合并两个表

Question

我有两个包含以下列的表（两个表中的列相同）：

1. 函数名，
2. 频率计数

我想将这两个表合并成一个带有列的最终表：

1. 函数名称
2. 基本频率计数
3. 比较频率计数
4. 频率计数增量

合并操作应该如下发生：

• 如果 [FunctionName] 在 Table1 中而不在 Table2 中，

  [Base Frequency Count] = Table1.[FrequencyCount]
  [Compared Frequency Count] = 0
  [Delta of Frequency Count] = Table1.[FrequencyCount]
  

• 如果 [FunctionName] 在 Table2 中而不在 Table1 中，

  [Base Frequency Count] = 0         
  [Compared Frequency Count] = Table2.[FrequencyCount]
  [Delta of Frequency Count] = Table2.[FrequencyCount]
  

• 如果 [FunctionName] 同时在 Table1 和 Table2 中，

  [Base Frequency Count] = Table1.[FrequencyCount]         
  [Compared Frequency Count] = Table2.[FrequencyCount]
  [Delta of Frequency Count] = Table1.[FrequencyCount]-Table2.[FrequencyCount]
  

希望查询具有良好的性能和最小的数量。的连接。 如果有人能给出好的指点，那就太好了。

Answer 1

SELECT ISNULL(fn.FunctionName, fc.FunctionName) AS FunctionName, 
       ISNULL(fn.FrequencyCount, 0) AS BaseFrequency,
       ISNULL(fc.FrequencyCount, 0) AS ComparedFrequencyCount,
       COALESCE((fn.FrequencyCount - fc.FrequencyCount), fn.FrequencyCount, fc.FrequencyCount) AS DeltaOfFrequencyCount
INTO FinalTable
FROM FunctionName fn FULL OUTER JOIN FrequencyCount fc ON fn.FunctionName = fc.FunctionName

请注意，如果 fn.FrequencyCount 或 fc.FrequencyCount 为 null（在 SQL 中，值 - null = null），COALESCE 将导致第一个表达式中的 null （移动到链中的下一个）。

Answer 2

SELECT *
  INTO new_table_name 
  FROM (SELECT t.frequencycount 'Base Frequency Count',
               0 'Compared Frequency Count',
               t.frequencycount 'Delta of Frequency Count'
          FROM TABLE1 t
          JOIN TABLE2 t2 ON t2.functionname != t.functionname
        UNION
        SELECT 0 'Base Frequency Count',
               t2.frequencycount 'Compared Frequency Count',
               t2.frequencycount 'Delta of Frequency Count'
          FROM TABLE2 t2
          JOIN TABLE1 t ON t.functionname != t2.functionname
        UNION
        SELECT t.frequencycount 'Base Frequency Count',
               t2.frequencycount 'Compared Frequency Count',
               t.frequencycount - t2.frequencycount 'Delta of Frequency Count'
          FROM TABLE1 t
          JOIN TABLE2 t2 ON t.functionname = t2.functionname)

Answer 3

不确定这是否会比 rexem 的解决方案慢（我没有对其进行基准测试）。但这对您来说可能更容易阅读。通过基于主键而不是 FunctionName 进行选择也可以显着提高性能。

--TABLE 1
SELECT FunctionName, FrequencyCount AS Base, 0 AS Compared, FrequencyCount AS Delta
FROM TableOne
WHERE FunctionName NOT IN (SELECT FunctionName FROM TableTwo)
UNION

--TABLE 2
SELECT FunctionName, 0 AS Base, FrequencyCount AS Compared, FrequencyCount AS Delta
FROM TableTwo
WHERE FunctionName NOT IN (SELECT FunctionName FROM TableOne)
UNION

--DELTA
SELECT FunctionName, 
(SELECT FrequencyCount FROM TableOne WHERE FunctionName = DeltaTable.FunctionName) AS Base,
(SELECT FrequencyCount FROM TableTwo WHERE FunctionName = DeltaTable.FunctionName) AS Compared,
(SELECT FrequencyCount FROM TableOne WHERE FunctionName = DeltaTable.FunctionName) - (SELECT FrequencyCount FROM TableTwo WHERE FunctionName = DeltaTable.FunctionName) AS Delta
FROM TableOne DeltaTable
WHERE FunctionName IN (SELECT FunctionName FROM TableOne)
AND FunctionName IN (SELECT FunctionName FROM TableTwo)

修正三角洲

--DELTA
SELECT One.FunctionName, 
One.FrequencyCount AS Base,
Two.FrequencyCount AS Compared,
One.FrequencyCount - Two.FrequencyCount AS Delta
FROM TableOne One
INNER JOIN TableTwo Two
    ON One.FunctionName = Two.FunctionName