Hierarchical query pseudo-columns are only valid in hierarchical queries. To define a hierarchical relationship in a query, you must use the CONNECT BY clause. This topic describes the three types of hierarchical query pseudo-columns: CONNECT_BY_ISCYCLE, CONNECT_BY_ISLEAF, and LEVEL.
CONNECT_BY_ISCYCLE pseudo-column
The CONNECT_BY_ISCYCLE pseudo-column is used to identify the starting row of a cycle.
If the current row's child node is also one of its ancestor nodes, CONNECT_BY_ISCYCLE returns 1; otherwise, it returns 0.
To use CONNECT_BY_ISCYCLE, you must also use the NOCYCLE option with the CONNECT BY clause. Otherwise, the query will return an error due to a cycle in the tree structure.
CONNECT_BY_ISLEAF pseudo-column
The CONNECT_BY_ISLEAF pseudo-column is used to identify leaf nodes in the hierarchical structure.
If the current row is a leaf node in the tree defined by the CONNECT BY condition, CONNECT_BY_ISLEAF returns 1; otherwise, it returns 0.
LEVEL pseudo-column
The LEVEL pseudo-column is used to identify the level of a node in the hierarchy.
In a hierarchical structure, the root is level 1, its child nodes are level 2, and so on. For example, the LEVEL value of the root node is 1, the LEVEL value of its child nodes is 2, and so on.
Consider a four-level inverted tree structure. The root row is the highest row in the inverted tree, and its LEVEL value is generally 1. Child rows are any non-root rows, and their LEVEL values are generally 2, 3, or 4. Parent rows are any rows that have child rows (excluding the root row), and their LEVEL values are generally 2 or 3. Leaf rows are any rows without child nodes, and their LEVEL values are generally 4.
Example of a hierarchical query
CREATE TABLE tbl1(col1 INT, col2 INT, col3 INT);
INSERT INTO tbl1 VALUES(1, 0, -1);
INSERT INTO tbl1 VALUES(2, 1, -2);
INSERT INTO tbl1 VALUES(4, 2, -4);
INSERT INTO tbl1 VALUES(5, 2, -5);
INSERT INTO tbl1 VALUES(3, 1, -3);
INSERT INTO tbl1 VALUES(6, 3, -6);
INSERT INTO tbl1 VALUES(7, 3, -7);
obclient> SELECT col1, col2, LEVEL, CONNECT_BY_ISLEAF, CONNECT_BY_ISCYCLE,
CONNECT_BY_ROOT col1,CONNECT_BY_ROOT col2 FROM tbl1 START WITH col1 = 1
CONNECT BY NOCYCLE PRIOR col1 = col2;
+------+------+-------+-------------------+--------------------+---------------------+---------------------+
| COL1 | COL2 | LEVEL | CONNECT_BY_ISLEAF | CONNECT_BY_ISCYCLE | CONNECT_BY_ROOTCOL1 | CONNECT_BY_ROOTCOL2 |
+------+------+-------+-------------------+--------------------+---------------------+---------------------+
| 1 | 0 | 1 | 0 | 0 | 1 | 0 |
| 2 | 1 | 2 | 0 | 0 | 1 | 0 |
| 4 | 2 | 3 | 1 | 0 | 1 | 0 |
| 5 | 2 | 3 | 1 | 0 | 1 | 0 |
| 3 | 1 | 2 | 0 | 0 | 1 | 0 |
| 6 | 3 | 3 | 1 | 0 | 1 | 0 |
| 7 | 3 | 3 | 1 | 0 | 1 | 0 |
+------+------+-------+-------------------+--------------------+---------------------+---------------------+
7 rows in set
obclient> SELECT col1, col2, LEVEL, CONNECT_BY_ISLEAF, CONNECT_BY_ISCYCLE,
CONNECT_BY_ROOT (col1 + col2) FROM tbl1 START WITH col1 = 1
CONNECT BY NOCYCLE PRIOR col1 = col2;
+------+------+-------+-------------------+--------------------+----------------------------+
| COL1 | COL2 | LEVEL | CONNECT_BY_ISLEAF | CONNECT_BY_ISCYCLE | CONNECT_BY_ROOT(COL1+COL2) |
+------+------+-------+-------------------+--------------------+----------------------------+
| 1 | 0 | 1 | 0 | 0 | 1 |
| 2 | 1 | 2 | 0 | 0 | 1 |
| 4 | 2 | 3 | 1 | 0 | 1 |
| 5 | 2 | 3 | 1 | 0 | 1 |
| 3 | 1 | 2 | 0 | 0 | 1 |
| 6 | 3 | 3 | 1 | 0 | 1 |
| 7 | 3 | 3 | 1 | 0 | 1 |
+------+------+-------+-------------------+--------------------+----------------------------+
7 rows in set
obclient> SELECT CONNECT_BY_ROOT col1, TO_NUMBER(CONNECT_BY_ROOT col1),
TO_CHAR(CONNECT_BY_ROOT col1), CONNECT_BY_ROOT(col1 + col2),
TO_NUMBER(CONNECT_BY_ROOT (col1 + col2)), TO_CHAR(CONNECT_BY_ROOT (col1 + col2))
FROM tbl1 START WITH col1 = 1 CONNECT BY NOCYCLE PRIOR col1 = col2;
+---------------------+--------------------------------+------------------------------+----------------------------+---------------------------------------+-------------------------------------+
| CONNECT_BY_ROOTCOL1 | TO_NUMBER(CONNECT_BY_ROOTCOL1) | TO_CHAR(CONNECT_BY_ROOTCOL1) | CONNECT_BY_ROOT(COL1+COL2) | TO_NUMBER(CONNECT_BY_ROOT(COL1+COL2)) | TO_CHAR(CONNECT_BY_ROOT(COL1+COL2)) |
+---------------------+--------------------------------+------------------------------+----------------------------+---------------------------------------+-------------------------------------+
| 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 | 1 |
+---------------------+--------------------------------+------------------------------+----------------------------+---------------------------------------+-------------------------------------+
7 rows in set