您需要使用IF...THEN语句来完成，大致如下（不确定db2的语法）：

SELECT @SomeValue = SomeValue FROM SomeTable

IF @SomeValue IS NULL
    INSERT INTO OtherTable VALUES (1, 2, 3)
    SELECT NewlyInsertedValue FROM OtherTable;
ELSE
    INSERT INTO OtherTable VALUES (1, 2, 3)
    SELECT SomeOtherValue FROM WeirdTable;
END IF;

你可以这样做两个语句。
somevalue is null时首先插入other

INSERT INTO othertable 
SELECT 1, 
       2, 
       3 
FROM   bigtable 
WHERE  somevalue IS NULL;

然后左联接到Somevalue上的两个表，null或not null

SELECT Coalesce(othertable.newlyinsertedvalue, weirdtable.someothervalue) foo, 
       column1, 
       column2 
FROM   bigtable 
       LEFT OUTER JOIN othertable 
         ON somevalue IS NULL 
       LEFT OUTER JOIN weirdtable 
         ON somevalue IS NOT NULL

我猜您实际上必须将连接修改为类似于

LEFT OUTER JOIN othertable 
         ON somevalue IS NULL 
           and bigtable.id = othertable.id
       LEFT OUTER JOIN weirdtable 
         ON somevalue IS NOT NULL 
           and bigtable.id = weirdtable .id

注意：我不确定在DB2中Coalesce的等价物是什么