pg 递归算法_PostgreSQL递归查询_20191212
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;
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