mySQL SELECT AS用於多個條件

Question

我需要將這兩個查詢合並為1：

SELECT user1ID AS friendID FROM friends WHERE user2ID = '$userid'
UNION
SELECT user2ID AS friendID FROM friends WHERE user1ID = '$userid'

然后

SELECT firstName, lastName FROM users WHERE ID = friendID (that we just established)

Answer 1

我猜您正在尋找MySQL的UNION （ Manual ）關鍵字。

由於已修改的問題而進行編輯：

您將需要一個子查詢來了解您要執行的操作（ JOIN也可能有效，但會更復雜）：

SELECT firstName, lastName FROM users WHERE ID IN
(
    SELECT user1ID AS friendID FROM friends WHERE user2ID = '$userid'
    UNION
    SELECT user2ID AS friendID FROM friends WHERE user1ID = '$userid'
)

請注意，子查詢可能會在非常大的表和/或復雜的查詢中導致性能問題。 在這種情況下，您可能需要將子查詢重構為JOIN 。