在SQL Server 2008中合并两个表

Question

I need to ask something is there any way combine two tables different count of columns like:

Select a,b,c, from x

 union 

Select d, e from y

Answer 1

you need to do something like this

Select a,b,c from x

 union all  -- ALL doesn't filter dups and is faster

Select d, e, '' as f from y

I used '' but you might want to use NULL or 0, NULL will be compatible will all data types, '' will not be

I also used UNION ALL not UNION since it will perform better because it doesn't have to do a sort operation to get rid of dups

the alias f is not needed here either because the top query determines the name of the columns in the resultset

Answer 2

Note that

select a, b, c from x
union
select d, e, '' as f from y;

and

select d, e, '' as f from y
union
select a, b, c from x;

will yield different results ie the attribute names from the first-appearing table will be used.

Perhaps better to be unequivocal by explicitly renaming the columns in the second table eg

select a, b, c from x
union
select d AS a, e AS b, '' as c from y;

Answer 3

first table 
id
name
second table 
name 
seatno

if you want to join on name & there are some duplicate names in both table the use ROW_NUMBER in join query!

Answer 4

select col1, col2, col3, col4
from mytable1
union all
select col5, col6, null as col7, '' as col8
from mytable2