title: How to efficiently check if two columns in the same table is one-to-one mapping?
body: <p>Even though the distinct value counts equals, it did not necessarily mean an one-to-one mapping relation.</p>
  
  <pre><code>SELECT COUNT(DISTINCT [Column_A]) FROM MyTable 
  SELECT COUNT( DISTINCT [Column_B]) FROM MyTable 
  </code></pre>
  
  <p>Column A: 1 2 3 4 5 6 7 8 9 10 1<br>
  Column B: a b c d e f g h i j  j</p>
  
  <p>The query above return a value of 10 for each column but they are not one-to-one mapping. </p>
  
  <p>How can I exactly check the existence of this <code>matching type?</code>
  Thanks in advance</p>