Question

I have two SQL Server tables. One table has four nvarchar(50) nullable columns and an ID column. The second table has two columns: ID, nvarchar(50) nullable column

I’m trying to insert the rows in the second table into the first table. How can I detect a null column in the first table and insert data from the second one into the null cell?

Ex:

Add from the second table

1   value3

to the first table

1     value1   value2   null    null

so the resulting row is

1     value1   value2   value3   null

Ex2:

Add from the second table

2   value1

to the first table

nothing for ID 2

so the resulting row is

2     value1   null   null   null

Basically I need to update existing rows or create new ones depending on the ID of the second table rows.

Answer 1

declare @T1 table
(
  ID int primary key,
  C1 varchar(50),
  C2 varchar(50),
  C3 varchar(50),
  C4 varchar(50)
);

declare @T2 table
(
  ID int primary key,
  C varchar(50)  
);

insert into @T1 values (1, 'value1', 'value2', null, null) ;

insert into @T2 values (1, 'value3');
insert into @T2 values (2, 'value1');

merge @T1 as T1
using @T2 as T2
on T1.ID = T2.ID
when not matched then
  insert (ID, C1)  values (T2.ID, T2.C)
when matched then
  update set
    C1 = case when T1.C1 is null 
           then T2.C 
           else T1.C1 
         end,
    C2 = case when T1.C1 is not null and 
                   T1.C2 is null 
           then T2.C 
           else T1.C2 
         end,
    C3 = case when T1.C1 is not null and 
                   T1.C2 is not null and 
                   T1.C3 is null 
           then T2.C 
           else T1.C3 
         end,
    C4 = case when T1.C1 is not null and 
                   T1.C2 is not null and 
                   T1.C3 is not null and 
                   T1.C4 is null 
           then T2.C 
           else T1.C4 
         end
;

select *
from @T1;

Result:

ID C1     C2     C3     C4
-- ------ ------ ------ ------
1  value1 value2 value3 NULL
2  value1 NULL   NULL   NULL