MySQL explicit_defaults_for_timestamp

默认情况下，timestamp类型字段所在数据行被更新时，该字段会自动更新为当前时间，而参数explicit_defaults_for_timestamp控制这一种行为。
explicit_defaults_for_timestamp=off，数据行更新时，timestamp类型字段更新为当前时间；
explicit_defaults_for_timestamp=on，数据行更新时，timestamp类型字段不更新为当前时间。

例如：

mysql> set explicit_defaults_for_timestamp=off;
Query OK, 0 rows affected (0.00 sec)

mysql> create table sam (id int,col1 timestamp);
Query OK, 0 rows affected (0.02 sec)

mysql> insert into sam(id) values (1);
Query OK, 1 row affected (0.01 sec)

mysql> select * from sam;
+------+---------------------+
| id   | col1                |
+------+---------------------+
|    1 | 2019-05-14 14:22:23 |
+------+---------------------+
1 row in set (0.00 sec)

mysql> set explicit_defaults_for_timestamp=on;
Query OK, 0 rows affected (0.00 sec)

mysql> create table tom(id int,col1 timestamp);
Query OK, 0 rows affected (0.01 sec)

mysql> insert into tom(id) values (1);
Query OK, 1 row affected (0.00 sec)

mysql> select * from tom;
+------+------+
| id   | col1 |
+------+------+
|    1 | NULL |
+------+------+
1 row in set (0.00 sec)

两表的表结构差异：

mysql> show create table sam\G
*************************** 1. row ***************************
       Table: sam
Create Table: CREATE TABLE `sam` (
  `id` int(11) DEFAULT NULL,
  `col1` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
1 row in set (0.00 sec)

mysql> 
mysql> 
mysql> show create table tom\G
*************************** 1. row ***************************
       Table: tom
Create Table: CREATE TABLE `tom` (
  `id` int(11) DEFAULT NULL,
  `col1` timestamp NULL DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
1 row in set (0.00 sec)