first
Returns the first value in a group. The function by default returns the first values it sees. It will return the first non-null value it sees when ignoreNulls is set to true. If all values are null, then null is returned. The function is non-deterministic because its results depends on the order of the rows which may be non-deterministic after a shuffle.
Syntax
Python
from pyspark.sql import functions as sf
sf.first(col, ignorenulls=False)
Parameters
Parameter | Type | Description |
|---|---|---|
|
| Column to fetch first value for. |
| bool | If first value is null then look for first non-null value. False by default. |
Returns
pyspark.sql.Column: first value of the group.
Examples
Python
from pyspark.sql import functions as sf
df = spark.createDataFrame([("Alice", 2), ("Bob", 5), ("Alice", None)], ("name", "age"))
df = df.orderBy(df.age)
df.groupby("name").agg(sf.first("age")).orderBy("name").show()
Output
+-----+----------+
| name|first(age)|
+-----+----------+
|Alice| NULL|
| Bob| 5|
+-----+----------+
To ignore any null values, set ignorenulls to True:
Python
df.groupby("name").agg(sf.first("age", ignorenulls=True)).orderBy("name").show()
Output
+-----+----------+
| name|first(age)|
+-----+----------+
|Alice| 2|
| Bob| 5|
+-----+----------+