Pseudo columns in Oracle

Pseudo columns in Oracle are special columns that provide specific metadata or context-related information about rows, tables, or other database objects. They are not actual columns stored in the database, but rather represent information generated or maintained by the Oracle database system. Here are some common pseudo columns in Oracle along with their examples:

1. ROWNUM: 

   - Example: `SELECT ROWNUM, column1 FROM table_name;`

   - This pseudo column returns the sequential row number of the row in the result set. It is particularly useful for pagination or limiting query results.

2. ROWID: 

   - Example: `SELECT ROWID, column1 FROM table_name;`

   - ROWID is a unique identifier for each row in a table, representing the physical address of the row in the database.

3. ROWIDTOCHAR: 

   - Example: `SELECT ROWIDTOCHAR(ROWID), column1 FROM table_name;`

   - This pseudo column converts the ROWID value of a row to a character string representation.

4. LEVEL: 

   - Example: `SELECT LEVEL, column1 FROM table_name CONNECT BY PRIOR parent_id = id;`

   - The LEVEL pseudo column is used in hierarchical queries to indicate the level of a row in a hierarchical data structure.

5. SYS_GUID(): 

   - Example: `SELECT SYS_GUID() AS guid FROM dual;`

   - SYS_GUID() generates a globally unique identifier (GUID) value for each row. It is commonly used to generate primary keys for tables.

6. SYSDATE: 

   - Example: `SELECT SYSDATE FROM dual;`

   - SYSDATE returns the current system date and time.

These examples demonstrate how pseudo columns can be used to retrieve metadata or provide context-specific information in Oracle queries. They are handy tools for various database operations and can enhance the functionality and efficiency of SQL queries.

Here are five frequently asked questions about pseudo columns in Oracle:

1. What are pseudo columns in Oracle?

   - This question seeks a general definition of pseudo columns, explaining that they are special columns in Oracle that provide metadata or context-related information about rows, tables, or other database objects.

2. How are pseudo columns different from regular columns?

   - This question aims to clarify the distinction between pseudo columns and regular columns, highlighting that pseudo columns are not stored in the database but are generated or maintained by the Oracle database system to provide specific information.

3. Can I modify or update pseudo columns in Oracle?

   - This question addresses whether pseudo columns are mutable, helping users understand that pseudo columns are typically read-only and cannot be modified directly.

4. In which scenarios are pseudo columns commonly used?

   - Users may inquire about common use cases or scenarios where pseudo columns are beneficial, such as pagination, row identification, hierarchical queries, or generating unique identifiers.

5. Are pseudo columns specific to certain types of database objects?

   - This question explores whether pseudo columns are universally applicable or specific to certain types of database objects. It helps users understand the scope and context in which pseudo columns can be used within the Oracle database ecosystem.