In Oracle Database 12c, when creating an external table using the CREATE TABLE ... ORGANIZATION EXTERNAL statement, there are certain clauses that are mandatory for the command to execute successfully.
* Statement C, the LOCATION clause, is required. The LOCATION clause specifies one or more external data source locations, typically a file or a directory that the external table will read from.
Without this, Oracle would not know where to find the external data for the table.
* Statement E, the access driver TYPE clause, is also required. The access driver tells Oracle how to interpret the format of the data files. The most common access driver is ORACLE_LOADER, which allows the reading of data files in a format compatible with the SQL*Loader utility. Another option could be ORACLE_DATAPUMP, which reads data in a Data Pump format.
* Statements A, B, and D are not strictly required for the command to execute successfully, although they are often used in practice:
* A, the DEFAULT DIRECTORY clause, is not mandatory if you have specified the full path in the LOCATION clause, but it is a best practice to use it to avoid hard-coding directory paths in the LOCATION clause.
* B, the REJECT LIMIT clause, is optional and specifies the maximum number of errors to allow during the loading of data. If not specified, the default is 0, meaning the load will fail upon the first error encountered.
* D, the ACCESS PARAMETERS clause, is where one would specify parameters for the access driver, such as field delimiters and record formatting details. While it is common to include this clause to define the format of the external data, it is not absolutely required for the command to execute; defaults would be used if this clause is omitted.
For reference, you can find more details in the Oracle Database SQL Language Reference for version 12c, under the CREATE TABLE statement for external tables.