Oracle - start with connect by 樹操做

Oracle - start with connect by 樹操做

準備工做：建表、插數據

create table tprior
( parentid number(10),
  subid    number(10)
);
 
insert into tprior values ( 1, 2 );
insert into tprior values ( 1, 3 );
insert into tprior values ( 2, 4 );
insert into tprior values ( 2, 5 );
insert into tprior values ( 3, 6 );
insert into tprior values ( 3, 7 );
insert into tprior values ( 5, 8 );
insert into tprior values ( 5, 9 );
insert into tprior values ( 7, 10 );
insert into tprior values ( 7, 11 );
insert into tprior values ( 10, 12 );
insert into tprior values ( 10, 13 );
 
commit;

 

表數據概況、B-Tree 結構

select * from tprior;

表數據

對應的 B-Tree 結構：

SQL

select parentid, subid, level
  from tprior
 start with subid = 7
connect by subid = prior parentid
 order by level;

• start with subid = 7，首先找出 level 1 記錄 (parentid, 7) 即 (3, 7)，做爲起始點
• connect by subid = prior parentid，表示從子節點往父節點方向遍歷，即 level 1 記錄 (3, 7) 中的父節點 3，做爲 level 2 記錄的子節點，查得 level 2 記錄 (1, 3)
• 一直查到根節點結束

 

select parentid, subid, level
  from tprior
 start with subid = 7
connect by prior subid = parentid
 order by level;

• start with subid = 7，level 1 記錄 (3, 7)
• connect by prior subid = parentid，從父節點向子節點方向遍歷，即 level 1 記錄 (3, 7) 中的子節點 7，做爲 level 2 記錄的父節點，查得 level 2 記錄 (7, 10)、(7, 11)
• 以此類推，一直查到沒有子節點的節點爲止

 

select parentid, subid, level
  from tprior
 start with parentid = 7
connect by subid = prior parentid
 order by level;

• start with parentid = 7，level 1 記錄 (7, 10)、(7, 11)
• connect by subid = prior parentid，子節點向節點方向遍歷，上一組數據的父節點 7，做爲 level 2 記錄的子節點，查得 (3, 7)、(3, 7) {爲何 level 2 是 2 組數據呢？由於 level 1 記錄有 2 個}
• 以此類推

 

select parentid, subid, level
  from tprior
 start with parentid = 7
connect by prior subid = parentid
 order by level;

• start with parentid = 7，level 1 記錄 (7, subid)、(7, subid) 即 (7, 10)、(7, 11)
• connect with prior parentid = subid，子節點向父節點遍歷，level 1 記錄的父節點 十、11，做爲 level 2 記錄的子節點，查得 (10, 13)、(10, 12)
• 以此類推

 

select parentid, subid, level
  from tprior
 where parentid > 3
 start with subid = 12
connect by subid = prior parentid
 order by level;

• SQL 執行順序是：先執行 start with，再執行 connect by，最後執行 where。
• 結論：where 只是樹結構的修剪，不改變樹的層次結構。