title: Composite Unique key constraint with multiple null value in one of the column
body: <p>I have a unique key on table x with 3 column (a,b,c) where a,b,c are foreign keys and c can be null in x table.</p>
  
  <pre><code>a b c
  - - ----
  1 1    1
  1 1    2
  1 1 NULL
  1 1 NUll
  </code></pre>
  
  <p>Above rows are valid on MySQL and inserting rows with multiple null does not violate constraint.But this is not true with Oracle , SQL-Server</p>
  
  <p>what are the best practices in this scenario?</p>
  
  <p>with every unique constraint unique index is getting created and if i disable unique index multiple nulls are allowed in SQL-Server also ( with filtered index)</p>
  
  <p>i need to set multiple value in column c with null as its foreign key.</p>
  
  <p>Please suggest should i remove the unique key constraint , should i remove foreign key from column c in table x. or we have any other solution.</p>