SELECT 1 vs. SELECT * in EXISTS Clauses: Does it Impact SQL Server Performance?

*SELECT 1 and SELECT in the EXISTS clause: SQL Server performance difference analysis**

In SQL Server, there is always a debate about whether to use SELECT 1 or SELECT * when using subqueries with the EXISTS operator. Some people think SELECT 1 is more efficient, but is the performance difference actually significant?

Conclusion:

According to Microsoft documentation and the ANSI SQL standard, there is little practical difference between using SELECT 1 or SELECT * in the EXISTS clause. SQL Server is optimized for both forms and interprets them as equivalent.

Reason:

In the EXISTS clause, the sole purpose of the subquery is to determine whether any rows satisfy the specified criteria. The results of the selection list are discarded by the system. Therefore, whether you specify SELECT 1 or SELECT *, the performance impact is insignificant.

Example:

Consider the following query:

<code class="language-sql">IF EXISTS (SELECT 1 FROM sometable WHERE a_valid_clause)
BEGIN
  ...
END</code>

Even if the subquery selects the literal 1, it does not attempt to perform any mathematical operations on it, so the presence of divide by zero (1/0) does not cause an error.

Summary:

While you may prefer to use SELECT 1 for consistency or clarity, doing so does not provide any real performance benefit when using the EXISTS operator in SQL Server.

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