prevent inserting duplicate records sql code example

Example 1: how to query without duplicate rows in sql

SELECT DISTINCT col1,col2... FROM table_name where Condition;

Example 2: sql query to delete duplicate records

--ID should be primary key

--get duplicate records using RANK
SELECT E.ID, 
    E.firstname, 
    E.lastname, 
    E.country, 
    T.rank
FROM [SampleDB].[dbo].[Employee] E
  INNER JOIN
(
 SELECT *, 
        RANK() OVER(PARTITION BY firstname, 
                                 lastname, 
                                 country
        ORDER BY id) rank
 FROM [SampleDB].[dbo].[Employee]
) T ON E.ID = t.ID;

--delete duplications
DELETE E
    FROM [SampleDB].[dbo].[Employee] E
         INNER JOIN
    (
        SELECT *, 
               RANK() OVER(PARTITION BY firstname, 
                                        lastname, 
                                        country
               ORDER BY id) rank
        FROM [SampleDB].[dbo].[Employee]
    ) T ON E.ID = t.ID
    WHERE rank > 1;