Looks nice! Thank you for the PR!

I have one question. What will it be like if we want to use subqueries like WHERE a IN (SELECT b FROM ...)?

I think it would be better to unnest explicitly like:

In(Ident("hoge"), Unnest([]string{"foo", "bar"}))

and define an interface that is responsible for converting memeduck values to ast.InCondition like:

type InCondition interface {
    ToASTInCondition() (ast.InCondition, error)
}

func Unnest(...) *UnnestInCondition { ... }

func (*UnnestInCondition) ToASTInCondition() (ast.InCondition, error) { ... }

for further extensibility.

Thanks for your review.
As you pointed out , I didn't consider the other InCondition type.
I fixed it. :)