MySQL：將多行（具有空值）合並為一行？

Question

我有一個名為code_status的表：

  Code     Message
    1     "Start"
    2     "In process"
    3     "Finish"

另一個表叫做歷史

 ID   Code    Name      time_date
 1     2      Jon       1/2/15
 31    1      Abby      2/1/15
 12    3      Sara      3/3/15
 31    2      Abby      2/3/15
 31    3      Abby      2/5/15
 8     2      Max       1/22/15

我想使用以下架構創建一個history_view：

history_view (id, name, start_date, process_date, finish_date)

如果未提供日期，則將為NULL

所以看起來像

ID   Name    start_date    process_date      finish_date
31   Abby     2/1/15          2/3/15           2/5/15
1    Jon      NULL            1/2/15            NULL
... etc

所以我開始做：

CREATE VIEW history_view
AS SELECT h.id, h.name, 
(CASE WHEN h.code = 1 THEN time_date) AS start_date,
(CASE when h.code = 2 THEN time_date) AS process_date,
(CASE when h.code = 3 THEN time_date) AS finish_date
FROM history h;

我將得到如下所示的結果：

ID   Name    start_date    process_date      finish_date
31   Abby     2/1/15         NULL               NULL
31   Abby      NULL          2/3/15             NULL
31   Abby      NULL          NULL             2/5/15
... ETC

有什么辦法將行合並在一起嗎？

Answer 1

當您說consolidate您指的是aggregation ，則可以使用聚合函數，例如min ：

CREATE VIEW history_view
AS SELECT h.id, h.name, 
min(CASE WHEN h.code = 1 THEN time_date) AS start_date,
min(CASE when h.code = 2 THEN time_date) AS process_date,
min(CASE when h.code = 3 THEN time_date) AS finish_date
FROM history h
group by h.id, h.name;