[sql] Avoid duplicates in INSERT INTO SELECT query in SQL Server

I have the following two tables:

Table1
----------
ID   Name
1    A
2    B
3    C

Table2
----------
ID   Name
1    Z

I need to insert data from Table1 to Table2. I can use the following syntax:

INSERT INTO Table2(Id, Name) SELECT Id, Name FROM Table1

However, in my case, duplicate IDs might exist in Table2 (in my case, it's just "1") and I don't want to copy that again as that would throw an error.

I can write something like this:

IF NOT EXISTS(SELECT 1 FROM Table2 WHERE Id=1)
INSERT INTO Table2 (Id, name) SELECT Id, name FROM Table1 
ELSE
INSERT INTO Table2 (Id, name) SELECT Id, name FROM Table1 WHERE Table1.Id<>1

Is there a better way to do this without using IF - ELSE? I want to avoid two INSERT INTO-SELECT statements based on some condition.

A simple DELETE before the INSERT would suffice:

DELETE FROM Table2 WHERE Id = (SELECT Id FROM Table1)
INSERT INTO Table2 (Id, name) SELECT Id, name FROM Table1

Switching Table1 for Table2 depending on which table's Id and name pairing you want to preserve.

I was facing the same problem recently...
Heres what worked for me in MS SQL server 2017...
The primary key should be set on ID in table 2...
The columns and column properties should be the same of course between both tables. This will work the first time you run the below script. The duplicate ID in table 1, will not insert...

If you run it the second time, you will get a

Violation of PRIMARY KEY constraint error

This is the code:

Insert into Table_2
Select distinct *
from Table_1
where table_1.ID >1

From SQL Server you can set a Unique key index on the table for (Columns that needs to be unique)

Using ignore Duplicates on the unique index as suggested by IanC here was my solution for a similar issue, creating the index with the Option WITH IGNORE_DUP_KEY

In backward compatible syntax
, WITH IGNORE_DUP_KEY is equivalent to WITH IGNORE_DUP_KEY = ON.

Ref.: index_option

I just had a similar problem, the DISTINCT keyword works magic:

INSERT INTO Table2(Id, Name) SELECT DISTINCT Id, Name FROM Table1

In my case, I had duplicate IDs in the source table, so none of the proposals worked. I don't care about performance, it's just done once. To solve this I took the records one by one with a cursor to ignore the duplicates.

So here's the code example:

DECLARE @c1 AS VARCHAR(12);
DECLARE @c2 AS VARCHAR(250);
DECLARE @c3 AS VARCHAR(250);


DECLARE MY_cursor CURSOR STATIC FOR
Select
c1,
c2,
c3
from T2
where ....;

OPEN MY_cursor
FETCH NEXT FROM MY_cursor INTO @c1, @c2, @c3

WHILE @@FETCH_STATUS = 0
BEGIN
    if (select count(1) 
        from T1
        where a1 = @c1
        and a2 = @c2
        ) = 0 
            INSERT INTO T1
            values (@c1, @c2, @c3)

    FETCH NEXT FROM MY_cursor INTO @c1, @c2, @c3
END
CLOSE MY_cursor
DEALLOCATE MY_cursor

In MySQL you can do this:

INSERT IGNORE INTO Table2(Id, Name) SELECT Id, Name FROM Table1

Does SQL Server have anything similar?

A little off topic, but if you want to migrate the data to a new table, and the possible duplicates are in the original table, and the column possibly duplicated is not an id, a GROUP BY will do:

INSERT INTO TABLE_2
(name)
  SELECT t1.name
  FROM TABLE_1 t1
  GROUP BY t1.name