fix(structure): emit NULL keyword for nullable MySQL timestamp columns

crates/dbx-core/src/table_structure_sql/column_format.rs

@@ -26,6 +26,12 @@ pub(super) fn column_definition(dialect: StructureDialect, column: &EditableStru
     }
     if !column.is_nullable && !is_oracle_like(dialect) && dialect != StructureDialect::ClickHouse {
         parts.push("NOT NULL".to_string());
+    } else if column.is_nullable
+        && dialect == StructureDialect::Mysql
+        && mysql_generated_clause.is_none()
+        && is_mysql_timestamp_type(&column.data_type)
+    {
+        parts.push("NULL".to_string());
     }
     if mysql_generated_clause.is_none() {
         if let Some(extra_clause) = column_extra_clause(dialect, column) {
@@ -438,3 +444,18 @@ pub(super) fn is_mysql_character_data_type(data_type: &str) -> bool {
     let normalized = base_type.split_whitespace().collect::<Vec<_>>().join(" ").to_ascii_lowercase();
     matches!(normalized.as_str(), "char" | "varchar" | "tinytext" | "text" | "mediumtext" | "longtext" | "enum" | "set")
 }
+
+/// MySQL silently rewrites a nullable `TIMESTAMP` column to `NOT NULL` when the
+/// generated DDL omits an explicit `NULL` keyword — regardless of whether a
+/// `DEFAULT` is present. With the (still common) `explicit_defaults_for_timestamp`
+/// server default of `OFF`, this can outright fail with `ERROR 1067 (42000):
+/// Invalid default value` for any non-first TIMESTAMP column. `DATETIME` is not
+/// affected and must not be touched.
+pub(super) fn is_mysql_timestamp_type(data_type: &str) -> bool {
+    let trimmed = data_type.trim();
+    let base_type = match trimmed.find('(') {
+        Some(open_index) => trimmed[..open_index].trim(),
+        None => trimmed,
+    };
+    base_type.eq_ignore_ascii_case("timestamp")
+}

crates/dbx-core/src/table_structure_sql/create_table.rs

@@ -1,6 +1,6 @@
 use super::column_format::{
     column_data_type, column_extra_clause, has_dameng_identity, is_dameng_identity_compatible_type,
-    is_mysql_character_data_type,
+    is_mysql_character_data_type, is_mysql_timestamp_type,
 };
 use super::comments::{build_sqlserver_column_comment_sql, build_sqlserver_table_comment_sql};
 use super::dialect::{capabilities_for, database_label, StructureDialect};
@@ -75,6 +75,12 @@ pub fn build_create_table_sql(mut options: TableStructureSqlOptions) -> TableStr
             && !matches!(dialect, StructureDialect::ClickHouse | StructureDialect::ManticoreSearch)
         {
             parts.push("NOT NULL".to_string());
+        } else if column.is_nullable
+            && !column.is_primary_key
+            && dialect == StructureDialect::Mysql
+            && is_mysql_timestamp_type(&column.data_type)
+        {
+            parts.push("NULL".to_string());
         }
         if let Some(extra_clause) = column_extra_clause(dialect, column) {
             parts.push(extra_clause);

crates/dbx-core/src/table_structure_sql/tests.rs

@@ -1746,7 +1746,7 @@ fn mysql_add_timestamp_column_drops_invalid_precision() {
     assert_eq!(result.warnings, Vec::<String>::new());
     assert_eq!(
         result.statements,
-        vec!["ALTER TABLE `users` ADD COLUMN `created_at` timestamp DEFAULT CURRENT_TIMESTAMP;"]
+        vec!["ALTER TABLE `users` ADD COLUMN `created_at` timestamp NULL DEFAULT CURRENT_TIMESTAMP;"]
     );
 }
 
@@ -1774,7 +1774,7 @@ fn mysql_add_timestamp_column_preserves_valid_precision() {
     assert_eq!(result.warnings, Vec::<String>::new());
     assert_eq!(
         result.statements,
-        vec!["ALTER TABLE `users` ADD COLUMN `created_at` timestamp(3) DEFAULT CURRENT_TIMESTAMP(3);"]
+        vec!["ALTER TABLE `users` ADD COLUMN `created_at` timestamp(3) NULL DEFAULT CURRENT_TIMESTAMP(3);"]
     );
 }
 
@@ -6551,3 +6551,120 @@ fn gaussdb_m_create_table_with_primary_key() {
     let sql = result.statements.join("\n");
     assert!(sql.contains("PRIMARY KEY (`id`)"));
 }
+
+#[test]
+fn mysql_create_table_nullable_timestamp_without_default_gets_explicit_null() {
+    // A second (or later) MySQL TIMESTAMP column that is nullable but has no
+    // DEFAULT must carry an explicit NULL keyword — otherwise MySQL either
+    // silently rewrites it to NOT NULL or, with the still-common
+    // explicit_defaults_for_timestamp=OFF server default, rejects it outright
+    // with ERROR 1067 (42000): Invalid default value. See issue #7416.
+    let mut created_at = column("created_at");
+    created_at.data_type = "timestamp".to_string();
+    created_at.is_nullable = false;
+
+    let mut updated_at = column("updated_at");
+    updated_at.data_type = "timestamp".to_string();
+    updated_at.is_nullable = true;
+
+    let result = build_create_table_sql(TableStructureSqlOptions {
+        database_type: Some(DatabaseType::Mysql),
+        schema: None,
+        table_name: "u7_game_order_step".to_string(),
+        columns: vec![created_at, updated_at],
+        indexes: Vec::new(),
+        foreign_keys: Vec::new(),
+        triggers: Vec::new(),
+        table_comment: None,
+        original_table_comment: None,
+        mysql_engine: None,
+        partitioned: false,
+        is_gaussdb_m_mode: false,
+    });
+
+    assert_eq!(result.warnings, Vec::<String>::new());
+    assert!(result.statements[0].contains("`updated_at` timestamp NULL"));
+    assert!(!result.statements[0].contains("`updated_at` timestamp NULL DEFAULT"));
+}
+
+#[test]
+fn mysql_create_table_nullable_timestamp_with_default_still_gets_explicit_null() {
+    // Even with an explicit DEFAULT, MySQL still needs the NULL keyword to
+    // keep the column nullable — omitting it silently produces NOT NULL.
+    let mut updated_at = column("updated_at");
+    updated_at.data_type = "timestamp".to_string();
+    updated_at.is_nullable = true;
+    updated_at.default_value = "CURRENT_TIMESTAMP".to_string();
+
+    let result = build_create_table_sql(TableStructureSqlOptions {
+        database_type: Some(DatabaseType::Mysql),
+        schema: None,
+        table_name: "events".to_string(),
+        columns: vec![updated_at],
+        indexes: Vec::new(),
+        foreign_keys: Vec::new(),
+        triggers: Vec::new(),
+        table_comment: None,
+        original_table_comment: None,
+        mysql_engine: None,
+        partitioned: false,
+        is_gaussdb_m_mode: false,
+    });
+
+    assert_eq!(result.warnings, Vec::<String>::new());
+    assert!(result.statements[0].contains("`updated_at` timestamp NULL DEFAULT CURRENT_TIMESTAMP"));
+}
+
+#[test]
+fn mysql_create_table_nullable_datetime_does_not_gain_null_keyword() {
+    // DATETIME is not subject to MySQL's TIMESTAMP-specific implicit-default
+    // quirk; the fix must not touch it.
+    let mut updated_at = column("updated_at");
+    updated_at.data_type = "datetime".to_string();
+    updated_at.is_nullable = true;
+
+    let result = build_create_table_sql(TableStructureSqlOptions {
+        database_type: Some(DatabaseType::Mysql),
+        schema: None,
+        table_name: "events".to_string(),
+        columns: vec![updated_at],
+        indexes: Vec::new(),
+        foreign_keys: Vec::new(),
+        triggers: Vec::new(),
+        table_comment: None,
+        original_table_comment: None,
+        mysql_engine: None,
+        partitioned: false,
+        is_gaussdb_m_mode: false,
+    });
+
+    assert_eq!(result.warnings, Vec::<String>::new());
+    assert!(!result.statements[0].contains("NULL"));
+}
+
+#[test]
+fn mysql_add_column_nullable_timestamp_without_default_gets_explicit_null() {
+    // The ADD COLUMN / MODIFY COLUMN / CHANGE COLUMN path shares
+    // column_definition() with CREATE TABLE and must carry the same fix.
+    let mut updated_at = column("updated_at");
+    updated_at.data_type = "timestamp".to_string();
+    updated_at.is_nullable = true;
+
+    let result = build_table_structure_change_sql(TableStructureSqlOptions {
+        database_type: Some(DatabaseType::Mysql),
+        schema: None,
+        table_name: "u7_game_order_step".to_string(),
+        columns: vec![updated_at],
+        indexes: Vec::new(),
+        foreign_keys: Vec::new(),
+        triggers: Vec::new(),
+        table_comment: None,
+        original_table_comment: None,
+        mysql_engine: None,
+        partitioned: false,
+        is_gaussdb_m_mode: false,
+    });
+
+    assert_eq!(result.warnings, Vec::<String>::new());
+    assert_eq!(result.statements, vec!["ALTER TABLE `u7_game_order_step` ADD COLUMN `updated_at` timestamp NULL;"]);
+}