SELECT source
FROM table
WHERE coalesce(source,'value1') = 'value1'

也许像这样：

WITH        -- Sample data
    list_tbl AS 
        (   Select  'value1'  "VALS" From Dual Union All 
            Select  'value9'  "VALS" From Dual Union All
            Select  'value7'  "VALS" From Dual Union All
            Select  'value1'  "VALS" From Dual Union All
            Select  NULL      "VALS" From Dual Union All
            Select  'value1'  "VALS" From Dual Union All
            Select  'valueX'  "VALS" From Dual Union All
            Select  NULL      "VALS" From Dual Union All
            Select  NULL      "VALS" From Dual Union All
            Select  'value2'  "VALS" From Dual Union All 
            Select  'value3'  "VALS" From Dual Union All
            Select  'value1'  "VALS" From Dual 
        )
--
-- SQL
Select  VALS From list_tbl Where Nvl(VALS, 'value1') = 'value1' 
--
-- Result
VALS  
------
value1
value1
null
value1
null
null
value1

Where子句Nvl（）函数将空值（如果有）视为您选择的值。..
您的第一次查询结果与上述示例数据相同。..

Select  VALS From list_tbl Where VALS = 'value1' Or VALS Is Null