sql - 如何将 oracle 查询转换为 PostgreSQL 以获取表计数

Question

我想将提到的 oracle 查询转换为 PostgreSQL 以获取表的计数

select count(*) from student connect by prior std_id=std_roll

Answer 1

Oracleconnect by可以重写为标准 SQL（和 Postgres）中的递归公用表表达式：

with recursive tree as (
  select std_id, std_roll
  from student
  where std_roll is null --<< I assume you are missing a START WITH in Oracle
  union all
  select c.std_id, c.std_roll
  from student c
    join tree p on p.std_id = c.std_roll
)
select count(*)
from tree;

然而，这并没有真正的意义。以上是一种复杂的写法（假设andselect count(*) from student之间有外键）std_rollstd_id