pyspark.sql.functions.first

pyspark.sql.functions.first(col, ignorenulls=False)

Aggregate function: 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.

New in version 1.3.0.

Changed in version 3.4.0: Supports Spark Connect.

Parameters

colColumn or str

column to fetch first value for.

ignorenullsColumn or str

if first value is null then look for first non-null value.

Returns

Column

first value of the group.

Notes

The function is non-deterministic because its results depends on the order of the rows which may be non-deterministic after a shuffle.

Examples

>>> df = spark.createDataFrame([("Alice", 2), ("Bob", 5), ("Alice", None)], ("name", "age"))
>>> df = df.orderBy(df.age)
>>> df.groupby("name").agg(first("age")).orderBy("name").show()
+-----+----------+
| name|first(age)|
+-----+----------+
|Alice|      NULL|
|  Bob|         5|
+-----+----------+

Now, to ignore any nulls we needs to set ignorenulls to True

>>> df.groupby("name").agg(first("age", ignorenulls=True)).orderBy("name").show()
+-----+----------+
| name|first(age)|
+-----+----------+
|Alice|         2|
|  Bob|         5|
+-----+----------+