PostgreSQL将非递归项的输出强制转换为正确的类型

我有以下PostgreSQL查询：

WITH RECURSIVE disease_tree AS
(
SELECT ref_disease_id, uid, parent, level, NULL AS subtype, NULL AS specific_subtype
FROM ref_disease
WHERE parent IS NULL

UNION ALL

SELECT d.ref_disease_id, d.uid, d.parent, d.level,  
  CASE 
      WHEN d.level = 2 THEN d.uid
      ELSE dtr.subtype END AS subtype,
  CASE 
      WHEN d.level = 3 THEN d.name
      ELSE NULL END AS specific_subtype
FROM ref_disease As d
INNER JOIN disease_tree AS dtr
ON d.parent = dtr.uid
)
select ref_disease_id, uid, level, subtype, specific_subtype from disease_tree 

到目前为止一切正常。但是根据我的应用程序逻辑，我必须在下一行返回

d.uid

而不是

d.name

：

当d.level=3时，则返回d.name

因此，新查询如下所示：

WITH RECURSIVE disease_tree AS
(
SELECT ref_disease_id, uid, parent, level, NULL AS subtype, NULL AS specific_subtype
FROM ref_disease
WHERE parent IS NULL

UNION ALL

SELECT d.ref_disease_id, d.uid, d.parent, d.level,  
  CASE 
      WHEN d.level = 2 THEN d.uid
      ELSE dtr.subtype END AS subtype,
  CASE 
      WHEN d.level = 3 THEN d.uid
      ELSE NULL END AS specific_subtype
FROM ref_disease As d
INNER JOIN disease_tree AS dtr
ON d.parent = dtr.uid
)
select ref_disease_id, uid, level, subtype, specific_subtype from disease_tree 

但它失败了，出现以下错误：

ERROR:  recursive query "disease_tree" column 6 has type text in non-recursive term but type character varying overall
LINE 3: ..._disease_id, uid, parent, level, NULL AS subtype, NULL AS sp...
                                                             ^
HINT:  Cast the output of the non-recursive term to the correct type.
********** Error **********

ERROR: recursive query "disease_tree" column 6 has type text in non-recursive term but type character varying overall
SQL state: 42804
Hint: Cast the output of the non-recursive term to the correct type.
Character: 107

---

如何强制转换输出并修复它？

一个

case

表达式返回一个类型，该类型派生自

然后
和
where
子句中的类型。默认情况下，
NULL
是文本（我认为）

您的案例似乎是混合类型。所以，转换这些值。Postgres有一个很好的缩写
：
：

您希望在
案例中执行转换，否则，当字符串试图转换为数字时，可能会出现运行时错误。
强制转换空值：
null:：character variable
@a_horse_with_no_name。这不会先尝试将字符串值转换为数字吗？这可能会导致错误。错误的原因是非递归部分中的
null
值的类型不正确
(CASE WHEN d.level = 2 THEN d.uid::text
      ELSE dtr.subtype::text
 END) AS subtype,