Support IF NOT EXISTS for CREATE SCHEMA (#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.

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(