De acordo com o manual :
Então, basta seguir o manual:
SELECT CAST(CAST(myColumn AS DECIMAL(5,2)) AS SIGNED) FROM myTable
ou
SELECT CAST(CAST(myColumn AS DECIMAL(5,2)) AS UNSIGNED) FROM myTable
SELECT CAST(CAST(myColumn AS DECIMAL(5,2)) AS SIGNED) FROM myTable
SELECT CAST(CAST(myColumn AS DECIMAL(5,2)) AS UNSIGNED) FROM myTable