title: how to avoid duplicates in a table check on multiple columns
body: <p>I want to make sure a combination of multiple columns is not inserted.</p>
  
  <p>Lets say.</p>
  
  <pre><code>Emloyee
  
  EmployeeId CatgoryId  BusinessId
  
  1          1          1
  
  2          2          1
  
  
  BusessCategory
  
  CategoryId BusinessId
  
  1          1
  1          2
  1          3
  2          1
  2          4
  2          3
  </code></pre>
  
  <p>I have to make sure no combination is repeated while inserting data in to the table.</p>
  
  <p>my sql query would me some thing like</p>
  
  <pre><code>insert into employee(EmployeeId, CatgoryId,  BusinessId)
  select e.employeeid,e.categoryid,bc.BusinessId from employee e inner join BusessCategory bc on bc.CatgoryId = e.CatgoryId 
  </code></pre>