sql - SQL中如何将单行拆分为多行

Question

我有一个如下所示的表格..它有多个列用于不同的延迟

Date   API    Latency1_Avg Latency1_Min Latency1_Max Latency2_Avg Latency2_Min Latency2_Max
----   ---    ------------ ------------ ------------ ------------ ------------ ------------
7/26/13  Foo    12              35          45           453           433         435
7/26/13  Bar    33              33          33           234           243         234

我想输出一个表格，将每一行分成多行，如下所示

Date    API   Latency  Aggregation  Value
----    ----  -------  -----------   -----
7/26/13 Foo   Latency1   Avg         12
7/26/13 Foo   Latency1   Min         35
7/26/13 Foo   Latency1   Max         45
7/26/13 Foo   Latency2   Avg         453
7/26/13 Foo   Latency2   Min         433
7/26/13 Foo   Latency2   Max         435

.....

现在，我正在做的是这个

    SELECT 
    Date,
    API, 
   "Latency1" AS Latency,
    "Avg" AS Calculation,
    Latency1_Avg AS Value  
    FROM Table UNION ALL
    SELECT 
    Date,
    API, 
    "Latency1" AS Latency,
    "Min" AS Calculation,
    Latency1_Min AS Value  
    FROM Table UNION ALL
    SELECT 
    Date,
    API, 
    "Latency1" AS Latency,
    "Max" AS Calculation,
    Latency1_Max AS Value  
    FROM Table UNION ALL

....等等

这在性能方面是非常低效的，因为我正在为每个 select 语句进行表扫描......当这个表很大时，查询需要很长时间......

是否有更好更快的方法来做到这一点？可能正在使用自定义函数？

Answer 1

您可以使用CROSS APPLY:

SELECT [Date]
      , API
      , LEFT(col,CHARINDEX('_',col)-1)'Latency'
      , RIGHT(col,CHARINDEX('_',REVERSE(col))-1)'Aggregation'
      , Value
FROM
(
  SELECT [Date],API,col,value
  FROM YourTable
  CROSS APPLY
  (
    VALUES ('Latency1_Avg', [Latency1_Avg]),('Latency1_Min', [Latency1_Min]),('Latency1_Max', [Latency1_Max]),('Latency2_Avg', [Latency2_Avg]),('Latency2_Min', [Latency2_Min]),('Latency2_Max', [Latency2_Max])
  ) C (COL, VALUE)
) SRC

演示:SQL Fiddle