当选择空字符串作为列时,SQL联合将考虑字符数

你似乎想要:

select t1.col1, t1.col2, t1.col3, null as col4
from table1 t1
union all
select t2.col1, t2.col2, null as col3, t2.col4
from table2 t2;

我建议使用 NULL 而不是 ''

将空值设为VARCHAR(50)就可以了。

select t1.col1, t1.col2, t1.col3, CAST(null as VARCHAR(50)) as col4
from table1 t1
union all
select t2.col1, t2.col2, CAST(null as VARCHAR(50)) as col3, t2.col4
from table2 t2;