php - MYSQL select, set all negative values to zero -

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let assume have mysql table called "test" in table have 2 columns "a", "b". want result of a-b, each row result negative, want set zero.

so can write : select test.a - test.b myresult

but don't know how set myresult 0 when myresult negative number. please 

select case when test.a - test.b >= 0 test.a - test.b else 0 end myresult test;

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