How to insert value in SQL table if not exists - sql

Question

I'm new in SQL. I'm trying to insert data into a SQL table. If I insert for the first time it works but if I try to do a new insert I get the error
Violation of UNIQUE KEY constraint.
The unique key constraint is on (uc, start_date). The error occurs because there is already the record (-1, 1900-01-01) in the table. Here is my code:
insert into tableA ([start_date],[end_date],[uc],[desc],[ind_sus])
select '1900-01-01' [start_date]
,'9999-12-31'[end_date]
,case when right([DIM], 1) = '|' then '-1'
else right([DIM], 2)
end [uc]
,'' [desc]
,0 [ind_sus]
from tableB b
where right([DIM], 2) NOT IN (select uc from tableA a
where a.uc = right([DIM], 2)
and convert(date,b.[DATETRIAL]) between a.start_date and a.end_date)
group by case when right([DIM], 1) = '|' then '-1'
else right([DIM], 2)
end
The data that I'm trying to insert fulfills the when condition of the case and therefore another (-1, 1900-01-01) as key. I tried:
case when not exists(select uc from tableA where uc = '-1') then '-1'
to insert only if the data doesn't exist. It didn't work because I have to put the same code in the group by clause and I realized that I cannot do a subquery in the group by clause. So, is there a way to check at the case clause level if a value already exist before inserting? Or should I rewrite the request in another way? Database can't be modified
I need suggestions please.

Answer 1

In order to be able to check and insert data when not exists you need to use MERGE operator (see docs for more info).
Since you did not provide us with test data, I'm not sure I can easily update your query, so here is an example I made up to show you how to do things you need using MERGE.
Create table and insert 1 row with id = 1:
create table some_table(id int primary key, text_col varchar(10));
insert into some_table values(1, 'first');
Try to insert a line with same id
insert into some_table values(1, 'another first'); -- it fails of course
same thing using merge
merge into some_table -- target table
using
(select 1 id, 'another first' text_col) new_data -- source data
on some_table.id = new_data.id -- column to check on
when not matched then
insert (id, text_col) values(new_data.id, new_data.text_col);
I hope this helps
My try to change your query to merge would be like that
(keep in mind I did not tested it so fixing syntax or possible data errors is up to you)
merge into tableA a
using (select uc
,'1900-01-01' [start_date]
,'9999-12-31'[end_date]
,case when right([DIM], 1) = '|' then '-1'
else right([DIM], 2) end [uc]
,'' [desc]
,0 [ind_sus]
from tableB b
group by case when right([DIM], 1) = '|' then '-1'
else right([DIM], 2)
end) b
on a.right([DIM], 2) = b.uc
and convert(date,b.[DATETRIAL]) between a.start_date and a.end_date
when not matched then
insert([start_date],[end_date],[uc],[desc],[ind_sus])
values(b.start_date, b.end_date, b.uc, b.desc, b.ind_sus)