組合2個SELECT查詢

Question

所以我有兩個查詢，我想一起使用以產生一個輸出結果給用戶。輸出向用戶描述從A到B的旅程。當我試圖將第一個查詢的結果加入到我的第二個查詢中時，我不斷收到語法錯誤。

Query failed: ERROR: syntax error at or near "UNION" LINE 6: UNION^

我有一個看一個類似的問題，但提供的解決方案&討論似乎並不管用。

$name = pg_escape_string($_POST['name']); // Start destination from user input 
$name2 = pg_escape_string($_POST['name2']); // End destination from user input 

//If no results are given, the following query below will execute instead. 

if (pg_num_rows($result) == 0) { 

$query = "SELECT dt1.name as name1,dt1.time as time1,dt2.name as name2,dt2.time as time2 
From departure_times as dt1 
inner join departure_times as dt2 on dt2.tram_id = dt1.tram_id 

UNION 

SELECT dt1.name as name3,dt1.time as time3,dt2.name as name4,dt2.time as time4 
From departure_times as dt1 
inner join departure_times as dt2 on dt2.tram_id = dt1.tram_id; 

$result = pg_query($query) or die('Query failed: ' . pg_last_error()); 

} ..... 


The desired output I am looking for is the following : 

     ------------------------------------------------------------------------ 
     name1 | time1 | name2 | time2 | name3 | time3 | name4 | time4 
     ------------------------------------------------------------------------ 

Answer 1

由外觀，你想創建一個輸出，列出從某個電臺到城市廣場的5個電車和從城市廣場到另一個電臺的5個電車。在這種情況下 - 你沒有在這兩個電臺之間建立任何連接（例如，準時）事情 - 你應該加入一個僞列，row_number() OVER()想到：

SELECT '$name' AS name1, w1.time1, 'City Square - arrival' AS name2, w1.time2, 
     'City Square - departure' AS name3, w2.time3, '$name2' AS name4, w2.time4 
FROM (
    SELECT dt1.time AS time1, dt2.time AS time2, row_number() OVER() AS rn 
    FROM departure_times AS dt1 
    JOIN departure_times AS dt2 USING (tram_id) 
    WHERE dt1.name = '$name' AND dt2.name = 'CitySquare' 
    LIMIT 5) w1 
JOIN (
    SELECT dt1.time AS time3, dt2.time AS time4, row_number() OVER() AS rn 
    FROM departure_times AS dt1 
    JOIN departure_times AS dt2 USING (tram_id) 
    WHERE dt1.name = 'CitySquare' AND dt2.name = '$name2' 
    LIMIT 5) w2 USING (rn); 

在每個子查詢中選擇您感興趣的五行由於沒有明顯的屬性，你CA。這兩組行上，你必須創建一個僞列，可以達到這個目的（你需要加入一些東西，否則你會得到一個CROSS JOIN導致輸出5 x 5行）。使用row_number() OVER() AS rn就是這樣做的：它創建一個別名爲rn的新列，其中包含整個行集合上的行號（OVER(),5行，因爲LIMIT子句）。您可以在兩個子查詢中執行此操作，以便您可以將它用作連接條件：USING (rn)。您不必在輸出中使用此列。

您無法控制將列出哪5個電車時間。如果你想要的話，你應該在這兩個子查詢中做一些類似WHERE dt1 > CURRENT_TIME和ORDER BY dt1的東西，或者沿着那條線。

Answer 2

Abelisto已經給了你第一部分，圍繞選擇查詢放置括號。

對於問題的第二部分，如果您想要顯示所有列，那麼您需要在兩個查詢中都具有​​所有列。因此，沿着添加null::text AS name3, null::timestamp AS time3等行（注意，我猜你的類型與類型鑄造，你可能不得不改變，根據你的查詢返回類型