Support IF NOT EXISTS for CREATE SCHEMA (apache#276)

This is a Postgres-specific clause: https://www.postgresql.org/docs/12/sql-createschema.html

Also add a test for `DROP SCHEMA IF EXISTS schema_name`, which is already supported in the parser.

Author: alex-dukhno

src/ast/mod.rs

@@ -558,7 +558,10 @@ pub enum Statement {
     /// `ROLLBACK [ TRANSACTION | WORK ] [ AND [ NO ] CHAIN ]`
     Rollback { chain: bool },
     /// CREATE SCHEMA
-    CreateSchema { schema_name: ObjectName },
+    CreateSchema {
+        schema_name: ObjectName,
+        if_not_exists: bool,
+    },
     /// `ASSERT <condition> [AS <message>]`
     Assert {
         condition: Expr,
@@ -829,7 +832,15 @@ impl fmt::Display for Statement {
             Statement::Rollback { chain } => {
                 write!(f, "ROLLBACK{}", if *chain { " AND CHAIN" } else { "" },)
             }
-            Statement::CreateSchema { schema_name } => write!(f, "CREATE SCHEMA {}", schema_name),
+            Statement::CreateSchema {
+                schema_name,
+                if_not_exists,
+            } => write!(
+                f,
+                "CREATE SCHEMA {if_not_exists}{name}",
+                if_not_exists = if *if_not_exists { "IF NOT EXISTS " } else { "" },
+                name = schema_name
+            ),
             Statement::Assert { condition, message } => {
                 write!(f, "ASSERT {}", condition)?;
                 if let Some(m) = message {

src/parser.rs

@@ -1078,8 +1078,12 @@ impl<'a> Parser<'a> {
     }
 
     pub fn parse_create_schema(&mut self) -> Result<Statement, ParserError> {
+        let if_not_exists = self.parse_keywords(&[Keyword::IF, Keyword::NOT, Keyword::EXISTS]);
         let schema_name = self.parse_object_name()?;
-        Ok(Statement::CreateSchema { schema_name })
+        Ok(Statement::CreateSchema {
+            schema_name,
+            if_not_exists,
+        })
     }
 
     pub fn parse_create_external_table(

tests/sqlparser_common.rs

@@ -1204,7 +1204,7 @@ fn parse_create_schema() {
     let sql = "CREATE SCHEMA X";
 
     match verified_stmt(sql) {
-        Statement::CreateSchema { schema_name } => {
+        Statement::CreateSchema { schema_name, .. } => {
             assert_eq!(schema_name.to_string(), "X".to_owned())
         }
         _ => unreachable!(),

tests/sqlparser_postgres.rs

@@ -300,6 +300,33 @@ fn parse_bad_if_not_exists() {
     );
 }
 
+#[test]
+fn parse_create_schema_if_not_exists() {
+    let sql = "CREATE SCHEMA IF NOT EXISTS schema_name";
+    let ast = pg_and_generic().verified_stmt(sql);
+    match ast {
+        Statement::CreateSchema {
+            if_not_exists: true,
+            schema_name,
+        } => assert_eq!("schema_name", schema_name.to_string()),
+        _ => unreachable!(),
+    }
+}
+
+#[test]
+fn parse_drop_schema_if_exists() {
+    let sql = "DROP SCHEMA IF EXISTS schema_name";
+    let ast = pg().verified_stmt(sql);
+    match ast {
+        Statement::Drop {
+            object_type,
+            if_exists: true,
+            ..
+        } => assert_eq!(object_type, ObjectType::Schema),
+        _ => unreachable!(),
+    }
+}
+
 #[test]
 fn parse_copy_example() {
     let sql = r#"COPY public.actor (actor_id, first_name, last_name, last_update, value) FROM stdin;