Oracle® Database SQL Language Reference 11g Release 1 (11.1) Part Number B28286-01 |
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If a table contains hierarchical data, then you can select rows in a hierarchical order using the hierarchical query clause:
hierarchical_query_clause::=
START
WITH
specifies the root row(s) of the hierarchy.
CONNECT
BY
specifies the relationship between parent rows and child rows of the hierarchy.
The NOCYCLE
parameter instructs Oracle Database to return rows from a query even if a CONNECT
BY
LOOP
exists in the data. Use this parameter along with the CONNECT_BY_ISCYCLE
pseudocolumn to see which rows contain the loop. Refer to CONNECT_BY_ISCYCLE Pseudocolumn for more information.
In a hierarchical query, one expression in condition
must be qualified with the PRIOR
operator to refer to the parent row. For example,
... PRIOR expr = expr or ... expr = PRIOR expr
If the CONNECT
BY
condition
is compound, then only one condition requires the PRIOR
operator, although you can have multiple PRIOR
conditions. For example:
CONNECT BY last_name != 'King' AND PRIOR employee_id = manager_id ... CONNECT BY PRIOR employee_id = manager_id and PRIOR account_mgr_id = customer_id ...
PRIOR
is a unary operator and has the same precedence as the unary + and - arithmetic operators. It evaluates the immediately following expression for the parent row of the current row in a hierarchical query.
PRIOR
is most commonly used when comparing column values with the equality operator. (The PRIOR
keyword can be on either side of the operator.) PRIOR
causes Oracle to use the value of the parent row in the column. Operators other than the equal sign (=) are theoretically possible in CONNECT
BY
clauses. However, the conditions created by these other operators can result in an infinite loop through the possible combinations. In this case Oracle detects the loop at run time and returns an error.
Both the CONNECT
BY
condition and the PRIOR
expression can take the form of an uncorrelated subquery. However, CURRVAL
and NEXTVAL
are not valid PRIOR
expressions, so the PRIOR
expression cannot refer to a sequence.
You can further refine a hierarchical query by using the CONNECT_BY_ROOT
operator to qualify a column in the select list. This operator extends the functionality of the CONNECT
BY
[PRIOR
] condition of hierarchical queries by returning not only the immediate parent row but all ancestor rows in the hierarchy.
See Also:
CONNECT_BY_ROOT for more information about this operator and "Hierarchical Query Examples"Oracle processes hierarchical queries as follows:
A join, if present, is evaluated first, whether the join is specified in the FROM
clause or with WHERE
clause predicates.
The CONNECT
BY
condition is evaluated.
Any remaining WHERE
clause predicates are evaluated.
Oracle then uses the information from these evaluations to form the hierarchy using the following steps:
Oracle selects the root row(s) of the hierarchy--those rows that satisfy the START
WITH
condition.
Oracle selects the child rows of each root row. Each child row must satisfy the condition of the CONNECT
BY
condition with respect to one of the root rows.
Oracle selects successive generations of child rows. Oracle first selects the children of the rows returned in step 2, and then the children of those children, and so on. Oracle always selects children by evaluating the CONNECT
BY
condition with respect to a current parent row.
If the query contains a WHERE
clause without a join, then Oracle eliminates all rows from the hierarchy that do not satisfy the condition of the WHERE
clause. Oracle evaluates this condition for each row individually, rather than removing all the children of a row that does not satisfy the condition.
Oracle returns the rows in the order shown in Figure 9-1. In the diagram, children appear below their parents. For an explanation of hierarchical trees, see Figure 3-1, "Hierarchical Tree".
To find the children of a parent row, Oracle evaluates the PRIOR
expression of the CONNECT
BY
condition for the parent row and the other expression for each row in the table. Rows for which the condition is true are the children of the parent. The CONNECT
BY
condition can contain other conditions to further filter the rows selected by the query. The CONNECT
BY
condition cannot contain a subquery.
If the CONNECT
BY
condition results in a loop in the hierarchy, then Oracle returns an error. A loop occurs if one row is both the parent (or grandparent or direct ancestor) and a child (or a grandchild or a direct descendent) of another row.
Note:
In a hierarchical query, do not specify eitherORDER
BY
or GROUP
BY
, as they will override the hierarchical order of the CONNECT
BY
results. If you want to order rows of siblings of the same parent, then use the ORDER
SIBLINGS
BY
clause. See order_by_clause .CONNECT BY Example The following hierarchical query uses the CONNECT
BY
clause to define the relationship between employees and managers:
SELECT employee_id, last_name, manager_id FROM employees CONNECT BY PRIOR employee_id = manager_id ORDER BY employee_id, last_name, manager_id; EMPLOYEE_ID LAST_NAME MANAGER_ID ----------- ------------------------- ---------- 100 King 101 Kochhar 100 101 Kochhar 100 102 De Haan 100 102 De Haan 100 103 Hunold 102 103 Hunold 102 103 Hunold 102 104 Ernst 103 104 Ernst 103 104 Ernst 103 ...
LEVEL Example The next example is similar to the preceding example, but uses the LEVEL
pseudocolumn to show parent and child rows:
SELECT employee_id, last_name, manager_id, LEVEL FROM employees CONNECT BY PRIOR employee_id = manager_id ORDER BY employee_id, last_name, manager_id, LEVEL; EMPLOYEE_ID LAST_NAME MANAGER_ID LEVEL ----------- ------------------------- ---------- ---------- 100 King 1 101 Kochhar 100 1 101 Kochhar 100 2 102 De Haan 100 1 102 De Haan 100 2 103 Hunold 102 1 103 Hunold 102 2 103 Hunold 102 3 104 Ernst 103 1 104 Ernst 103 2 104 Ernst 103 3 ...
START WITH Examples The next example adds a START
WITH
clause to specify a root row for the hierarchy and an ORDER
BY
clause using the SIBLINGS
keyword to preserve ordering within the hierarchy:
SELECT last_name, employee_id, manager_id, LEVEL FROM employees START WITH employee_id = 100 CONNECT BY PRIOR employee_id = manager_id ORDER SIBLINGS BY last_name; LAST_NAME EMPLOYEE_ID MANAGER_ID LEVEL ------------------------- ----------- ---------- ---------- King 100 1 Cambrault 148 100 2 Bates 172 148 3 Bloom 169 148 3 Fox 170 148 3 Kumar 173 148 3 Ozer 168 148 3 Smith 171 148 3 De Haan 102 100 2 Hunold 103 102 3 Austin 105 103 4 Ernst 104 103 4 Lorentz 107 103 4 Pataballa 106 103 4 Errazuriz 147 100 2 Ande 166 147 3 Banda 167 147 3 ...
In the hr.employees
table, the employee Steven King is the head of the company and has no manager. Among his employees is John Russell, who is the manager of department 80. If you update the employees
table to set Russell as King's manager, you create a loop in the data:
UPDATE employees SET manager_id = 145 WHERE employee_id = 100; SELECT last_name "Employee", LEVEL, SYS_CONNECT_BY_PATH(last_name, '/') "Path" FROM employees WHERE level <= 3 AND department_id = 80 START WITH last_name = 'King' CONNECT BY PRIOR employee_id = manager_id AND LEVEL <= 4; ERROR: ORA-01436: CONNECT BY loop in user data
The NOCYCLE
parameter in the CONNECT
BY
condition causes Oracle to return the rows in spite of the loop. The CONNECT_BY_ISCYCLE
pseudocolumn shows you which rows contain the cycle:
SELECT last_name "Employee", CONNECT_BY_ISCYCLE "Cycle", LEVEL, SYS_CONNECT_BY_PATH(last_name, '/') "Path" FROM employees WHERE level <= 3 AND department_id = 80 START WITH last_name = 'King' CONNECT BY NOCYCLE PRIOR employee_id = manager_id AND LEVEL <= 4 ORDER BY "Employee", "Cycle", LEVEL, "Path"; Employee Cycle LEVEL Path ------------------------- ---------- ---------- ------------------------- Abel 0 3 /King/Zlotkey/Abel Ande 0 3 /King/Errazuriz/Ande Banda 0 3 /King/Errazuriz/Banda Bates 0 3 /King/Cambrault/Bates Bernstein 0 3 /King/Russell/Bernstein Bloom 0 3 /King/Cambrault/Bloom Cambrault 0 2 /King/Cambrault Cambrault 0 3 /King/Russell/Cambrault Doran 0 3 /King/Partners/Doran Errazuriz 0 2 /King/Errazuriz Fox 0 3 /King/Cambrault/Fox ...
CONNECT_BY_ROOT Examples The following example returns the last name of each employee in department 110, each manager above that employee in the hierarchy, the number of levels between manager and employee, and the path between the two:
SELECT last_name "Employee", CONNECT_BY_ROOT last_name "Manager", LEVEL-1 "Pathlen", SYS_CONNECT_BY_PATH(last_name, '/') "Path" FROM employees WHERE LEVEL > 1 and department_id = 110 CONNECT BY PRIOR employee_id = manager_id ORDER BY "Employee", "Manager", "Pathlen", "Path"; Employee Manager Pathlen Path --------------- --------------- ---------- ------------------------------ Gietz Higgins 1 /Higgins/Gietz Gietz King 3 /King/Kochhar/Higgins/Gietz Gietz Kochhar 2 /Kochhar/Higgins/Gietz Higgins King 2 /King/Kochhar/Higgins Higgins Kochhar 1 /Kochhar/Higgins
The following example uses a GROUP
BY
clause to return the total salary of each employee in department 110 and all employees below that employee in the hierarchy:
SELECT name, SUM(salary) "Total_Salary" FROM ( SELECT CONNECT_BY_ROOT last_name as name, Salary FROM employees WHERE department_id = 110 CONNECT BY PRIOR employee_id = manager_id) GROUP BY name ORDER BY name, "Total_Salary"; NAME Total_Salary ------------------------- ------------ Gietz 8300 Higgins 20300 King 20300 Kochhar 20300
See Also:
LEVEL Pseudocolumn and CONNECT_BY_ISCYCLE Pseudocolumn for a discussion of how these pseudocolumns operate in a hierarchical query
SYS_CONNECT_BY_PATH for information on retrieving the path of column values from root to node
order_by_clause for more information on the SIBLINGS
keyword of ORDER
BY
clauses