README: update for recent improvements

Remove outdated bits that claim shoddy SQL support and code
structure--we're much better on those fronts now! Also add a few
paragraphs about the current state of SQL compliance, why it's hard to
say anything detailed about SQL compliance, and what our long-term goals
are.

Author: benesch

README.md

@@ -6,15 +6,22 @@
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-The goal of this project is to build a SQL lexer and parser capable of parsing SQL that conforms with the [ANSI SQL:2011](https://jakewheat.github.io/sql-overview/sql-2011-foundation-grammar.html#_5_1_sql_terminal_character) standard but also making it easy to support custom dialects so that this crate can be used as a foundation for vendor-specific parsers.
+The goal of this project is to build a SQL lexer and parser capable of parsing
+SQL that conforms with the [ANSI/ISO SQL standard][sql-standard] while also
+making it easy to support custom dialects so that this crate can be used as a
+foundation for vendor-specific parsers.
 
-This parser is currently being used by the [DataFusion](https://github.com/andygrove/datafusion) query engine and [LocustDB](https://github.com/cswinter/LocustDB).
+This parser is currently being used by the [DataFusion] query engine and
+[LocustDB].
 
 ## Example
 
-The current code is capable of parsing some trivial SELECT and CREATE TABLE statements.
+To parse a simple `SELECT` statement:
 
 ```rust
+use sqlparser::dialect::GenericDialect;
+use sqlparser::parser::Parser;
+
 let sql = "SELECT a, b, 123, myfunc(b) \
            FROM table_1 \
            WHERE a > b AND b < 100 \
@@ -33,23 +40,88 @@ This outputs
 AST: [Query(Query { ctes: [], body: Select(Select { distinct: false, projection: [UnnamedExpr(Identifier("a")), UnnamedExpr(Identifier("b")), UnnamedExpr(Value(Long(123))), UnnamedExpr(Function(Function { name: ObjectName(["myfunc"]), args: [Identifier("b")], over: None, distinct: false }))], from: [TableWithJoins { relation: Table { name: ObjectName(["table_1"]), alias: None, args: [], with_hints: [] }, joins: [] }], selection: Some(BinaryOp { left: BinaryOp { left: Identifier("a"), op: Gt, right: Identifier("b") }, op: And, right: BinaryOp { left: Identifier("b"), op: Lt, right: Value(Long(100)) } }), group_by: [], having: None }), order_by: [OrderByExpr { expr: Identifier("a"), asc: Some(false) }, OrderByExpr { expr: Identifier("b"), asc: None }], limit: None, offset: None, fetch: None })]
 ```
 
+## SQL compliance
+
+SQL was first standardized in 1987, and revisions of the standard have been
+published regularly since. Most revisions have added significant new features to
+the language, and as a result no database claims to support the full breadth of
+features. This parser currently supports most of the SQL-92 syntax, plus some
+syntax from newer versions that have been explicitly requested, plus some MSSQL-
+and PostgreSQL-specific syntax. Whenever possible, the [online SQL:2011
+grammar][sql-2011-grammar] is used to guide what syntax to accept. (We will
+happily accept changes that conform to the SQL:2016 syntax as well, but that
+edition's grammar is not yet available online.)
+
+Unfortunately, stating anything more specific about compliance is difficult.
+There is no publicly available test suite that can assess compliance
+automatically, and doing so manually would strain the project's limited
+resources. Still, we are interested in eventually supporting the full SQL
+dialect, and we are slowly building out our own test suite.
+
+If you are assessing whether this project will be suitable for your needs,
+you'll likely need to experimentally verify whether it supports the subset of
+SQL that you need. Please file issues about any unsupported queries that you
+discover. Doing so helps us prioritize support for the portions of the standard
+that are actually used. Note that if you urgently need support for a feature,
+you will likely need to write the implementation yourself. See the
+[Contributing](#Contributing) section for details.
+
+### Supporting custom SQL dialects
+
+This is a work in progress, but we have some notes on [writing a custom SQL
+parser](docs/custom_sql_parser.md).
+
 ## Design
 
-This parser is implemented using the [Pratt Parser](https://tdop.github.io/) design, which is a top-down operator-precedence parser.
+The core expression parser uses the [Pratt Parser] design, which is a top-down
+operator-precedence (TDOP) parser, while the surrounding SQL statement parser is
+a traditional, hand-written recursive descent parser. Eli Bendersky has a good
+[tutorial on TDOP parsers][tdop-tutorial], if you are interested in learning
+more about the technique.
 
-I am a fan of this design pattern over parser generators for the following reasons:
+We are a fan of this design pattern over parser generators for the following
+reasons:
 
-- Code is simple to write and can be concise and elegant (this is far from true for this current implementation unfortunately, but I hope to fix that using some macros)
+- Code is simple to write and can be concise and elegant
 - Performance is generally better than code generated by parser generators
 - Debugging is much easier with hand-written code
-- It is far easier to extend and make dialect-specific extensions compared to using a parser generator
-
-## Supporting custom SQL dialects
-
-This is a work in progress but I started some notes on [writing a custom SQL parser](docs/custom_sql_parser.md).
+- It is far easier to extend and make dialect-specific extensions
+  compared to using a parser generator
 
 ## Contributing
 
-Contributors are welcome! Please see the [current issues](https://github.com/andygrove/sqlparser-rs/issues) and feel free to file more!
-
-Please run [cargo fmt](https://github.com/rust-lang/rustfmt#on-the-stable-toolchain) to ensure the code is properly formatted.
+Contributions are highly encouraged!
+
+Pull requests that add support for or fix a bug in a feature in the SQL
+standard, or a feature in a popular RDBMS, like Microsoft SQL Server or
+PostgreSQL, will almost certainly be accepted after a brief review. For
+particularly large or invasive changes, consider opening an issue first,
+especially if you are a first time contributor, so that you can coordinate with
+the maintainers. CI will ensure that your code passes `cargo test`,
+`cargo fmt`, and `cargo clippy`, so you will likely want to run all three
+commands locally before submitting your PR.
+
+If you are unable to submit a patch, feel free to file an issue instead. Please
+try to include:
+
+  * some representative examples of the syntax you wish to support or fix;
+  * the relevant bits of the [SQL grammar][sql-2011-grammar], if the syntax is
+    part of SQL:2011; and
+  * links to documentation for the feature for a few of the most popular
+    databases that support it.
+
+Please be aware that, while we strive to address bugs and review PRs quickly, we
+make no such guarantees for feature requests. If you need support for a feature,
+you will likely need to implement it yourself. Our goal as maintainers is to
+facilitate the integration of various features from various contributors, but
+not to provide the implementations ourselves, as we simply don't have the
+resources.
+
+[tdop-tutorial]: https://eli.thegreenplace.net/2010/01/02/top-down-operator-precedence-parsing
+[`cargo fmt`]: https://github.com/rust-lang/rustfmt#on-the-stable-toolchain
+[current issues]: https://github.com/andygrove/sqlparser-rs/issues
+[DataFusion]: https://github.com/apache/arrow/tree/master/rust/datafusion
+[LocustDB]: https://github.com/cswinter/LocustDB
+[Pratt Parser]: https://tdop.github.io/
+[sql-2011-grammar]: https://jakewheat.github.io/sql-overview/sql-2011-foundation-grammar.html
+[sql-standard]: https://en.wikipedia.org/wiki/ISO/IEC_9075