lag.md 8.1 KB
title: "Lag over" description: "Use idiomatic Materialize SQL to access the previous row's value (lag) when ordered by a field that advances in a regular pattern, such as in regular intervals." menu: main:
parent: idiomatic-materialize-sql
identifier: idiomatic-materialize-lag
weight: 20
Overview
The "lag over (order by )" query pattern accesses the field value of the previous row as determined by some ordering.
For "lag 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.
{{< callout >}}
Materialize and window functions
{{< idiomatic-sql/materialize-window-functions >}}
{{</ callout >}}
Idiomatic Materialize SQL
{{< important >}}
Do not use if the "lag over (order by)" ordering cannot be represented by an equality match.
{{</ important >}}
Exclude the first row in results
Idiomatic Materialize SQL: To access the lag (previous row's field value)
ordered by some field that increases in a regular pattern, 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 first row since it does not have a previous
row.
| Idiomatic Materialize SQL |
Use a self join 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., `WHERE t1.fieldA =
t2.fieldA + ...`). The
query *excludes* the first row in the results since it does not have a previous
row.
{{< important >}}
The idiomatic Materialize SQL applies only to those "lag over" queries whose
ordering can be represented by some **equality condition**.
{{}}
```mzsql -- Excludes the first row in the results -- SELECT t1.fieldA, t2.fieldB as previous_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 [`LAG(fieldZ) OVER (ORDER BY ...)`](/sql/functions/#lag) window
function when the order by field increases in a regular pattern.
```nofmt
-- Anti-pattern. Avoid. --
SELECT fieldA, ...
LAG(fieldZ) OVER (ORDER BY fieldA) as previous_row_value
FROM tableA;
```
|
Include the first row in results
Idiomatic Materialize SQL: To access the lag (previous row's field value)
ordered by some field that increases in a regular pattern, use a self
LEFT 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 first row, returning null as its
lag 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 first row, returning `null` as its lag value.
{{< important >}}
The idiomatic Materialize SQL applies only to those "lag over" queries whose
ordering can be represented by some **equality condition**.
{{}}
```mzsql -- Includes the first row in the results -- SELECT t1.fieldA, t2.fieldB as previous_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 [`LAG(fieldZ) OVER (ORDER BY ...) window
function`](/sql/functions/#lag) when the order by field increases in a regular
pattern.
```nofmt
SELECT fieldA, ...
LAG(fieldZ) OVER (ORDER BY fieldA) as previous_row_value
FROM tableA;
```
|
Examples
{{< note >}}
The example data can be found in the Appendix.
{{</ note >}}
Find previous row's value (exclude the first row in results)
Using idiomatic Materialize SQL, the following example finds the previous 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. The
query excludes the first row in the results since the first row does not have a
previous row.
| Materialize SQL ✅ | ```mzsql -- Excludes the first row in results -- SELECT o1.order_date, o1.daily_total, o2.daily_total as previous_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 "lag over" queries whose ordering can be represented by some **equality condition**. {{}} |
| Anti-pattern ❌ |
Avoid the use of [`LAG() OVER (ORDER BY ...)` window
function](/sql/functions/#lag) to access previous row's value if the order by
field increases in a regular pattern.
```nofmt
-- Anti-pattern. Includes the first row's value. --
SELECT order_date, daily_total,
LAG(daily_total) OVER (ORDER BY order_date) as previous_daily_total
FROM orders_daily_totals;
```
|
| Materialize SQL ✅ | ```mzsql -- Include the first row in results -- SELECT o1.order_date, o1.daily_total, o2.daily_total as previous_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 "lag over" queries whose ordering can be represented by some **equality condition**. {{}} |
| Anti-pattern ❌ |
Avoid the use of [`LAG() OVER (ORDER BY ...)`
window function](/sql/functions/#lag) to access previous row's value if the
order by field increases in a regular pattern.
```nofmt
-- Anti-pattern. Includes the first row's value. --
SELECT order_date, daily_total,
LAG(daily_total) OVER (ORDER BY order_date) as previous_daily_total
FROM orders_daily_totals;
```
|