Order of Execution on SELECT Expressions

Bert Wagner digs into an interesting topic:

Success! But as I was celebrating my dynamic SQL victory, I realized I was making an assumption about SQL Server that I had never thought about before:

The above query only works because SQL Server is executing the variables in the SELECT list sequentially. I’m incrementing @CurrentRow only after processing my @RowQuery variable, and this logic only works correctly if SQL Server executes the variable expressions in the order they appear in the SELECT list. If SQL Server was executing items in the SELECT list in reverse or random order, @CurrentRow could potentially get set BEFORE @RowQuery was evaluated, causing the logic of adding “UNION ALL SELECT” in the right location to fail.

I have a vague recollection that you couldn’t always count on this, though I admit to never having seen a counter-example. I don’t believe expression execution order is defined in the ANSI SQL standard—the idea is that everything is typically resolved “at once.”

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