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>

tests/sqlparser_common.rs

@@ -1044,6 +1044,7 @@ fn parse_create_table() {
             external: false,
             file_format: None,
             location: None,
+            query: _query,
         } => {
             assert_eq!("uk_cities", name.to_string());
             assert_eq!(
@@ -1177,6 +1178,36 @@ fn parse_drop_schema() {
     }
 }
 
+#[test]
+fn parse_create_table_as() {
+    let sql = "CREATE TABLE t AS SELECT * FROM a";
+
+    match verified_stmt(sql) {
+        Statement::CreateTable { name, query, .. } => {
+            assert_eq!(name.to_string(), "t".to_string());
+            assert_eq!(query, Some(Box::new(verified_query("SELECT * FROM a"))));
+        }
+        _ => unreachable!(),
+    }
+
+    // BigQuery allows specifying table schema in CTAS
+    // ANSI SQL and PostgreSQL let you only specify the list of columns
+    // (without data types) in a CTAS, but we have yet to support that.
+    let sql = "CREATE TABLE t (a INT, b INT) AS SELECT 1 AS b, 2 AS a";
+    match verified_stmt(sql) {
+        Statement::CreateTable { columns, query, .. } => {
+            assert_eq!(columns.len(), 2);
+            assert_eq!(columns[0].to_string(), "a INT".to_string());
+            assert_eq!(columns[1].to_string(), "b INT".to_string());
+            assert_eq!(
+                query,
+                Some(Box::new(verified_query("SELECT 1 AS b, 2 AS a")))
+            );
+        }
+        _ => unreachable!(),
+    }
+}
+
 #[test]
 fn parse_create_table_with_on_delete_on_update_2in_any_order() -> Result<(), ParserError> {
     let sql = |options: &str| -> String {
@@ -1245,6 +1276,7 @@ fn parse_create_external_table() {
             external,
             file_format,
             location,
+            query: _query,
         } => {
             assert_eq!("uk_cities", name.to_string());
             assert_eq!(
@@ -1307,12 +1339,6 @@ fn parse_create_external_table_lowercase() {
     assert_matches!(ast, Statement::CreateTable{..});
 }
 
-#[test]
-fn parse_create_table_empty() {
-    // Zero-column tables are weird, but supported by at least PostgreSQL.
-    let _ = verified_stmt("CREATE TABLE t ()");
-}
-
 #[test]
 fn parse_alter_table() {
     let add_column = "ALTER TABLE tab ADD COLUMN foo TEXT";