How to simplify a repeatable pattern in SQL query?

Is that possible to simplify:

SELECT GREATEST(a,b)
FROM my_table 
WHERE GREATEST(a,b) > 25

to something like:

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SELECT GREATEST(a,b) AS max_a_b 
FROM my_table 
WHERE max_a_b > 25

This try returns:

Unknown column 'max_a_b' in 'where clause'

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SELECT GREATEST(a,b) AS max_a_b 
FROM my_table 
HAVING max_a_b > 25

At where level your alias doesn't exist while it exists if you use having clause.

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SELECT max_a_b
FROM (SELECT GREATEST(a,b) AS max_a_b 
      FROM my_table)
WHERE max_a_b > 25

This should work, but it's questionable if this simplifies the query. Repeating the GREATEST(a,b) in the WHERE clause is fine, imo.

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MySQL doesn't run the GREATEST(a,b) twice, for what it's worth.

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The statement is still simple. And when your question is about optimization then this is better done by DBMS rather than you.