Tsql T-SQL中嵌套select查询的自连接

是否可以将嵌套的select语句与其自身连接起来（无需写出两次并运行两次）

像这样的东西会很理想

SELECT P.Child, P.Parent, Q.Parent AS GrandParent
FROM (SELECT Child, Parent FROM something-complex) AS P
LEFT JOIN P AS Q ON Q.Child = P.Parent

---

您可以使用公共表表达式：

WITH P AS (SELECT Child, Parent FROM something-complex)
SELECT P.Child, P.Parent, Q.Parent as GrandParent
LEFT JOIN P AS Q ON Q.Child = P.Parent

---

您可以使用公共表表达式：

WITH P AS (SELECT Child, Parent FROM something-complex)
SELECT P.Child, P.Parent, Q.Parent as GrandParent
LEFT JOIN P AS Q ON Q.Child = P.Parent

---

50%的可能性。您可以使用CTE避免写入两次，但它仍将执行两次

;WITH p
     AS (SELECT child,
                parent
         FROM   something-complex)
SELECT p.child,
       p.parent,
       q.parent AS grandparent
FROM   p
       LEFT JOIN p AS q
         ON q.child = p.parent  

---

如果查询很昂贵，则需要将其具体化为一个表变量或

#temp

表，以避免自连接导致对基础查询的两次调用。

50%可能。您可以使用CTE避免写入两次，但它仍将执行两次

;WITH p
     AS (SELECT child,
                parent
         FROM   something-complex)
SELECT p.child,
       p.parent,
       q.parent AS grandparent
FROM   p
       LEFT JOIN p AS q
         ON q.child = p.parent  

如果查询很昂贵，则需要将其具体化为一个表变量或

#temp

表，以避免自连接导致对基础查询的两次调用