# Lead over Use idiomatic Materialize SQL to access the next row's value (lead) when ordered by a field that advances in a regular pattern, such as in regular intervals. ## Overview The "lead over" query pattern accesses the field value of the next row as determined by some ordering. For "lead over (order by)" queries whose ordering can be represented by some equality condition (such as when ordering by a field that increases at a regular interval), Materialize provides an idiomatic SQL as an alternative to the window function. > ### Materialize and window functions > For [window functions](/sql/functions/#window-functions), when an input record > in a partition (as determined by the `PARTITION BY` clause of your window > function) is added/removed/changed, Materialize recomputes the results for the > entire window partition. This means that when a new batch of input data arrives > (that is, every second), **the amount of computation performed is proportional > to the total size of the touched partitions**. > For example, assume that in a given second, 20 input records change, and these > records belong to **10** different partitions, where the average size of each > partition is **100**. Then, amount of work to perform is proportional to > computing the window function results for **10\*100=1000** rows. > To avoid performance issues that may arise as the number of records grows, > consider rewriting your query to use idiomatic Materialize SQL instead of window > functions. If your query cannot be rewritten without the window functions and > the performance of window functions is insufficient for your use case, please > [contact our team](/support/). ## Idiomatic Materialize SQL > **Important:** Do not use if the "lead over (order by)" ordering cannot be represented by an > equality match. ### Exclude the last row in results **Idiomatic Materialize SQL:** To access the lead (next row's field value) ordered by some field that increases in **regular** intervals, use a self join that specifies an **equality condition** on the order by field (e.g., `WHERE t1.order_field = t2.order_field - 1`, `WHERE t1.order_field = t2.order_field * 2`, etc.). The query *excludes* the last row in the results since it does not have a next row.


Idiomatic Materialize SQL	Use a self join that specifies an equality match on the lead's order by field (e.g., `fieldA`). The order by field must increment in a regular pattern in order to be represented by an equality condition (e.g., `WHERE t1.fieldA = t2.fieldA - ...`). The query excludes the last row in the results since it does not have a next row. > Important: The idiomatic Materialize SQL applies only to those "lead over" queries whose > ordering can be represented by some equality condition. ```mzsql -- Excludes the last row in the results -- SELECT t1.fieldA, t2.fieldB as next_row_value FROM tableA t1, tableA t2 WHERE t1.fieldA = t2.fieldA - ... -- or some other operand ORDER BY fieldA; ```
Anti-pattern ❌	Avoid the use of [`LEAD(fieldZ) OVER (ORDER BY ...) window function`](/sql/functions/#lead) when the order by field increases in a regular pattern. ```nofmt -- Anti-pattern. Avoid. -- SELECT fieldA, ... LEAD(fieldZ) OVER (ORDER BY fieldA) as next_row_value FROM tableA; ```

Idiomatic Materialize SQL

Use a self join that specifies an **equality match** on the lead's order by field (e.g., `fieldA`). The order by field must increment in a regular pattern in order to be represented by an equality condition (e.g., `WHERE t1.fieldA = t2.fieldA - ...`). The query *excludes* the last row in the results since it does not have a next row. > **Important:** The idiomatic Materialize SQL applies only to those "lead over" queries whose > ordering can be represented by some **equality condition**.
```mzsql -- Excludes the last row in the results -- SELECT t1.fieldA, t2.fieldB as next_row_value FROM tableA t1, tableA t2 WHERE t1.fieldA = t2.fieldA - ... -- or some other operand ORDER BY fieldA; ```

Anti-pattern ❌

Avoid the use of [`LEAD(fieldZ) OVER (ORDER BY ...) window function`](/sql/functions/#lead) when the order by field increases in a regular pattern.

```nofmt -- Anti-pattern. Avoid. -- SELECT fieldA, ... LEAD(fieldZ) OVER (ORDER BY fieldA) as next_row_value FROM tableA; ```

### Include the last row in results **Idiomatic Materialize SQL:** To access the lead (next row's field value) ordered by some field that increases in **regular** intervals, use a self [`LEFT JOIN/LEFT OUTER JOIN`](/sql/select/join/#left-outer-join) that specifies an **equality condition** on the order by field (e.g., `ON t1.order_field = t2.order_field - 1`, `ON t1.order_field = t2.order_field * 2`, etc.). The `LEFT JOIN/LEFT OUTER JOIN` query *includes* the last row, returning `null` as its lead value.


Idiomatic Materialize SQL	Use a self [`LEFT JOIN/LEFT OUTER JOIN`](/sql/select/join/#left-outer-join) (e.g., `FROM tableA t1 LEFT JOIN tableA t2`) that specifies an equality match on the lag's order by field (e.g., `fieldA`). The order by field must increment in a regular pattern in order to be represented by an equality condition (e.g., `ON t1.fieldA = t2.fieldA - ...`). The query includes the last row, returning `null` as its lead value. > Important: The idiomatic Materialize SQL applies only to those "lead over" queries whose > ordering can be represented by some equality condition. ```mzsql -- Includes the last row in the response -- SELECT t1.fieldA, t2.fieldB as next_row_value FROM tableA t1 LEFT JOIN tableA t2 ON t1.fieldA = t2.fieldA - ... -- or some other operand ORDER BY fieldA; ```
Anti-pattern ❌	Avoid the use of [`LEAD(fieldZ) OVER (ORDER BY ...) window function`](/sql/functions/#lead) when the order by field increases in regular intervals. ```nofmt SELECT fieldA, ... LEAD(fieldZ) OVER (ORDER BY fieldA) as next_row_value FROM tableA; ```

Idiomatic Materialize SQL

Use a self [`LEFT JOIN/LEFT OUTER JOIN`](/sql/select/join/#left-outer-join) (e.g., `FROM tableA t1 LEFT JOIN tableA t2`) that specifies an **equality match** on the lag's order by field (e.g., `fieldA`). The order by field must increment in a regular pattern in order to be represented by an equality condition (e.g., `ON t1.fieldA = t2.fieldA - ...`). The query *includes* the last row, returning `null` as its lead value. > **Important:** The idiomatic Materialize SQL applies only to those "lead over" queries whose > ordering can be represented by some **equality condition**. ```mzsql -- Includes the last row in the response -- SELECT t1.fieldA, t2.fieldB as next_row_value FROM tableA t1 LEFT JOIN tableA t2 ON t1.fieldA = t2.fieldA - ... -- or some other operand ORDER BY fieldA; ```

Anti-pattern ❌

Avoid the use of [`LEAD(fieldZ) OVER (ORDER BY ...) window function`](/sql/functions/#lead) when the order by field increases in regular intervals.

```nofmt SELECT fieldA, ... LEAD(fieldZ) OVER (ORDER BY fieldA) as next_row_value FROM tableA; ```

## Examples > **Note:** The example data can be found in the > [Appendix](/transform-data/idiomatic-materialize-sql/appendix/example-orders). ### Find next row's value (exclude the last row in results) Using idiomatic Materialize SQL, the following example finds the next day's order total. That is, the example uses a self join on `orders_daily_totals`. The row ordering on the `order_date` field is represented by an **equality condition** using an [interval of `1 DAY`](/sql/types/interval/#valid-operations). The query excludes the last row in the results since the last row does not have a next row.


Materialize SQL ✅	```mzsql -- Excludes the last row in results -- SELECT o1.order_date, o1.daily_total, o2.daily_total as next_daily_total FROM orders_daily_totals o1, orders_daily_totals o2 WHERE o1.order_date = o2.order_date - INTERVAL '1' DAY ORDER BY order_date; ``` > Important: The idiomatic Materialize SQL applies only to those "lead over" queries whose > ordering can be represented by some equality condition.
Anti-pattern ❌	Avoid the use of [`LEAD() OVER (ORDER BY ...)` window function](/sql/functions/#lead) to access next row's value if the order by field increases in regular intervals. ```nofmt -- Anti-pattern. Includes the last row's value. -- SELECT order_date, daily_total, LEAD(daily_total) OVER (ORDER BY order_date) as next_daily_total FROM orders_daily_totals; ```

Materialize SQL ✅

```mzsql -- Excludes the last row in results -- SELECT o1.order_date, o1.daily_total, o2.daily_total as next_daily_total FROM orders_daily_totals o1, orders_daily_totals o2 WHERE o1.order_date = o2.order_date - INTERVAL '1' DAY ORDER BY order_date; ``` > **Important:** The idiomatic Materialize SQL applies only to those "lead over" queries whose > ordering can be represented by some **equality condition**.

Anti-pattern ❌

Avoid the use of [`LEAD() OVER (ORDER BY ...)` window function](/sql/functions/#lead) to access next row's value if the order by field increases in regular intervals.

```nofmt -- Anti-pattern. Includes the last row's value. -- SELECT order_date, daily_total, LEAD(daily_total) OVER (ORDER BY order_date) as next_daily_total FROM orders_daily_totals; ```

### Find next row's value (include the last row in results) Using idiomatic Materialize SQL, the following example finds the next day's order total. The example uses a self [`LEFT JOIN/LEFT OUTER JOIN`](/sql/select/join/#left-outer-join) on `orders_daily_totals`. The row ordering on the `order_date` field is represented by an **equality condition** using an [interval of `1 DAY`](/sql/types/interval/#valid-operations)). The query includes the last row in the results, using `null` as the next row's value.


Materialize SQL ✅	```mzsql -- Include the last row in the results -- SELECT o1.order_date, o1.daily_total, o2.daily_total as next_daily_total FROM orders_daily_totals o1 LEFT JOIN orders_daily_totals o2 ON o1.order_date = o2.order_date - INTERVAL '1' DAY ORDER BY order_date; ``` > Important: The idiomatic Materialize SQL applies only to those "lead over" queries whose > ordering can be represented by some equality condition.
Anti-pattern ❌	Avoid the use of [`LEAD() OVER (ORDER BY ...)` window function](/sql/functions/#lead) to access next row's value if the order by field increases in a regular pattern. ```nofmt -- Anti-pattern. Includes the last row in results. -- SELECT order_date, daily_total, LEAD(daily_total) OVER (ORDER BY order_date) as next_daily_total FROM orders_daily_totals; ```

Materialize SQL ✅

```mzsql -- Include the last row in the results -- SELECT o1.order_date, o1.daily_total, o2.daily_total as next_daily_total FROM orders_daily_totals o1 LEFT JOIN orders_daily_totals o2 ON o1.order_date = o2.order_date - INTERVAL '1' DAY ORDER BY order_date; ``` > **Important:** The idiomatic Materialize SQL applies only to those "lead over" queries whose > ordering can be represented by some **equality condition**.

Anti-pattern ❌

Avoid the use of [`LEAD() OVER (ORDER BY ...)` window function](/sql/functions/#lead) to access next row's value if the order by field increases in a regular pattern.

```nofmt -- Anti-pattern. Includes the last row in results. -- SELECT order_date, daily_total, LEAD(daily_total) OVER (ORDER BY order_date) as next_daily_total FROM orders_daily_totals; ```

## See also - [Lag over](/transform-data/idiomatic-materialize-sql/lag) - [`INTERVAL`](/sql/types/interval/) - [`LEFT JOIN/LEFT OUTER JOIN`](/sql/select/join/#left-outer-join) - [`LEAD()`](/sql/functions/#lead) - [Window functions](/sql/functions/#window-functions)