How to prevent Duplicate records from my table Insert ignore does not work here

Question

mysql> select * from emp;

    +-----+---------+------+------+------+
    | eno | ename   | dno  | mgr  | sal  |
    +-----+---------+------+------+------+
    |          


        

Answer 1

This will work regardless of whether you clean up your table first (i.e. you can stop inserting duplicates immediately and clean up on separate schedule) and without having to add any unique constraints or altering table in any other way:

INSERT INTO
    emp (ename, dno, mgr, sal)
SELECT
    e.ename, 2, 2, 2000
FROM
    (SELECT 'dlksdgj' AS ename) e
    LEFT JOIN emp ON e.ename = emp.ename
WHERE
    emp.ename IS NULL

The above query assumes you want to use ename as a "unique" field, but in the same way you could define any other fields or their combinations as unique for the purposes of this INSERT.

It works because it's an INSERT ... SELECT format where the SELECT part only produces a row (i.e. something to insert) if its left joined emp does not already have that value. Naturally, if you wanted to change which field(s) defined this "uniqueness" you would modify the SELECT and the LEFT JOIN accordingly.