[mssql] Support DECLARE statements

[FreeMasen]

I am running into a few errors trying to parse MsSql.

The following sql

DECLARE @Var1 INTEGER;

reports the following error:

ParserError("Unexpected keyword \"DECLARE\" at the beginning of a statement")

This is a pretty normal thing to appear in t-sql scripts from my experience, though I maybe misunderstanding why this might be an error.

[benesch]

No one has implemented support for that statement is all. PRs welcome!

[AugustoFKL]

@nickolay @alamb just to make sure, since adding this results in a breaking change on the Statement API. Implementing this is still accepted?

[alamb]

@AugustoFKL -- given that sqlparser uses an enum (that isn't explicitly declared non_exhaustive https://doc.rust-lang.org/reference/attributes/type_system.html) pretty much any change that adds a variant to one of the existing enums will be a breaking change, from a semver perspective.

However, in terms of my take on "easy to upgrade changes" new enum variants to expand the sql support are fine (users have the choice to match _ if they want or they can be explicit)

[alamb]

So "yes adding support for a DECLARE" statement is definitely acceptable from my perspective (like I will review it and help merge it)

[lovasoa]

Just adding a comment here because that feature would be very welcome in the SQLPage web application builder: sqlpage/SQLPage#49

[devanbenz]

@alamb while working on apache/datafusion#9726 I was running in to this. I often use MSSQL for work so the ScalarVariable syntax that I'm used to is DECLARE @foo AS INT; etc. Could I potentially pick up a PR to modify this library to support that in my endeavor to work on the issue linked? I read through the source code for this lib and see that syntax was still not supported.

[alamb]

Sure -- thank you @devanbenz -- feel free to make a PR and then ping me (or @iffyio ). And thank you!

[devanbenz]

@alamb or @iffyio #1235 please take a look when you can. Also, please let me know if there's anything to improve. I'm a tad bit newer to Rust so always happy to improve via constructive critiques :D