Support BigQuery CASE expr flavor

[shay-rf]

BigQuery supports two flavors of CASE.
The regular is analogous to Clojure's 'cond' - where we provide pairs of condition and expression, with the optional default expression at the end.

The second flavor is similar to Clojure's 'case' - where we provide a single expression and then pairs of value and expression, with the optional default expression at the end.
https://cloud.google.com/bigquery/docs/reference/standard-sql/conditional_expressions#case_expr

So, one can provide something that computes a value once and then based on the result value provides the target value ( or default ).

I am wondering how could this be supported? For example something like:
[:case-val my-expression
1 "foo"
2 "bar"
"baz" ]

Is it something that better be added internally, or should a user registered op be constructed ?

[seancorfield]

I'm happy to add support for this. :case is currently "special syntax" so I'll make that into a reusable function and add :case-expr (which I think is more expressive and more accurate than :case-val since it accepts an expression).

[seancorfield]

How does this look @shay-rf ?

(deftest test-case-expr
  (is (= ["SELECT CASE foo WHEN ? THEN ? WHEN ? THEN foo / ? ELSE ? END FROM bar"
          1 -1 2 2 0]
         (sut/format
          {:select [[[:case-expr :foo
                      1 -1
                      2 [:/ :foo 2]
                      :else 0]]]
           :from [:bar]}))))

[shay-rf]

Works perfectly :)
Thanks!