In Oracle, start with connect by prior recursive query is used to query multi-level subordinate employees.

One 、 The source of the problem ：

Previously in a database group , A question was raised ： Now there's a watch user,user There are fields in the table username,leader, Now find out ,leader yes boss,boss Of all subordinate employees username. however boss My position is very high , His subordinates may also have subordinates , Subordinates and subordinates , There will even be many subordinates .

Two 、 The train of thought of the false great God ：

at that time , When I saw someone answer like this, I immediately vomited blood , Really treat programmers as coders , Can only move bricks ！！！

-- Results without vision ：
select  username  from user
where leader = 'boss'
union 
select username from user 
where leader in（select  username  from user
where leader = 'boss';

If there is 10 Subordinate employees , Do you have to write 10 individual union all Do you ？ Subqueries are nested on 9 Tier? ？ Once the amount of data is large , The database must not be down .

The leader immediately let the bag go . It's a small query of subordinate employees , The database crashed .

3、 ... and 、 Use Oracle Medium start with connect by prior

start with connect by prior Is a recursive query usage .

We'll take Oracle Employee table in the self-contained database emp For example ：

-- A simple line of code ：
select * from emp start with empno = 7839 connect by prior empno = mgr;

give the result as follows ：

  This structure is emp Employee surface level chart ： Four 、 Tell me about start with connect by prior Usage of ：

1、connect by The grammar of ：

select ... from tablename
start by cond1
connect by prior cond2
where cond3

2、 Introduction of each part ：

start with Clause is optional , It is used to identify which node is the root node of the tree structure . If the clause is omitted , It means that all rows that meet the query criteria are taken as root nodes .

cond1 Is the root node qualifier , Of course, you can relax the restrictions , To get multiple root nodes , It's actually multiple trees .

prior The operator must be placed before one of the two columns of the join relationship . For parent-child relationships between nodes ,prior Operator represents the parent node on one side , On the other side is the child node , To determine whether the search tree structure is top-down or bottom-up . In a connected relationship , In addition to using column names , List expressions are also allowed .

cond2 It's the connection condition , Among them prior Indicates the previous record , such as connect by prior id=praentid That is, the last record id It's from this record praentid, That is, the father of this record is the last record .

cond3 It's filter conditions , Used to filter all returned records .

5、 ... and 、 Final use ：

start with connect by prior Recursive query usage ： This clause is mainly used for B Data recursive query of tree structure type , give B Any node in the tree structure type , Traverse its final parent node or child node .