UNION/UNION ALL不会删除其中包含NULL的行.

The UNION/UNION ALL does not remove rows with NULL in them.

UNION删除重复的行； UNION ALL不会，并且比UNION快.

UNION removes duplicate rows; UNION ALL does not, and is faster than UNION for it.

检查您的数据&查询您为什么看不到空值的原因.

Check your data & your query for why NULLs you expect to see are not visible.