CREATE EXTERNAL TABLE (Transact-SQL)

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Questions and Answers

What is the purpose of the CREATE EXTERNAL DATA SOURCE statement in the provided text?

  • To define a connection to a Hadoop cluster (correct)
  • To define a connection to a SQL Server database
  • To define a connection to an Azure Blob Storage account
  • To define a connection to an Amazon S3 bucket

What is the purpose of the CREATE EXTERNAL FILE FORMAT statement in the provided text?

  • To define the format of data files in a SQL Server database
  • To define the format of data files in an Amazon S3 bucket
  • To define the format of data files in an Azure Blob Storage account
  • To define the format of data files in a Hadoop cluster (correct)

What is the purpose of the CREATE EXTERNAL TABLE statement in the provided text?

  • To create a table in a SQL Server database
  • To create a table in an Azure Blob Storage account
  • To create a table in an Amazon S3 bucket
  • To create a table in a Hadoop cluster (correct)

What is the purpose of the CREATE DATABASE SCOPED CREDENTIAL statement in the provided text?

<p>To create a credential for accessing a SQL Server database (A)</p> Signup and view all the answers

What is the purpose of the CREATE MASTER KEY statement in the provided text?

<p>To create a master key for encrypting data in a SQL Server database (C)</p> Signup and view all the answers

What is the purpose of the SELECT INTO statement in the provided text?

<p>To create a new SQL Server table from the results of a query (B)</p> Signup and view all the answers

What is the purpose of the JOIN statement in the provided text?

<p>To join data from a Hadoop cluster with data from a SQL Server database (A)</p> Signup and view all the answers

What is the purpose of the PUSHDOWN option in the CREATE EXTERNAL DATA SOURCE statement?

<p>To enable push-down of predicates and aggregations to the Hadoop cluster (C)</p> Signup and view all the answers

What is the purpose of the LOCATION parameter in the CREATE EXTERNAL DATA SOURCE statement?

<p>To specify the location of the Hadoop cluster (C)</p> Signup and view all the answers

What is the purpose of the CREDENTIAL parameter in the CREATE EXTERNAL DATA SOURCE statement?

<p>To specify the credentials for accessing the SQL Server database (D)</p> Signup and view all the answers

What does SQL store for an external table?

<p>Only the table metadata (C)</p> Signup and view all the answers

Why is it recommended to provide a three-part name for an external table?

<p>For best performance (C)</p> Signup and view all the answers

What happens if the column definitions in an external table don't match the data in the external files?

<p>File rows will be rejected (A)</p> Signup and view all the answers

In PolyBase, what happens when a query selects from an external table with a specified folder location?

<p>Files are retrieved from the folder and subfolders (D)</p> Signup and view all the answers

What does PolyBase not return when querying from an external table?

<p>.hidden files (B)</p> Signup and view all the answers

Where is the core-site.xml configuration file located for changing PolyBase settings?

<p>\PolyBase\Hadoop\Conf\bin root of SQL Server (A)</p> Signup and view all the answers

Which clause defines the external data source (a shard map) that is used for the external table?

<p>DATA_SOURCE (D)</p> Signup and view all the answers

Which data distribution method indicates that an application-specific method is used to distribute the data?

<p>ROUND_ROBIN (A)</p> Signup and view all the answers

Which data type is not supported for columns in external tables for Azure SQL Database?

<p>text (B)</p> Signup and view all the answers

What is the purpose of the SCHEMA_NAME and OBJECT_NAME clauses?

<p>To map the external table definition to a table in a different schema (A)</p> Signup and view all the answers

Which data distribution method specifies that identical copies of the table are present on each database?

<p>REPLICATED (D)</p> Signup and view all the answers

What is the purpose of the DISTRIBUTION clause in the CREATE EXTERNAL TABLE statement?

<p>To specify the data distribution method (A)</p> Signup and view all the answers

What happens if the column definitions, including the data types and number of columns, do not match the data in the external files?

<p>The file rows will be rejected when querying the actual data (A)</p> Signup and view all the answers

Which data distribution method specifies that the data from different tables don't overlap?

<p>SHARDED (A)</p> Signup and view all the answers

Which clause allows you to configure column name, data type, nullability, and collation for an external table?

<p>[ ,...n ] (A)</p> Signup and view all the answers

Which of the following is true about the DEFAULT CONSTRAINT on external tables in Azure SQL Database?

<p>You can't use the DEFAULT CONSTRAINT on external tables (B)</p> Signup and view all the answers

What is the purpose of using GRANT or REVOKE statements for external tables?

<p>To control access privileges to the external data source (C)</p> Signup and view all the answers

How does SQL Database handle data retrieval from an external data source in an ad hoc query scenario?

<p>It creates a temporary table to store the retrieved data during query execution (C)</p> Signup and view all the answers

What happens to the temporary table created during an ad hoc query against an external table?

<p>It is automatically dropped after the query completes (B)</p> Signup and view all the answers

In the import scenario, where does SQL Database store the data retrieved from an external data source?

<p>In a permanent SQL table created during query execution (A)</p> Signup and view all the answers

What is a limitation of querying external tables compared to querying regular SQL Server tables?

<p>Queries against external tables do not adhere to isolation semantics (B)</p> Signup and view all the answers

Which Data Definition Language (DDL) statements are allowed on external tables?

<p>CREATE TABLE, DROP TABLE, CREATE VIEW, DROP VIEW (B)</p> Signup and view all the answers

Which of the following is the correct syntax for creating a master key in SQL Server?

<p>CREATE MASTER KEY ENCRYPTION BY PASSWORD = 'password'; (C)</p> Signup and view all the answers

What is the purpose of creating a database scoped credential in SQL Server?

<p>To grant access to external data sources (A)</p> Signup and view all the answers

Which of the following is the correct syntax for creating an external data source in SQL Server?

<p>CREATE EXTERNAL DATA SOURCE source_name WITH (LOCATION = 'oracle://[:]', CREDENTIAL = credential_name); (D)</p> Signup and view all the answers

What is the purpose of the PUSHDOWN option when creating an external data source in SQL Server?

<p>To enable or disable query pushdown optimization (C)</p> Signup and view all the answers

Which of the following is the correct syntax for creating an external table in SQL Server for Oracle?

<p>CREATE EXTERNAL TABLE customers(...) WITH (LOCATION = 'DB1.mySchema.customer', DATA_SOURCE = external_data_source_name); (A)</p> Signup and view all the answers

What is the purpose of the LOCATION option when creating an external table in SQL Server?

<p>To specify the location or path of the external data (B)</p> Signup and view all the answers

Which of the following is the correct syntax for creating an external file format in SQL Server for Parquet files?

<p>CREATE EXTERNAL FILE FORMAT ParquetFileFormat WITH (FORMAT_TYPE = PARQUET); (C)</p> Signup and view all the answers

What is the purpose of the FILE_FORMAT option when creating an external table in SQL Server?

<p>To specify the file format of the external data (C)</p> Signup and view all the answers

Which of the following is the correct syntax for querying a Parquet file stored in S3-compatible object storage via an external table in SQL Server?

<p>SELECT * FROM Region; (C)</p> Signup and view all the answers

What is the purpose of the sp_data_source_objects system stored procedure in SQL Server?

<p>To retrieve metadata about external data sources and external tables (A)</p> Signup and view all the answers

What is the purpose of the matching suffix for corresponding data and reason files?

<p>To ensure the files are related and can be easily identified (A)</p> Signup and view all the answers

Which of the following permissions is not required to create an external table in SQL Server?

<p>CREATE TABLE (B)</p> Signup and view all the answers

What is the purpose of the DATABASE SCOPED CREDENTIAL used in the CREATE EXTERNAL TABLE command?

<p>To provide authentication credentials for the external data source (B)</p> Signup and view all the answers

When using an external table to export data to a Hadoop or Azure Storage external data source, what additional permission is required for the login specified in the DATABASE SCOPED CREDENTIAL?

<p>WRITE (C)</p> Signup and view all the answers

What is the purpose of the ALTER ANY EXTERNAL DATA SOURCE permission?

<p>Both a and b (D)</p> Signup and view all the answers

What happens if the attempt to connect to the external data source fails when executing the CREATE EXTERNAL TABLE statement?

<p>The statement will fail, and the external table won't be created (D)</p> Signup and view all the answers

What is the difference between ad hoc query scenarios and import scenarios when using an external table in SQL Server?

<p>In ad hoc queries, PolyBase stores the retrieved rows in a temporary table, while in import scenarios the new table is created during query execution (C)</p> Signup and view all the answers

What is the purpose of the Hadoop resource manager location option in the CREATE EXTERNAL DATA SOURCE statement?

<p>To enable predicate pushdown to improve query performance (A)</p> Signup and view all the answers

What is a limitation of using external tables in SQL Server?

<p>Query results against an external table are not guaranteed to be deterministic (D)</p> Signup and view all the answers

What is the restriction on simultaneously running queries against different Hadoop data sources in SQL Server?

<p>The queries must use the same Hadoop connectivity server configuration setting (A)</p> Signup and view all the answers

What is the purpose of the REJECT_TYPE option in PolyBase?

<p>To specify whether the REJECT_VALUE option is a literal value or a percentage (C)</p> Signup and view all the answers

What is the purpose of the REJECT_VALUE option in PolyBase?

<p>To specify the value or the percentage of rows that can be rejected before the query fails (A)</p> Signup and view all the answers

What is the purpose of the REJECT_SAMPLE_VALUE option in PolyBase?

<p>To determine the number of rows to attempt to retrieve before PolyBase recalculates the percentage of rejected rows (C)</p> Signup and view all the answers

What happens when the percentage of rejected rows exceeds the REJECT_VALUE in PolyBase?

<p>The query will return partial results until the reject threshold is exceeded, then fail with an error message (D)</p> Signup and view all the answers

What is the purpose of the REJECTED_ROW_LOCATION option in PolyBase?

<p>To specify the directory within the External Data Source where the rejected rows and error files should be written (C)</p> Signup and view all the answers

What is the format of the directory created for the rejected rows and error files in PolyBase?

<p>The directory is named based on the time of load submission in the format YearMonthDay-HourMinuteSecond (D)</p> Signup and view all the answers

What is the purpose of the "_" character in the REJECTED_ROW_LOCATION directory name?

<p>To ensure the directory is escaped for other data processing (C)</p> Signup and view all the answers

Which external data sources can use the REJECTED_ROW_LOCATION option in PolyBase?

<p>Only external data sources where TYPE = HADOOP and for external tables using DELIMITEDTEXT FORMAT_TYPE (B)</p> Signup and view all the answers

What types of files are written in the REJECTED_ROW_LOCATION directory in PolyBase?

<p>Both the rejected rows files and the error files (C)</p> Signup and view all the answers

How does PolyBase calculate the percentage of failed rows when using the REJECT_TYPE = percentage option?

<p>PolyBase calculates the percentage of failed rows at intervals, after attempting to retrieve the number of rows specified by the REJECT_SAMPLE_VALUE option (C)</p> Signup and view all the answers

What happens if the percentage of failed rows exceeds the REJECT_VALUE when using the REJECT_TYPE = percentage option in PolyBase?

<p>The query will return partial results until the reject threshold is exceeded, then fail with an error message (B)</p> Signup and view all the answers

What is the primary purpose of creating an external table using the CREATE EXTERNAL TABLE command?

<p>To provide access to data stored in a Hadoop cluster or Azure Blob Storage (C)</p> Signup and view all the answers

Which of the following is a primary use case for external data sources when used with an external table?

<p>Data virtualization and data load using PolyBase (C)</p> Signup and view all the answers

What is the minimum version of SQL Server required to use the CREATE EXTERNAL TABLE command?

<p>SQL Server 2016 (B)</p> Signup and view all the answers

Which of the following statements is true about the syntax conventions used in the CREATE EXTERNAL TABLE command?

<p>The syntax conventions are specific to Transact-SQL (D)</p> Signup and view all the answers

Which of the following is a secondary use case for external data sources when used with an external table?

<p>Bulk load operations using SQL Server or SQL Database using BULK INSERT or OPENROWSET (D)</p> Signup and view all the answers

What is the primary purpose of the DROP EXTERNAL TABLE command mentioned in the text?

<p>To remove the external table definition from the SQL Server database (C)</p> Signup and view all the answers

Which of the following statements is NOT allowed on external tables in PolyBase?

<p>INSERT (D)</p> Signup and view all the answers

What is the maximum number of files PolyBase can consume per folder when running 32 concurrent queries?

<p>33k files (D)</p> Signup and view all the answers

Which of the following data types CANNOT be used in PolyBase external tables?

<p>Both geography and text (B)</p> Signup and view all the answers

What is the row width limit for PolyBase in SQL Server 2016?

<p>32 KB (B)</p> Signup and view all the answers

Which of the following statements is true about creating external tables to MongoDB collections that contain arrays?

<p>Both of the above approaches are valid (C)</p> Signup and view all the answers

What type of lock is acquired on the SCHEMARESOLUTION object when working with PolyBase external tables?

<p>Shared lock (B)</p> Signup and view all the answers

Which of the following statements is true about the security of data files for an external table in PolyBase?

<p>It is your responsibility to manage the security of the external data files (C)</p> Signup and view all the answers

In the provided example for creating an external table with data in text-delimited format, what is the value of the FIELD_TERMINATOR option in the myfileformat external file format?

<p>'|' (C)</p> Signup and view all the answers

Which of the following statements is true about the LOCATION option when creating an external table in PolyBase?

<p>It specifies the path to the external data files (B)</p> Signup and view all the answers

In the provided example for creating an external table with data in RCFile format, what is the value of the FORMAT_TYPE option in the myfileformat_rc external file format?

<p>RCFILE (D)</p> Signup and view all the answers

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Study Notes

External Data Sources and Tables

  • CREATE EXTERNAL DATA SOURCE statement creates a reference to an external data source.
  • CREATE EXTERNAL FILE FORMAT statement defines the format of an external file.
  • CREATE EXTERNAL TABLE statement creates an external table, which is a virtual table that references data in an external data source.

Security and Authentication

  • CREATE DATABASE SCOPED CREDENTIAL statement creates a credential that can be used to authenticate to an external data source.
  • CREATE MASTER KEY statement creates a master key, which is used to encrypt the credential.
  • CREDENTIAL parameter in CREATE EXTERNAL DATA SOURCE statement specifies the credential to use for authentication.

External Tables

  • External tables store metadata about the external data, not the data itself.
  • Providing a three-part name for an external table is recommended for clarity and unambiguity.
  • If column definitions in an external table don't match the data in the external files, the query will fail.
  • PolyBase returns only the rows that match the specified folder location when querying an external table.

Querying External Tables

  • SELECT INTO statement is used to import data from an external table into a regular SQL Server table.
  • JOIN statement can be used to combine data from an external table with data from a regular SQL Server table.
  • PolyBase does not return data that is not present in the external files when querying an external table.

PolyBase Settings

  • The core-site.xml configuration file is used to change PolyBase settings.
  • The PUSHDOWN option in CREATE EXTERNAL DATA SOURCE statement enables pushdown operations.
  • The LOCATION parameter in CREATE EXTERNAL DATA SOURCE statement specifies the location of the external data source.

Data Distribution

  • The DISTRIBUTION clause in CREATE EXTERNAL TABLE statement specifies how the data is distributed.
  • The SCHEMA_NAME and OBJECT_NAME clauses define the schema and name of the external table.
  • The REPLICATED distribution method specifies that identical copies of the table are present on each database.

Data Types and Constraints

  • The DEFAULT CONSTRAINT on external tables in Azure SQL Database is not supported.
  • The GRANT and REVOKE statements are used to manage permissions on external tables.
  • External tables have limitations compared to regular SQL Server tables, such as data type restrictions.

Ad Hoc Queries and Import Scenarios

  • In ad hoc query scenarios, SQL Database stores the data in a temporary table.
  • In import scenarios, SQL Database stores the data in a regular SQL Server table.

Creating External Tables

  • The correct syntax for creating an external table in SQL Server for Oracle is CREATE EXTERNAL TABLE ... FOR Oracle.
  • The LOCATION option specifies the location of the external table.
  • The FILE_FORMAT option specifies the format of the external files.

Querying External Tables

  • The correct syntax for querying a Parquet file stored in S3-compatible object storage via an external table in SQL Server is SELECT ... FROM ... LOCATION ....
  • The sp_data_source_objects system stored procedure is used to manage external data sources.

Security and Permissions

  • The DATABASE SCOPED CREDENTIAL is used to authenticate to an external data source.
  • The ALTER ANY EXTERNAL DATA SOURCE permission is required to create an external table.
  • Additional permissions are required to export data to a Hadoop or Azure Storage external data source.

PolyBase Options

  • The REJECT_TYPE option specifies the type of error handling for rejected rows.
  • The REJECT_VALUE option specifies the value for error handling.
  • The REJECTED_ROW_LOCATION option specifies the location for rejected rows and error files.

External Data Sources

  • The Hadoop resource manager location option specifies the location of the Hadoop resource manager.
  • External data sources can include Hadoop, Azure Storage, and Oracle.
  • External tables can be used to export data to external data sources.

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