僅使用不同列的條件從同一表中選擇相同的數據

Question

我認為最好先向你展示他的查詢：

    SELECT 
    Positive.Amount AS PosAmount, 
    Negative.Amount AS NegAmount, 
    booking.Correction
FROM (booking)
LEFT JOIN ( SELECT ID, Amount FROM booking WHERE Amount < 0 ) 
    AS Negative ON booking.ID = Negative.ID
LEFT JOIN ( SELECT ID, Amount FROM booking WHERE Amount > 0 ) 
    AS Positive ON booking.ID = Positive.ID

我想在這里實現的是以下內容。 我希望每個預訂的金額取決於它在不同列中是正數還是負數。 這可能是另一種更便宜的方式嗎？ 我的意思是加入一張桌子本身不是最好的方法，可以嗎？

該表看起來像：

而期望的結果是這樣的：

提前致謝！

Answer 1

你只需要在這里使用CASE語句。 不需要子查詢和自連接：

SELECT
    CASE WHEN amount > 0 THEN amount END as posAmount,
    CASE WHEN amount < 0 THEN amount END as negAmount,
    correction
FROM booking

Answer 2

我不太清楚你想要什么。 也許它是聚合：

SELECT SUM(CASE WHEN Amount > 0 THEN b.Amount ELSE 0 END) as PosAmount, 
       SUM(CASE WHEN Amount < 0 THEN b.Amount ELSE 0 END) as NegAmount, 
       b.Correction
FROM booking b
GROUP BY b.correction;

也許它只是兩個不同的列：

SELECT (CASE WHEN Amount > 0 THEN b.Amount ELSE 0 END) as PosAmount, 
       (CASE WHEN Amount < 0 THEN b.Amount ELSE 0 END) as NegAmount, 
       b.Correction
FROM booking b;