Query to insert data into columns and check for pre-existing values for those columns data I have table t1 and t2 with same column names in both tables, column a and b. I want to enter data of column a and b from t2 into t1 at the same time check weather  any data pre exist in t1. If data exist already in a and b column then don't enter that data.
The main challenge in my case is , I want to use combination of (a and b) column data to check for pre existing values.
Thanks for your precious time in advance.
SQL Server:

INSERT INTO t1 (a,b) 
SELECT a, b 
FROM t2
EXCEPT
SELECT a, b 
FROM t1 


The EXCEPT operator compares the result sets of two queries and eliminates those rows in the first result set that match in the second result set. It uses an exact match to identify and eliminate rows.

There are other options of course, such as MERGE or using NOT EXISTS in your WHERE clause, however, for a simple query such as this EXCEPT will likely perform better.