# Window Partitioning

Optionally specified in an analytic function's `OVER`

clause, a partition (`PARTITION BY`

) clause groups input rows before the function processes them. Window partitioning is similar to an aggregate function's `GROUP BY`

clause, except it returns exactly one result row per input row. If you omit the window partition clause, the function treats all input rows as a single partition.

## Specifying Window Partitioning

You specify window partitioning in the analytic function's `OVER`

clause, as follows:

`{ PARTITION BY expression[,…] | PARTITION BEST | PARTITION NODES }`

For syntax details, see Window Partition Clause.

## Examples

The examples in this topic use the `allsales`

schema defined in Invoking Analytic Functions.

CREATE TABLE allsales(state VARCHAR(20), name VARCHAR(20), sales INT); INSERT INTO allsales VALUES('MA', 'A', 60); INSERT INTO allsales VALUES('NY', 'B', 20); INSERT INTO allsales VALUES('NY', 'C', 15); INSERT INTO allsales VALUES('MA', 'D', 20); INSERT INTO allsales VALUES('MA', 'E', 50); INSERT INTO allsales VALUES('NY', 'F', 40); INSERT INTO allsales VALUES('MA', 'G', 10); COMMIT;

**Median of sales within each state**

The following query uses the analytic window-partition-clause to calculate the median of sales within each state. The analytic function is computed per partition and starts over again at the beginning of the next partition.

=> SELECT state, name, sales, MEDIAN(sales) OVER (PARTITION BY state) AS median from allsales;

Results are grouped into partitions for MA (35) and NY (20) under the median column.

state | name | sales | median -------+------+-------+-------- NY | C | 15 | 20 NY | B | 20 | 20 NY | F | 40 | 20 ------------------------------- MA | G | 10 | 35 MA | D | 20 | 35 MA | E | 50 | 35 MA | A | 60 | 35 (7 rows)

**Median of sales among all states**

The following query calculates the median of total sales among states. When you use `OVER()`

with no parameters, there is one partition—the entire input:

=> SELECT state, sum(sales), median(SUM(sales)) OVER () AS median FROM allsales GROUP BY state; state | sum | median -------+-----+-------- NY | 75 | 107.5 MA | 140 | 107.5 (2 rows)

**Sales larger than median (evaluation order)**

Analytic functions are evaluated *after *all other clauses except the query's final SQL `ORDER BY`

clause. So a query that asks for all rows with sales larger than the median returns an error because the `WHERE`

clause is applied before the analytic function and column `m`

does not yet exist:

=> SELECT name, sales, MEDIAN(sales) OVER () AS m FROM allsales WHERE sales > m; ERROR 2624: Column "m" does not exist

You can work around this by placing in a subquery the predicate `WHERE sales > m`

:

=> SELECT * FROM (SELECT name, sales, MEDIAN(sales) OVER () AS m FROM allsales) sq WHERE sales > m; name | sales | m ------+-------+---- F | 40 | 20 E | 50 | 20 A | 60 | 20 (3 rows)

For more examples, see Analytic Query Examples.