Add CREATE TABLE AS support (#206)

We parse it as a regular `CREATE TABLE` statement
followed by an `AS <query>`, which is how BigQuery works:
https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language#create_table_statement


ANSI SQL and PostgreSQL only support a plain list of columns
after the table name in a CTAS
    `CREATE TABLE t (a) AS SELECT a FROM foo`

We currently only allow specifying a full schema with data
types, or omitting it altogether.

https://www.postgresql.org/docs/12/sql-createtableas.html
https://jakewheat.github.io/sql-overview/sql-2016-foundation-grammar.html#as-subquery-clause


Finally, when no schema is specified, we print empty parens after a
plain `CREATE TABLE t ();` as required by PostgreSQL, but skip them
in a CTAS: `CREATE TABLE t AS ...`. This affects serialization only,
the parser allows omitting the schema in a regular `CREATE TABLE` too
since the first release of the parser:
7d27abdfb4/src/sqlparser.rs (L325-L332)

Co-authored-by: Nickolay Ponomarev <asqueella@gmail.com>

src/parser.rs

@@ -1020,6 +1020,7 @@ impl Parser {
             external: true,
             file_format: Some(file_format),
             location: Some(location),
+            query: None,
         })
     }
 
@@ -1108,8 +1109,17 @@ impl Parser {
         let table_name = self.parse_object_name()?;
         // parse optional column list (schema)
         let (columns, constraints) = self.parse_columns()?;
+
+        // PostgreSQL supports `WITH ( options )`, before `AS`
         let with_options = self.parse_with_options()?;
 
+        // Parse optional `AS ( query )`
+        let query = if self.parse_keyword(Keyword::AS) {
+            Some(Box::new(self.parse_query()?))
+        } else {
+            None
+        };
+
         Ok(Statement::CreateTable {
             name: table_name,
             columns,
@@ -1119,6 +1129,7 @@ impl Parser {
             external: false,
             file_format: None,
             location: None,
+            query,
         })
     }