title: check value does not already exist in a column before inserting new record
body: <p>I have a table with two columns.</p>
  
  <p>The first column is the primary key and SQL auto generates the value.</p>
  
  <p>When a new record is inserted into my table, myTbl, I want to make sure that the ExternalId value does not currently exist in my table.</p>
  
  <p>I was about to create an insert trigger but was thinking there might be a better / more efficient way than creating a trigger?</p>
  
  <pre><code>create table myTbl
  (
      TeamId int not null identity(1000, 1),
      ExternalId nvarchar(14)
      constraint PK_myTbl primary key(TeamId)
  )
  
  
       TeamId     ExternalId
       1000       DFJK
       1001       LMKG
       1002       PLKM
  </code></pre>