Transpiling from SQL to PRQL

[max-sixty]

This is a longer-term idea — but now we're using sqlparser to write SQL, we could also use it to read SQL and produce PRQL.

We'd need to write:

1. a fairly simple compiler from sqlparser AST to PRQL AST
2. Display implementations which convert PRQL AST to PRQL code

This would be useful for people transitioning from SQL. At least initially, the main users are likely to be engineers who are frustrated by SQL because they've written a lot of it.

We wouldn't need to support all of SQL for it to be useful — there's a fairly small subset that's used a lot — and possibly a larger subset which we could implement with s-strings.

We also wouldn't need to automatically do things like variable "creation" (e.g. replacing salary + payroll_tax + benefits_cost with gross_cost), which would be much more difficult (but great if we could!).

[aljazerzen]

Update: this is currently not possible yet, but we are not far off.

The Display trait for prql's AST Node is done, we just need to parse SQL and convert it to AST, which should't be a major problem.

[ryanrussell]

A SQL -> PRQL transpiler would REALLY ease adoption for PRQL.

Even if it had some disclaimers, it lowers a lot of barriers if a new user is trying to learn/adopt PRQL and could copy/paste a bunch of SQL queries.

[florent-martineau]

Using Apache Calcite for parsing queries and turning them into an AST could be an idea.

Calcite is written in Java, though, and not in Rust.

[max-sixty]

We'd likely use sqlparser-rs, which we currently use to represent SQL AST, and it builds the SQL string.