PySpark之选择特征select、筛选filter、聚合运算、group by、join table、inner join 、left join、right join、full outer join，如下所示：
from __future__ import print_function, division
from pyspark import SparkConf, SparkContext
from pyspark.sql import SparkSession
## 启动 Spark （如果你已经启动就不需要）
spark = SparkSession.builder.master("local[2]").appName("test").enableHiveSupport().getOrCreate()
sc = spark.sparkContext
## 读取数据集csv
df = spark.read.csv('../data/rating.csv', sep = ',', header = True) #自动判断格式interSchema=True
df.show()
## 选择特征 ---select
#select userid from data -- sql语句
df.select('userid').show() #筛选userid
#select userid ,movieid from data -- sql语句
df.select('userid','movieid').show()
## 对特征进行操作 ---selectExpr
#select userid as id from data
df.selectExpr('userid as id').show()
#select movieid,rating * 2 as rating_2 from data
df.selectExpr('movieid', 'rating * 2 as rating_2').show()
df.printSchema()
df.selectExpr('cast(taring as DOUBLE)').printSchema() #转换类型cast
## 筛选userid --- filter
#select * from data where rating > 3
df.filter('rating > 3').show()
#select * from data where userid = 2 and rating > 3
df.filter('userid == 2 and rating > 3').show()
#select userid, rating from data where userid = 2 and rating > 3
df.filter('userid == 2 and rating > 3').select('userid', 'rating').show()
df.select("userID", "rating").filter("userID = 2 and rating > 3").show()
## 聚合运算
#select count(*) from data
df.count()
df.agg({'userid':'count'}).show()
#select count(*) from data where userid = 1
df.filter('userid = 1').count()
#select count(userid) from data,select avg(rating) from data
df.agg({'userid':'count','rating':'avg'}).show()
## group by
##计算每个user评比了多少部电影，平均分数如何？
# select userid,count(*),avg(rating) from data group by userid
df.groupBy('userid').agg({'movieid':'count','rating':'avg'}).show()
from pyspark.sql.function import *
df.groupBy('userid').agg(count('movieid'), round(avg(df.rating), 2)).show()
## join table、inner join 、left join、right join、full outer join
# 创建数据框df_profile
d = [{'name': 'Alice', 'age': 1}, {'name': 'Bryan', 'age': 3}, {'name': 'Cool', 'age':2}]
df_profile = spark.createDataFrame(d) #转换为数据框
df_profile.show()
# 创建数据框df_parents
d = [{'name': 'Jason', 'child': 'Alice'},
{'name': 'Bill', 'child': 'Bryan'},
{'name': 'Sera', 'child': 'Bryan'},
{'name': 'Jill', 'child': 'Ken'}]
df_parents = spark.createDataFrame(d) #转换为数据框
df_parents.show()
#inner join
df_profile.join(df_parents, df_profile.name == df_parents.child).show()
#left join
df_profile.join(df_parents, df_profile.name == df_parents.child, 'left').show()
#right join
df_profile.join(df_parents, df_profile.name == df_parents.child, 'right').show()
#full outer join
df_profile.join(df_parents, df_profile.name == df_parents.child, 'outer').show()
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