start with，connect by主要目的：从表中取出树状数据。可以假想成表中存成的各条数据是分别是树中的一个结点。
主要用法：
select ... start with initial-condition connect by nocycle recurse-condition
select ... connect by recurse-condition
select ... start with initial-condition connect by nocycle recurse-condition
select ... connect by recurse-condition
The
start with .. connect by clause can be used to select data that has a hierarchical relationship (usually some sort of parent->child (boss->employee or thing->parts).
It is also being used when an sql
execution plan is
explained.
recurse-condition can make use of the keyword
prior:
connect by prior foo = bar
This construct establishes the recursion. All records that are part of the next lower hierarchical level are found by having bar = foo. foo is a value found in the current hierarchical level.
A simple example
In the following example, the table from which that data is selected consists of just these attributes: parent and child. We make sure (by means of a unique constraint) that the child is uniqe within the table. This is just like in the real life where (as of yet) a child cannot have two different mothers.
The data filled into the table is such that a the sum over the children with the same parent is the value of the parent:
set feedback off create table test_connect_by ( parent number, child number, constraint uq_tcb unique (child) );
5 = 2+3
insert into test_connect_by values ( 5, 2);
insert into test_connect_by values ( 5, 3);
18 = 11+7
insert into test_connect_by values (18,11);
insert into test_connect_by values (18, 7);
17 = 9+8
insert into test_connect_by values (17, 9);
insert into test_connect_by values (17, 8);
26 = 13+1+12
insert into test_connect_by values (26,13);
insert into test_connect_by values (26, 1);
insert into test_connect_by values (26,12);
15=10+5
insert into test_connect_by values (15,10);
insert into test_connect_by values (15, 5);
38=15+17+6
insert into test_connect_by values (38,15);
insert into test_connect_by values (38,17);
insert into test_connect_by values (38, 6);
38, 26 and 18 have no parents (the parent is null)
insert into test_connect_by values (null, 38);
insert into test_connect_by values (null, 26);
insert into test_connect_by values (null, 18);
Now, let's select the data hierarchically:
select lpad(' ',2*(level-1)) || to_char(child) s from test_connect_by start with parent is null connect by prior child = parent; //prior child=parent是指子row的parent如果和父row的child值相等，即连接。
This select statement results in:
38
15
10
5
2
3
17
9
8
6
26
13
1
12
18
11
7
Interpreting connect by statements
How must a
start with ... connect by select statement be read and interpreted? If Oracle encounters such an SQL statement, it proceeds as described in the following pseude code.
for rec in (select * from some_table) loop
if FULLFILLS_START_WITH_CONDITION(rec) then
RECURSE(rec, rec.child);
end if;
end loop;
procedure RECURSE (rec in MATCHES_SELECT_STMT, parent_id IN field_type) is
begin
APPEND_RESULT_LIST(rec);
for rec_recurse in (select * from some_table) loop
if FULLFILLS_CONNECT_BY_CONDITION(rec_recurse.id, parent_id) then
RECURSE(rec_recurse,rec_recurse.id);
end if;
end loop;
end procedure RECURSE;
Thanks to
Frank Trenkamp who spotted an error in the logic in the above pseudo code and corrected it.
Thanks also to
Abhishek Ghose who made me think about a better way to describe the logic.
Pruning branches
Sometimes, it might be a requirement to only partially retrieve a hierarchical tree and to prune branches. Here, a tree is filled. Each child is the number of its parent plus a new digit on the right side.
create table prune_test ( parent number, child number ); insert into prune_test values (null, 1); insert into prune_test values (null, 6); insert into prune_test values (null, 7); insert into prune_test values ( 1, 12); insert into prune_test values ( 1, 14); insert into prune_test values ( 1, 15); insert into prune_test values ( 6, 61); insert into prune_test values ( 6, 63); insert into prune_test values ( 6, 65); insert into prune_test values ( 6, 69); insert into prune_test values ( 7, 71); insert into prune_test values ( 7, 74); insert into prune_test values ( 12, 120); insert into prune_test values ( 12, 124); insert into prune_test values ( 12, 127); insert into prune_test values ( 65, 653); insert into prune_test values ( 71, 712); insert into prune_test values ( 71, 713); insert into prune_test values ( 71, 715); insert into prune_test values ( 74, 744); insert into prune_test values ( 74, 746); insert into prune_test values ( 74, 748); insert into prune_test values ( 712,7122); insert into prune_test values ( 712,7125); insert into prune_test values ( 712,7127); insert into prune_test values ( 748,7481); insert into prune_test values ( 748,7483); insert into prune_test values ( 748,7487);
Now, we want to retrieve the tree, but prune everything below the branch 1 and 71. It would be false to put these into a where clause of the sql statement, rather, it belongs to the connect by clause:
select lpad(' ', 2*level) || child from prune_test start with parent is null connect by prior child=parent //prior child=parent是指子row的parent如果和父row的child值相等，即连接 and parent not in (1, 71); //这里对继续寻找孩子进行了过滤，即如果某个parent不在(1, 71)之间，那么再继续找孩子了。
This returns:
1
6
61
63
65
653
69
7
71
74
744
746
748
7481
7483
7487
|
在取出数据的时候，同时也可以把leve取出来，比如下面的例子：
SELECT last_name "Employee", CONNECT_BY_ISLEAF "IsLeaf", LEVEL, SYS_CONNECT_BY_PATH(last_name, '/') "Path" FROM employees WHERE LEVEL <= 3 AND department_id = 80 START WITH employee_id = 100 CONNECT BY PRIOR employee_id = manager_id AND LEVEL <= 4; Employee IsLeaf LEVEL Path --------------- ---------- ---------- ----------------------------------- Russell 0 2 /King/Russell Tucker 1 3 /King/Russell/Tucker Bernstein 1 3 /King/Russell/Bernstein Hall 1 3 /King/Russell/Hall Olsen 1 3 /King/Russell/Olsen Cambrault 1 3 /King/Russell/Cambrault Tuvault 1 3 /King/Russell/Tuvault Partners 0 2 /King/Partners King 1 3 /King/Partners/King Sully 1 3 /King/Partners/Sully McEwen 1 3 /King/Partners/McEwen Smith 1 3 /King/Partners/Smith Doran 1 3 /King/Partners/Doran Sewall 1 3 /King/Partners/Sewall Errazuriz 0 2 /King/Errazuriz Vishney 1 3 /King/Errazuriz/Vishney ... 34 rows selected.