使用別名在選擇查詢

Question

我要問，如何能在SELECT查詢中使用別名，

我需要這個

SELECT (Complex SubQuery) AS A, (Another Sub Query WHERE ID = A) FROM TABLE 

Answer 1

你不能做到這一點：

SELECT (Complex SubQuery) AS A, (Another Sub Query WHERE ID = A) FROM TABLE 

但是，您可以做此：

SELECT (Another Sub Query WHERE ID = A.somecolumn) 
FROM table 
JOIN SELECT (Complex SubQuery) AS A on (A.X = TABLE.Y) 

或

SELECT (Another Sub Query) 
FROM table 
WHERE table.afield IN (SELECT Complex SubQuery.otherfield) 

問題在於，您不能在SELECT和WHERE子句中引用像這樣的別名，因爲它們不會在執行select或where部分時進行評估。
也可以使用having子句，但子句不使用索引，應儘可能避免。

Answer 2

你可以重寫查詢這樣

SELECT Complex.A, (Another Sub Query WHERE ID = Complex.A) 
FROM TABLE 
CROSS JOIN ((Complex SubQuery) AS A) Complex 

Answer 3

另一種解決方案，您可以使用：

SELECT (Complex SubQuery) AS A, (Another Sub Query WHERE ID = A) 
FROM 
TABLE MAIN 
OUTER APPLY (SELECT (Complex SubQuery) AS A)