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First value in group

doc/user/content/transform-data/idiomatic-materialize-sql/first-value.md

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Overview

The "first value in each group" query pattern returns the first value, according to some ordering, in each group.

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Materialize and window functions

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Idiomatic Materialize SQL

Idiomatic Materialize SQL: To find the first value in each group, use MIN() or MAX() aggregate function in a subquery.

<table> <thead> <tr> <th></th> <th></th> </tr> </thead> <tbody> <tr> <td><blue>Materialize SQL</blue></td> <td class="copyableCode">

Use a subquery that uses the MIN() or MAX() aggregate function.

<div style="background-color: var(--code-block)">
mzsql
SELECT tableA.fieldA, tableA.fieldB, minmax.Z
 FROM tableA,
 (SELECT fieldA,
    MIN(fieldZ),
    MAX(fieldZ)
 FROM tableA
 GROUP BY fieldA) minmax
WHERE tableA.fieldA = minmax.fieldA
ORDER BY fieldA ... ;
</td> </tr> <tr> <td><red>Anti-pattern</red> ❌</td> <td>

<red>Avoid the use of FIRST_VALUE() OVER (PARTITION BY ... ORDER BY ...) window function for first value within groups queries.</red>

<div style="background-color: var(--code-block)">
nofmt
-- Anti-pattern. Avoid. --
SELECT fieldA, fieldB,
 FIRST_VALUE(fieldZ) OVER (PARTITION BY fieldA ORDER BY ...),
 FIRST_VALUE(fieldZ) OVER (PARTITION BY fieldA ORDER BY ... DESC)
FROM tableA
ORDER BY fieldA, ...;
</div> </td> </tr> </tbody> </table>

Query hints

To further improve the memory usage of the idiomatic Materialize SQL, you can specify a AGGREGATE INPUT GROUP SIZE query hint in the idiomatic Materialize SQL.

mzsql
SELECT tableA.fieldA, tableA.fieldB, minmax.Z
 FROM tableA,
 (SELECT fieldA,
    MIN(fieldZ),
    MAX(fieldZ)
 FROM tableA
 GROUP BY fieldA
 OPTIONS (AGGREGATE INPUT GROUP SIZE = ...)
 ) minmax
WHERE tableA.fieldA = minmax.fieldA
ORDER BY fieldA ... ;

For more information on setting AGGREGATE INPUT GROUP SIZE, see Optimization.

Examples

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The example data can be found in the Appendix.

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Use MIN() to find the first value

Using idiomatic Materialize SQL, the following example finds the lowest item price in each order and calculates the difference between the price of each item in the order and the lowest price. The example uses a subquery that groups by the order_id and selects MIN(price) to find the lowest price (i.e., first value if ordered by ascending price values).

<table> <thead> <tr> <th></th> <th></th> </tr> </thead> <tbody> <tr> <td><blue>Materialize SQL</blue> ✅</td> <td class="copyableCode">
mzsql
SELECT o.order_id, minmax.lowest_price, o.item, o.price,
  o.price - minmax.lowest_price AS diff_lowest_price
FROM orders_view o,
      (SELECT order_id,
         MIN(price) AS lowest_price
      FROM orders_view
      GROUP BY order_id) minmax
WHERE o.order_id = minmax.order_id
ORDER BY o.order_id, o.item;
</td> </tr> <tr> <td><red>Anti-pattern</red> ❌</td> <td>

<red>Avoid the use of FIRST_VALUE() OVER (PARTITION BY ... ORDER BY ...) window function for first value within groups queries.</red>

<div style="background-color: var(--code-block)">
nofmt
-- Anti-pattern --
SELECT order_id,
  FIRST_VALUE(price)
    OVER (PARTITION BY order_id ORDER BY price) AS lowest_price,
  item,
  price,
  price - FIRST_VALUE(price)
    OVER (PARTITION BY order_id ORDER BY price) AS diff_lowest_price
FROM orders_view
ORDER BY order_id, item;
</div> </td> </tr> </tbody> </table>

Use MAX() to find the first value

Using idiomatic Materialize SQL, the following example finds the highest item price in each order and calculates the difference between the price of each item in the order and the highest price. The example uses a subquery that groups by the order_id and selects MAX(price) to find the highest price (i.e., first value if ordered by descending price values).

<table> <thead> <tr> <th></th> <th></th> </tr> </thead> <tbody> <tr> <td><blue>Materialize SQL</blue> ✅</td> <td class="copyableCode">
mzsql
SELECT o.order_id, minmax.highest_price, o.item, o.price,
  o.price - minmax.highest_price AS diff_highest_price
FROM orders_view o,
      (SELECT order_id,
         MAX(price) AS highest_price
      FROM orders_view
      GROUP BY order_id) minmax
WHERE o.order_id = minmax.order_id
ORDER BY o.order_id, o.item;
</td> </tr> <tr> <td><red>Anti-pattern</red> ❌</td> <td>

<red>Avoid the use of FIRST_VALUE() OVER (PARTITION BY ... ORDER BY ...) window function for first value within groups queries.</red>

<div style="background-color: var(--code-block)">
nofmt
-- Anti-pattern --
SELECT order_id,
  FIRST_VALUE(price)
    OVER (PARTITION BY order_id ORDER BY price DESC) AS highest_price,
  item,
  price,
  price - FIRST_VALUE(price)
    OVER (PARTITION BY order_id ORDER BY price DESC) AS diff_highest_price
FROM orders_view
ORDER BY order_id, item;
</div> </td> </tr> </tbody> </table>

Use MIN() and MAX() to find the first values

Using idiomatic Materialize SQL, the following example finds the lowest and the highest item price in each order and calculates the difference between each item in the order and these prices. The example uses a subquery that groups by the order_id and selects MIN(price) as the lowest price (i.e., first value if ordered by price values) and MAX(price) as the highest price (i.e., first value if ordered by descending price values)

<table> <thead> <tr> <th></th> <th></th> </tr> </thead> <tbody> <tr> <td><blue>Materialize SQL</blue> ✅</td> <td class="copyableCode">
mzsql
SELECT o.order_id, minmax.lowest_price, minmax.highest_price, o.item, o.price,
  o.price - minmax.lowest_price AS diff_lowest_price,
  o.price - minmax.highest_price AS diff_highest_price
FROM orders_view o,
      (SELECT order_id,
         MIN(price) AS lowest_price,
         MAX(price) AS highest_price
      FROM orders_view
      GROUP BY order_id) minmax
WHERE o.order_id = minmax.order_id
ORDER BY o.order_id, o.item;
</td> </tr> <tr> <td><red>Anti-pattern</red> ❌</td> <td>

<red>Avoid the use of FIRST_VALUE() OVER (PARTITION BY ... ORDER BY ...) window function for first value within groups queries.</red>

<div style="background-color: var(--code-block)">
nofmt
-- Anti-pattern --
SELECT order_id,
  FIRST_VALUE(price)
    OVER (PARTITION BY order_id ORDER BY price) AS lowest_price,
  FIRST_VALUE(price)
    OVER (PARTITION BY order_id ORDER BY price DESC) AS highest_price,
  item,
  price,
  price - FIRST_VALUE(price)
    OVER (PARTITION BY order_id ORDER BY price) AS diff_lowest_price,
  price - FIRST_VALUE(price)
    OVER (PARTITION BY order_id ORDER BY price DESC) AS diff_highest_price
FROM orders_view
ORDER BY order_id, item;
</div> </td> </tr> </tbody> </table>

See also