS next_amount,
       LEAD(amount) OVER (PARTITION BY region ORDER BY amount) - amount AS amount_difference
FROM sales;

Output:

sale_id	region	amount	next_amount	amount_difference
1	North	100	150	50
3	North	150	200	50
2	North	200	NULL	NULL
5	South	250	300	50
4	South	300	NULL	NULL

Use Cases

1. Comparative Analysis:

   • Compare current values to future values within a dataset.

2. Trend Analysis:

   • Analyze changes in metrics like sales or prices over time.

3. Gaps in Data:

   • Identify and analyze gaps between consecutive rows.

4. Forecasting:

   • Use future values to derive trends or project outcomes.

Conclusion

The LEAD() function is a powerful tool for accessing subsequent rows in a result set. It simplifies tasks like comparisons, trend analysis, and gap detection without requiring complex subqueries or joins. Its flexibility with partitioning and ordering makes it invaluable for advanced analytics in MySQL.