Pseudocolumn in Oracle SQL

Pseudocolumn: A pseudo-column behaves like a table column but is not actually stored in the table. You can select from pseudo-columns, but you cannot insert, update, or delete their values. A pseudo-column is also similar to a function without arguments. This section describes these pseudo-columns: 

• CURRVAL and NEXTVAL
• LEVEL
• ROWID
• ROWNUM

1. CURRVAL and NEXTVAL: A sequence is a schema object that can generate unique sequential values. These values are often used for primary and unique keys. You can refer to sequence values in SQL statements with these pseudocolumns: 

• CURRVAL : Returns the current value of a sequence.
• NEXTVAL : Increments the sequence and returns the next value.

Examples:
SELECT STUDENTSEQ.currval FROM DUAL;
INSERT INTO STUDENT VALUES (STUDENTSEQ.nextval, ‘BISHAL’, ‘JAVA’, 7902);

2. LEVEL:For each row returned by a hierarchical query, the LEVEL pseudocolumn returns 1 for a root node, 2 for a child of a root, and so on. 

3. ROWNUM: Oracle engine maintains the number of each record inserted by users in table. By the help of ROWNUM clause we can access the data according to the record inserted. 

Example: 
SELECT * FROM EMP WHERE ROWNUM <= 3; 

4. ROWID:For each row in the database, the ROWID pseudocolumn returns a row\’s address. The ROWID contains 3 information about row address: 

• FileNo : FileNo means Table Number.
• DataBlockNo : DataBlockNo means the space assigned by the oracle SQL engine to save the record.
• RecordNo : Oracle engine maintains the record number for each record.

Example:
SELECT ROWID, ename FROM emp WHERE deptno = 20;