SQL SERVER – Puzzle – Solution – Why Does UNION ALL Work but UNION Produces Error?

Last week, I asked a very simple puzzle about SQL SERVER – Puzzle – Why Does UNION ALL Work but UNION Produces Error?. The puzzle was very simple and interesting both together. I got quite a few valid answers to the puzzle so far. You may check original puzzle post to see all the valid answers. In this blog post we will quickly discuss the answer of our puzzle.

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SQL SERVER – Puzzle – Why Does UNION ALL Work but UNION Produces Error?

You already know what UNION and UNION ALL operators do. They combine datasets while UNION keeps the distinct data, UNION ALL will retain all the data. You can read more about that over here: SQL SERVER – Difference Between Union vs. Union All – Optimal Performance Comparison. Let us see why does anyone all work, but union produces error in this puzzle. 

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