前言：Ora2pg處理遞歸查詢并不太好使，特別是當遞歸查詢作為子查詢存在，且引用外層 表字段時，遞歸查詢在pg里的語法是with recursive，寫法不如oracle簡潔，下面介紹轉換語法：

1. 測試表結構

create table trees

(

id int,

code int,--本節點編號

pcode int,--父節點編號

info text

);

-- 插入測試數據

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. 查詢節點編號10010本級節點下級節點

-- 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. 查詢節點編號10010本級節點上級節點

-- 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替換

-- 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. 計算300到500之間的偶數平均值

--不使用遞歸

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;

--使用遞歸

with recursive t(n) as(

values(300)

union all

select n+2 from t where n<500

)select avg(n) from t;

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