Apache Spark SQL is a module for working with structured data using Spark. It provides a programming abstraction called DataFrames and can also act as a distributed SQL query engine. Spark SQL allows developers to query structured data inside Spark programs, using either SQL or a familiar DataFrame API.
For example, Spark SQL can be used to query data stored in a variety of data sources, including Hive, Avro, Parquet, ORC, JSON, and JDBC. It can also be used to join data from different sources, such as joining a Hive table with data from a JSON file. Spark SQL can also be used to access data from external databases, such as Apache Cassandra, MySQL, PostgreSQL, and Oracle.