diff --git a/src/frontend/src/handler/create_source.rs b/src/frontend/src/handler/create_source.rs
index 3616997d384cb..b62eab3dd55f8 100644
--- a/src/frontend/src/handler/create_source.rs
+++ b/src/frontend/src/handler/create_source.rs
@@ -676,6 +676,38 @@ pub fn handle_addition_columns(
     Ok(())
 }
 
+/// Merge the column defs from external and user's column definition in `CREATE` SQL.
+pub(crate) fn merge_columns_defs(
+    cols_from_source: Vec<ColumnCatalog>,
+    cols_from_sql: Vec<ColumnCatalog>,
+    col_defs_from_sql: &[ColumnDef],
+    wildcard_idx: Option<usize>,
+) -> Result<Vec<ColumnCatalog>> {
+    if cols_from_sql.is_empty() {
+        Ok(cols_from_source)
+    } else if let Some(wildcard_idx) = wildcard_idx {
+        if col_defs_from_sql.iter().any(|c| !c.is_generated()) {
+            Err(RwError::from(NotSupported(
+                "Only generated columns are allowed in user-defined schema from SQL".to_string(),
+                "Remove the non-generated columns".to_string(),
+            )))
+        } else {
+            // Replace `*` with `cols_from_source`
+            let mut cols_from_sql = cols_from_sql;
+            let mut cols_from_source = cols_from_source;
+            let mut cols_from_sql_r = cols_from_sql.split_off(wildcard_idx);
+            cols_from_sql.append(&mut cols_from_source);
+            cols_from_sql.append(&mut cols_from_sql_r);
+            Ok(cols_from_sql)
+        }
+    } else {
+        Err(RwError::from(ProtocolError(
+            "User-defined schema from SQL is not allowed here, since RisingWave has got the column definitions from the external system. \
+            You can try to use `CREATE TABLE/SOURCE obj_name (*, [generated column defs])`\
+            Please refer to https://www.risingwave.dev/docs/current/sql-create-source/ for more information.".to_string())))
+    }
+}
+
 /// Bind columns from both source and sql defined.
 pub(crate) fn bind_all_columns(
     source_schema: &ConnectorSchema,
@@ -685,30 +717,12 @@ pub(crate) fn bind_all_columns(
     wildcard_idx: Option<usize>,
 ) -> Result<Vec<ColumnCatalog>> {
     if let Some(cols_from_source) = cols_from_source {
-        if cols_from_sql.is_empty() {
-            Ok(cols_from_source)
-        } else if let Some(wildcard_idx) = wildcard_idx {
-            if col_defs_from_sql.iter().any(|c| !c.is_generated()) {
-                Err(RwError::from(NotSupported(
-                    "Only generated columns are allowed in user-defined schema from SQL"
-                        .to_string(),
-                    "Remove the non-generated columns".to_string(),
-                )))
-            } else {
-                // Replace `*` with `cols_from_source`
-                let mut cols_from_sql = cols_from_sql;
-                let mut cols_from_source = cols_from_source;
-                let mut cols_from_sql_r = cols_from_sql.split_off(wildcard_idx);
-                cols_from_sql.append(&mut cols_from_source);
-                cols_from_sql.append(&mut cols_from_sql_r);
-                Ok(cols_from_sql)
-            }
-        } else {
-            // TODO(yuhao): https://github.com/risingwavelabs/risingwave/issues/12209
-            Err(RwError::from(ProtocolError(
-                    format!("User-defined schema from SQL is not allowed with FORMAT {} ENCODE {}. \
-                    Please refer to https://www.risingwave.dev/docs/current/sql-create-source/ for more information.", source_schema.format, source_schema.row_encode))))
-        }
+        merge_columns_defs(
+            cols_from_source,
+            cols_from_sql,
+            col_defs_from_sql,
+            wildcard_idx,
+        )
     } else {
         if wildcard_idx.is_some() {
             return Err(RwError::from(NotSupported(