转调字符串

Question

我正在探索 MLB 棒球统计数据，这是在球场级别的。我想总结一下板材外观层面的音高。相关栏目：

game_pk | pitcher_id | batter_id | inning | pitch_number | pitch_type | plt_apprnc_pk (unique)
----------------------------------------------------------------------------------------------
492321  |   542881   |   518466  |    2   |       1      |    FF      |   4923215428815184662
492321  |   543475   |   571448  |    1   |       2      |    SL      |   4923215434755714481
492321  |   543475   |   571448  |    1   |       1      |    FF      |   4923215434755714481
492321  |   543475   |   571448  |    1   |       3      |    FF      |   4923215434755714481

我正在研究一个仅显示每个板外观（plt_apprnc_pk）的一次迭代的查询，后面是pitch_number列（第一个音高，第二个音高，第三个音高等...）显示相应音高编号的每种音高类型。本质上，我想通过音高数字调换音高类型。

plt_apprnc_pk (unique)| first_pitch | second_pitch | third_pitch
----------------------------------------------------------------
  4923215434755714481 |     SL      |      FF      |     FF

我尝试使用 Case 语句（即 CASE WHENitch_number = 1 THENpitch_type END）来执行此操作。我不断为每个 plt_apprnc_pk 获取多行。

plt_apprnc_pk (unique)| first_pitch | second_pitch | third_pitch
------------------------------------------------------------------
  4923215434755714481 |     SL      |      NULL    |    NULL
  4923215434755714481 |    NULL     |      FF      |    NULL
  4923215434755714481 |    NULL     |      NULL    |     FF

基于pitch_number 转置这些字符串同时仅保留唯一键的一次迭代的最佳方法是什么？

Answer 1

我尝试使用 Case 语句（即 CASE WHENitch_number = 1 THENpitch_type END）来执行此操作。我不断为每个 plt_apprnc_pk 获取多行。

这里的模式称为条件聚合：

SELECT plt_apprnc_pk
     ,MAX(CASE WHEN pitch_number = 1 THEN pitch_type END) AS first_pitch 
     ,MAX(CASE WHEN pitch_number = 2 THEN pitch_type END) AS second_pitch 
     ,MAX(CASE WHEN pitch_number = 3 THEN pitch_type END) AS thrid_pitch 
     -- ...
     --,MAX(CASE WHEN pitch_number = n THEN pitch_type END) AS nth_pitch 
FROM tab
GROUP BY plt_apprnc_pk;

注意：必须提前知道最大“pitch_number”。