MYSQL variabla v query

SELECT a.*, @a:=200.50, @a+EUR AS end_value FROM ...

Res je, ustvari se nov stolpec. Potencialno - če res želiš - bi lahko okoli tega zavil še en SELECT, kjer bi izbral vse razen tega stolpca.

Naj pa vsekakor omenim, da dokumentacija MySQL tvoje idejo odsvetuje.

Previous releases of MySQL made it possible to assign a value to a user variable in statements other than SET. This functionality is supported in MySQL 8.0 for backward compatibility but is subject to removal in a future release of MySQL.

Prav tako:

The order of evaluation for expressions involving user variables is undefined. For example, there is no guarantee that SELECT @a, @a:=@a+1 evaluates @a first and then performs the assignment.