oracle 条件查询不为空 不生效

Question

UPDATE a 
SET a.num = ( SELECT b.nnnum FROM b WHERE a.id = b.id and b.id is not null )
WHERE
    EXISTS ( SELECT 1 FROM b WHERE a.num = b.nnnum );

这样写的话 b表为空的话 他直接把a表中的num已经有的值 给制空了 and条件不为空 未生效 麻烦各位指教 谢谢~

Answer 1

UPDATE 表A a 
SET a.num = ( SELECT b.nnnum FROM 表B b WHERE a.id = b.id)
WHERE
    EXISTS ( SELECT 1 FROM 表B b WHERE a.id = b.id and b.nnnum is not null);