(Oracle DB) - 21. Pseudo Column

/* Pseudo Column
	#	Pseudo column doesn't store any data, but works like a table
	#	Pseudo column can be used in SELECT, but not in INSERT, UPDATE, DELETE
	#	Sequences NEXTVAL, CURRVAL are one of pseudo column 
*/

	/* Pseudo Column in Hierarchical Query */
CONNECT_BY_ISCYCLE
CONNECT_BY_ISLEAF
LEVEL

	/* Pseudo Column in Sequence */
NEXTVAL, CURRVAL

	/* ROWNUM, ROWID */
ROWNUM, ROWID
	--	ROWNUM은, query에서 return되는 각 rows에 대한 순서값을 표현하는 pseudo column
SELECT ROWNUM, employee_id
	FROM employees
WHERE ROWNUM < 5
;
	--	ROWID represents the address value of stored rows.
	--	ROWID is unique_key and identifies each rows by the address values.
SELECT ROWNUM, ROWID, employee_id
	FROM employees
;