如何防止查询在SQL中生成多个不必要的列？

Question

这里是 SQL 初学者。所以到目前为止，当我尝试根据“一代”列查找总旅行时间时，我得到了这个：

SELECT
    generation,
    SUM(CASE WHEN generation = 'Millennials' THEN Rental_Duration_seconds END) AS MillennialTT,
    SUM(CASE WHEN generation = 'Generation Z' THEN Rental_Duration_seconds END) AS GenZTT,
    SUM(CASE WHEN generation = 'Generation X' THEN Rental_Duration_seconds END) AS GenXTT,
    SUM(CASE WHEN generation = 'Baby Boomers' THEN Rental_Duration_seconds END) AS BBYBoomerTT,
    SUM(CASE WHEN generation = 'Silent Generation' THEN Rental_Duration_seconds END) AS SilentGenTT
FROM
    dbo.2019_Q2_Trips
GROUP BY
    generation;

此查询将生成一个包含多个列的表，这些列具有不必要的 NULL 值。我可以使用 ELSE 语句将 NULL 值设置为 0，但这不会使列消失。

有没有更好的方法来做到这一点，不会生成多列？

Answer 1

是的，有。

SELECT generation, SUM(Rental_Duration_Seconds) FROM 2019_Q2_Trips GROUP BY generation;