前言：Ora2pg处理递归查询并不太好使，特别是当递归查询作为子查询存在，且引用外层 表字段时，递归查询在pg里的语法是with recursive，写法不如oracle简洁，下面介绍转换语法：

1. 测试表结构

create table trees

(

id int,

code int,--本节点编号

pcode int,--父节点编号

info text

);

-- 插入测试数据

insert into trees values(1,100,null,'course A');

insert into trees values(2,10010,100,'course A10');

insert into trees values(3,1001010,10010,'course A1010');

insert into trees values(4,1001011,10010,'course A1011');

insert into trees values(5,10011,100,'course A11');

insert into trees values(6,10012,100,'course A12');

insert into trees values(7,200,null,'course B');

insert into trees values(8,300,null,'course C');

2. 查询节点编号10010本级节点下级节点

-- oracle

select *

from trees t

start with t.code = 10010

connect by prior t.code = pcode ;

-- pg

with recursive cte as(

select x.*

from trees x

where x.code = 10010

union all

select y.*

from trees y

join cte c on c.code = y.pcode

)select * from cte;

3. 查询节点编号10010本级节点上级节点

-- oracle

select *

from trees t

start with t.code = 10010

connect by t.code = prior pcode ;

-- pg

with recursive cte as(

select x.*

from trees x

where x.code = 10010

union all

select y.*

from trees y

join cte c on c.pcode = y.code

)select * from cte;

4. connect by level替换

-- oracle

SELECT REGEXP_SUBSTR(nn, '[^,]+', 1, LEVEL) AS AM

FROM insertmp

CONNECT BY LEVEL <= LENGTH(nn)-LENGTH(REPLACE(nn, ','))+1

-- pg

select distinct regexp_split_to_table(nn,',') am from insertmp;

5. 计算300到500之间的偶数平均值

--不使用递归

with cet1 as(

select generate_series(300,500) as a

),cet2 as(

select a from cet1 where a%2=0

)select avg(a) from cet2;

--使用递归

with recursive t(n) as(

values(300)

union all

select n+2 from t where n<500

)select avg(n) from t;

往期回顾