Why do I get sql error "Violation of PRIMARY KEY constraint" - sql

Question

I have a table
CREATE TABLE [dbo].[DocGenVariable](
[userkey_sessionid] [varchar](38) NOT NULL,
[is_session] [bit] NULL,
[var_name] [nvarchar](255) NOT NULL,
[var_value] [nvarchar](1000) NOT NULL,
[topic_id] [varchar](38) NOT NULL,
[resource_id] [int] NULL,
[added] [datetime] NOT NULL,
CONSTRAINT [PK_DocGenVariable] PRIMARY KEY CLUSTERED
(
[userkey_sessionid] ASC,
[var_name] ASC,
[topic_id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
Sometimes (attention > sometimes) I get an error
"Violation of PRIMARY KEY constraint 'PK_DocGenVariable'.
Cannot insert duplicate key in object 'dbo.DocGenVariable'.
when I try to add an entry into this table
delete from DocGenVariable where userkey_sessionid = #key_session and var_name = #var_name and topic_id = #topic_id
if (LTRIM(RTRIM(#var_value)) <> '')
begin
insert into DocGenVariable(userkey_sessionid, is_session, var_name, var_value, topic_id, resource_id, added)
values (#key_session, #is_session, #var_name, #var_value, #topic_id, #resource_id, GetDate())
end
What I do first is deleting an entry which must be updated, then add a new entry.
Why do I get this constraint error?

Answer 1

Why delete then insert? Just update it.
IF EXISTS(select 1 from DocGenVariable where userkey_sessionid = #key_session
and
var_name = #var_name and topic_id = #topic_id)
BEGIN
UPDATE ....
END ELSE BEGIN
INSERT ...
END

Answer 2

Try upsert
using DocGenVariable target
using DocGenVariable source
on source.userkey_sessionid = #key_session
and source.var_name = #var_name
and source.topic_id = #topic_id
and target.userkey_sessionid = #key_session
and target.var_name = #var_name
and target.topic_id = #topic_id
when match
set target.resource_id = #resource_id
, target .added = GetDate()
when not matched then
insert into DocGenVariable(userkey_sessionid, is_session, var_name, var_value, topic_id, resource_id, added)
values (#key_session, #is_session, #var_name, #var_value, #topic_id, #resource_id, GetDate())