Snowflake: Support ALTER TABLE ADD COLUMN IF NOT EXISTS

src/dialect/bigquery.rs

@@ -179,4 +179,9 @@ impl Dialect for BigQueryDialect {
     fn supports_comma_separated_trim(&self) -> bool {
         true
     }
+
+    /// See <https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language#alter_table_add_column_statement>
+    fn supports_add_column_if_not_exists(&self) -> bool {
+        true
+    }
 }

src/dialect/duckdb.rs

@@ -146,4 +146,9 @@ impl Dialect for DuckDbDialect {
     fn supports_nested_comments(&self) -> bool {
         true
     }
+
+    /// See <https://duckdb.org/docs/stable/sql/statements/alter_table>
+    fn supports_add_column_if_not_exists(&self) -> bool {
+        true
+    }
 }

src/dialect/generic.rs

@@ -332,4 +332,8 @@ impl Dialect for GenericDialect {
     fn supports_alter_column_position(&self) -> bool {
         true
     }
+
+    fn supports_add_column_if_not_exists(&self) -> bool {
+        true
+    }
 }

src/dialect/mod.rs

@@ -1617,6 +1617,15 @@ pub trait Dialect: Debug + Any {
         false
     }
 
+    /// Returns true if the dialect supports `ALTER TABLE ... ADD [COLUMN] IF NOT EXISTS <col>`.
+    /// Example:
+    ///  ```sql
+    ///  ALTER TABLE tbl ADD COLUMN IF NOT EXISTS c INT
+    /// ```
+    fn supports_add_column_if_not_exists(&self) -> bool {
+        false
+    }
+
     /// Returns true if the dialect considers the specified ident as a function
     /// that returns an identifier. Typically used to generate identifiers
     /// programmatically.

src/dialect/postgresql.rs

@@ -368,4 +368,9 @@ impl Dialect for PostgreSqlDialect {
     fn supports_comment_optimizer_hint(&self) -> bool {
         true
     }
+
+    /// See <https://www.postgresql.org/docs/current/sql-altertable.html>
+    fn supports_add_column_if_not_exists(&self) -> bool {
+        true
+    }
 }

src/dialect/snowflake.rs

@@ -721,6 +721,11 @@ impl Dialect for SnowflakeDialect {
     fn supports_comma_separated_trim(&self) -> bool {
         true
     }
+
+    /// See <https://docs.snowflake.com/en/sql-reference/sql/alter-table>
+    fn supports_add_column_if_not_exists(&self) -> bool {
+        true
+    }
 }
 
 // Peeks ahead to identify tokens that are expected after

src/parser/mod.rs

@@ -10818,8 +10818,7 @@ impl<'a> Parser<'a> {
                 } else {
                     let column_keyword = self.parse_keyword(Keyword::COLUMN);
 
-                    let if_not_exists = if dialect_of!(self is PostgreSqlDialect | BigQueryDialect | DuckDbDialect | GenericDialect)
-                    {
+                    let if_not_exists = if self.dialect.supports_add_column_if_not_exists() {
                         self.parse_keywords(&[Keyword::IF, Keyword::NOT, Keyword::EXISTS])
                             || if_not_exists
                     } else {

tests/sqlparser_common.rs

@@ -5453,12 +5453,7 @@ fn parse_alter_table_add_column() {
 
 #[test]
 fn parse_alter_table_add_column_if_not_exists() {
-    let dialects = TestedDialects::new(vec![
-        Box::new(PostgreSqlDialect {}),
-        Box::new(BigQueryDialect {}),
-        Box::new(GenericDialect {}),
-        Box::new(DuckDbDialect {}),
-    ]);
+    let dialects = all_dialects_where(|d| d.supports_add_column_if_not_exists());
 
     match alter_table_op(dialects.verified_stmt("ALTER TABLE tab ADD IF NOT EXISTS foo TEXT")) {
         AlterTableOperation::AddColumn { if_not_exists, .. } => {