Overview of Sub Queries¶

Let us recap about Sub Queries.

Let us start spark context for this Notebook so that we can execute the code provided. You can sign up for our 10 node state of the art cluster/labs to learn Spark SQL using our unique integrated LMS.

val username = System.getProperty("user.name")

import org.apache.spark.sql.SparkSession

val username = System.getProperty("user.name")
val spark = SparkSession.
    builder.
    config("spark.ui.port", "0").
    config("spark.sql.warehouse.dir", s"/user/${username}/warehouse").
    enableHiveSupport.
    appName(s"${username} | Spark SQL - Windowing Functions").
    master("yarn").
    getOrCreate

If you are going to use CLIs, you can use Spark SQL using one of the 3 approaches.

Using Spark SQL

spark2-sql \
    --master yarn \
    --conf spark.ui.port=0 \
    --conf spark.sql.warehouse.dir=/user/${USER}/warehouse

Using Scala

spark2-shell \
    --master yarn \
    --conf spark.ui.port=0 \
    --conf spark.sql.warehouse.dir=/user/${USER}/warehouse

Using Pyspark

pyspark2 \
    --master yarn \
    --conf spark.ui.port=0 \
    --conf spark.sql.warehouse.dir=/user/${USER}/warehouse

We typically have Sub Queries in FROM Clause.
We need not provide alias to the Sub Queries in FROM Clause in Spark SQL. In earlier versions, you might have to provide alias for the Sub Query.
We use Sub Queries quite often over queries using Analytics/Windowing Functions

%%sql

SELECT * FROM (SELECT current_date)

%%sql

SELECT * FROM (SELECT current_date) AS q

Let us see few more examples with respect to Sub Queries.

%%sql

USE itversity_retail

%%sql

SELECT * FROM (
    SELECT order_date, count(1) AS order_count
    FROM orders
    GROUP BY order_date
) q
LIMIT 10

Note

Here is an example of how we can filter based up on the derived columns using sub query. However, this can be achieved with direct query as well using HAVING.

%%sql

SELECT * FROM (
    SELECT order_date, count(1) AS order_count
    FROM orders
    GROUP BY order_date
) q
WHERE q.order_count > 10

%%sql

SELECT order_date, count(1) AS order_count
FROM orders
GROUP BY order_date
    HAVING count(1) > 10

Apache Spark using SQL

Overview of Sub Queries¶