sql - 具有 uniqueidentifier 的递归 CTE 必须在 anchor 中具有值

Question

;WITH Companies(pkCompanyID,name,parentCompanyID,SomeId) AS

        (
            SELECT
                tblCompany.pkCompanyID,
                tblCompany.name,
                NULL,--this is a int with no value and it works in the anchor
                NULL AS SomeId--But this uniqueidentifier do not work why?
            FROM
                tblCompany
            WHERE
                tblCompany.fkCompToCompID IS NULL
            UNION ALL
            SELECT
                tblCompany.pkCompanyID,
                tblCompany.name,
                tblCompany.fkCompToCompID,
                NEWID()
            FROM
                tblCompany
                JOIN Companies ON tblCompany.fkCompToCompID=Companies.pkCompanyID
        )
        SELECT
            *
        FROM
            Companies

因此该函数将给出错误:“ anchor 与递归查询“Companies”的列“SomeId”中的递归部分之间的类型不匹配”。我知道我可以通过将“NULL AS SomeId”替换为“CAST(NULL AS uniqueidentifier)”来修复它。但是为什么当parentCompanyId不需要知道它是一个int时，anchor需要知道它是一个uniqueidentifier呢？是因为uniqueidentifier是对象而int是值类型吗？

Answer 1

NULL 默认情况下被假定为 int，这就是为什么您不必显式转换该类型。

您可以从

看到这一点

SELECT NULL AS FOO INTO BAR;

SELECT DATA_TYPE
FROM INFORMATION_SCHEMA.COLUMNS
WHERE COLUMN_NAME='FOO'AND TABLE_NAME='BAR';