title: Sql merging two text columns into single column
body: <p>I have two tables t1 &amp; t2.</p>
  
  <pre><code>table t1
   user_id  tags
     1      a,b,c
     2      b,c
  
  table t2
   user_id  tags
     1      d,c
     2      c,d
  </code></pre>
  
  <p>I want to merge this into table t1. How can i do this.</p>
  
  <pre><code> table t1
    user_id  tags
      1     a,b,c,d
      2     b,c,d
  </code></pre>
  
  <p>I am new in sql.</p>