Question

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.

Answer 1

Using NOT EXISTS:

INSERT INTO TABLE_2
  (id, name)
SELECT t1.id,
       t1.name
  FROM TABLE_1 t1
 WHERE NOT EXISTS(SELECT id
                    FROM TABLE_2 t2
                   WHERE t2.id = t1.id)

Using NOT IN:

INSERT INTO TABLE_2
  (id, name)
SELECT t1.id,
       t1.name
  FROM TABLE_1 t1
 WHERE t1.id NOT IN (SELECT id
                       FROM TABLE_2)

Using LEFT JOIN/IS NULL:

INSERT INTO TABLE_2
  (id, name)
   SELECT t1.id,
          t1.name
     FROM TABLE_1 t1
LEFT JOIN TABLE_2 t2 ON t2.id = t1.id
    WHERE t2.id IS NULL

Of the three options, the LEFT JOIN/IS NULL is less efficient. See this link for more details.