Expr to Sql : Case

[yyy1000]

Which issue does this PR close?

Related #9726.

Rationale for this change

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[yyy1000]

I'm trying to make these two iterations to one, but I failed. ☹️
I tried

1. unzip, but I got two vec which consist of Result<Expr> but not Expr
2. match, got type incompatible error 🥲
3. collect, but don't know how to collect a (Result<Expr>, Result<Expr>)

[alamb]

I likewise tried to use izip and came up with this, which wasn't particularly nice

                let (conditions, results): (Vec<_>, Vec<_>) = when_then_expr
                    .iter()
                    .map(|(w, t)| (self.expr_to_sql(w), self.expr_to_sql(t)))
                    .unzip();

                let conditions = conditions.into_iter().collect::<Result<Vec<_>>>()?;
                let results = results.into_iter().collect::<Result<Vec<_>>>()?;

This is the best I could come up with but I am not sure how much better it is:

                let mut conditions = vec![];
                let mut results = vec[]!;
                for (w, t) in when_then_expr {
                    conditions.push(self.expr_to_sql(w)?);
                    results.push(self.expr_to_sql(t)?);
                }

[yyy1000]

Yeah, I also tried two pushes before but I removed it for declaring two mut vec 😆

[yyy1000]

I also changed some of these tests to make them looks concise.

[alamb]

Looks good to me @yyy1000 -- thank you for the contribution.

[alamb]

I likewise tried to use izip and came up with this, which wasn't particularly nice

                let (conditions, results): (Vec<_>, Vec<_>) = when_then_expr
                    .iter()
                    .map(|(w, t)| (self.expr_to_sql(w), self.expr_to_sql(t)))
                    .unzip();

                let conditions = conditions.into_iter().collect::<Result<Vec<_>>>()?;
                let results = results.into_iter().collect::<Result<Vec<_>>>()?;

This is the best I could come up with but I am not sure how much better it is:

                let mut conditions = vec![];
                let mut results = vec[]!;
                for (w, t) in when_then_expr {
                    conditions.push(self.expr_to_sql(w)?);
                    results.push(self.expr_to_sql(t)?);
                }