Hola Yoi,
Textualmente del manual de MySQL:
Cita: E.1. Restrictions on Stored Programs
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The restrictions for stored functions also apply to triggers.
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Restrictions for Stored Functions
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- Statements that return a result set. This includes SELECT statements that do not have an INTO var_list clause and other statements such as SHOW, EXPLAIN, and CHECK TABLE. A function can process a result set either with SELECT ... INTO var_list or by using a cursor and FETCH statements.