sql - 在 SELECT 列表中优先

Question

我不明白它向查询结果添加了什么。从我正在学习的书中:

If you prefix a column name with PRIOR in the select list (SELECT PRIOR EMPLOYEE_ID, ...), you specify the “prior” row’s value.

SELECT PRIOR EMPLOYEE_ID, MANAGER_ID, LPAD(' ', LEVEL * 2) || EMPLOYEES.JOB_ID
  FROM EMPLOYEES
  START WITH EMPLOYEE_ID = 100
  CONNECT BY PRIOR EMPLOYEE_ID =  MANAGER_ID;

我看到的唯一区别是，它在第​​一行添加了一个 NULL 值并将员工 ID 递增 1。

Answer 1

PRIOR 只是从遍历的层次结构中的前一条记录中获取一条记录。
我认为理解它如何工作的最好方法是玩一个简单的层次结构:

create table qwerty(
   id int,
   name varchar2(100),
   parent_id int
);

insert all
into qwerty values( 1, 'Grandfather', null )
into qwerty values( 2, 'Father', 1 )
into qwerty values( 3, 'Son', 2 )
into qwerty values( 4, 'Grandson', 3 )
select 1234 from dual;

下面的查询遍历上面的层次结构:

select level, t.*
from qwerty t
start with name = 'Grandfather'
connect by prior id = parent_id

     LEVEL         ID NAME                  PARENT_ID
---------- ---------- -------------------- ----------
         1          1 Grandfather                     
         2          2 Father                        1 
         3          3 Son                           2 
         4          4 Grandson                      3 

如果我们在上述查询中添加“PRIOR name”，则会显示“parent”的名称。此值取自层次结构中的 prevoius 记录(来自 LEVEL-1)

select level, prior name as parent_name, t.*
from qwerty t
start with name = 'Grandfather'
connect by prior id = parent_id;

     LEVEL PARENT_NAME                  ID NAME                  PARENT_ID
---------- -------------------- ---------- -------------------- ----------
         1                               1 Grandfather                     
         2 Grandfather                   2 Father                        1 
         3 Father                        3 Son                           2 
         4 Son                           4 Grandson                      3