除了你的一般判断力，我会使用一个案件时，把你的第一个标准第一，然后继续与后续的下一个优先条件。

-- column1 is >= 3, use it as first.
case when column1 >= 3
          then column1
-- if it gets this far, then column1 is LESS than 3
     when column1 < 3
      AND column2 = 1
      AND column3 = column4 
      AND column3 = column5
     then column2

    when column1 < 3
      AND column2 = 1
      etc
    then ...
    else ... last choice
end

将WHEN条件保留在应该处理它们的优先级中。

我不知道你的一些条件是什么（“etc etc”），但其中一些条件（如2和3）是相同的，我怀疑你有19个条件。
您需要知道哪些条件具有优先级。
下面是基于您所提供的内容的查询的基本代码结构。

select
  case
    when col1 >= 3 then col1
    else 
      case
        when col2 = 1 and not null in (col4, col5) then 2
        when col2 = 1 and not null in (col3, col4) and col5 is null then 1
        when col2 = 2 and not null in (col3, col4, col5) then 3
        else null -- col2 not in (1,2)
      end
  end

存储它们的方式如下：

insert into my_table (mycolumn)

select
  case
    when col1 >= 3 then col1
    else
      case
        when col2 = 1 and not null in (col4, col5) then 2
        when col2 = 1 and not null in (col3, col4) and col5 is null then 1
        when col2 = 2 and not null in (col3, col4, col5) then 3
        else null -- col2 not in (1,2)
      end
  end

使用CASE表达式时要小心，因为它是按顺序工作的，并且在第一个结果为TRUE时退出。因此，条件的顺序至关重要。

WITH
    tbl (C1, C2, C3, C4, C5) AS
        (
            Select 1, 1, 3, 4, 5 From Dual Union All
            Select 1, 1, 3, 3, 5 From Dual Union All
            Select 1, 1, 3, 3, Null From Dual Union All
            Select 2, 2, 3, 3, 5 From Dual Union All
            Select 2, 2, 3, Null, Null From Dual Union All
            Select 2, 2, Null, Null, Null From Dual Union All
            Select 4, 2, 3, 3, 5 From Dual
        )
Select t.*,
    CASE 
        WHEN C1 >= 3 THEN C1
    ELSE
        CASE 
            WHEN C2 = 1 And     C3 Is Not Null And      C4 Is Not Null And      C5 Is Not Null THEN 2
            WHEN C2 = 1 And     C3 Is Null And          C4 Is Not Null And      C5 Is Not Null THEN 2
            WHEN C2 = 1 And     C3 Is Not Null And      C4 Is Not Null And      C5 Is Null THEN 1
            WHEN C2 = 1 And     C3 Is Not Null And      C4 Is Null And          C5 Is Null THEN 1
            --
            WHEN C2 = 2 And     C3 Is Not Null And      C4 Is Not Null And      C5 Is Not Null THEN 3
            WHEN C2 = 2 And     C3 Is Null And          C4 Is Not Null And      C5 Is Not Null THEN 3
            WHEN C2 = 2 And     C3 Is Not Null And      C4 Is Not Null And      C5 Is Null THEN 2
            WHEN C2 = 2 And     C3 Is Not Null And      C4 Is Null And         C5 Is Null THEN 2
        ELSE 
            -1
        END
    END "RESULT"
From tbl t
Order By 1

R e s u l t :
        C1         C2         C3         C4         C5     RESULT
---------- ---------- ---------- ---------- ---------- ----------
         1          1          3          4          5          2 
         1          1          3          3          5          2 
         1          1          3          3                     1 
         2          2          3          3          5          3 
         2          2          3                                2 
         2          2                                          -1 
         4          2          3          3          5          4

结果-1表示没有匹配条件