Feat: add terse coalesce operator

[z3z1ma]

One of the most common operations we perform in queries other than casting is coalescing. Offering a terse syntax available to all dialects just as you do with the :: casting is hugely beneficial. This feature is also immediately attractive within a SQLMesh project and is just generally a pretty neat showcase of the power of sqlglot.

SELECT x ?? y FROM z -> SELECT COALESCE(x, y) FROM z
"SELECT a, b ?? 'No Data' FROM z" -> SELECT a, COALESCE(b, 'No Data') FROM z

cc: @tobymao

[z3z1ma]

While you could, probably detect ?? chained one after another and put them all into a single COALESCE(.., .., ..) function, its significantly simpler this way, probably more resilient, and just as correct as far as I can tell. And the code footprint to enable this is beautifully simple. 🤓

[georgesittas]

LGTM, good idea Alex 👍

By the way, implementing the ?? operator in this way means that expressions like

a + b ?? c

will be parsed as a + COALESCE(b, c). Perhaps this is something obvious, but I wanted to point it out in case people don't feel this should be the operator's precedence. Of course, one can always wrap a + b in parentheses to produce COALESCE((a + b), c).

[tobymao]

@z3z1ma i'm not sure this is right, i would have thought this was closer to a bitwise operation like concat x || y. can we move it out of column ops?