mysql配置文件--explicit_defaults_for_timestamp

In MySQL, the TIMESTAMP data type differs in nonstandard ways from other data types: mysql中TIMESTAMP类型和其他的类型有点不一样(在没有设置explicit_defaults_for_timestamp=1的情况下）

• TIMESTAMP columns not explicitly declared with the NULL attribute are assigned the NOT NULL attribute. (Columns of other data types, if not explicitly declared as NOT NULL, permit NULL values.) Setting such a column to NULL sets it to the current timestamp.在默认情况下，如果TIMESTAMP列没有显式的指明null属性，那么该列会被自动加上not null属性(而其他类型的列如果没有被显式的指定not null，那么是允许null值的)，如果往这个列中插入null值，会自动的设置该列的值为current timestamp值

• The first TIMESTAMP column in a table, if not declared with the NULL attribute or an explicit DEFAULT or ON UPDATE clause, is automatically assigned the DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP attributes.表中的第一个TIMESTAMP列，如果没有指定null属性或者没有指定默认值，也没有指定ON UPDATE语句。那么该列会自动被加上DEFAULT CURRENT_TIMESTAMP和ON UPDATE CURRENT_TIMESTAMP属性。

• TIMESTAMP columns following the first one, if not declared with the NULL attribute or an explicit DEFAULT clause, are automatically assigned DEFAULT '0000-00-00 00:00:00' (the “zero” timestamp). For inserted rows that specify no explicit value for such a column, the column is assigned '0000-00-00 00:00:00' and no warning occurs.第一个TIMESTAMP列之后的其他的TIMESTAMP类型的列，如果没有指定null属性，也没有指定默认值，那么该列会被自动加上DEFAULT '0000-00-00 00:00:00'属性。如果insert语句中没有为该列指定值，那么该列中插入'0000-00-00 00:00:00'，并且没有warning

Those nonstandard behaviors remain the default for TIMESTAMP but as of MySQL 5.6.6 are deprecated and this warning appears at startup:在5.6.6及以后的版本中，如果在配置文件中没有指定explicit_defaults_for_timestamp参数，启动时error日志中会报如下错误

[Warning] TIMESTAMP with implicit DEFAULT value is deprecated.
Please use --explicit_defaults_for_timestamp server option (see
documentation for more details).

As indicated by the warning, to turn off the nonstandard behaviors, enable the explicit_defaults_for_timestamp system variable at server startup. With this variable enabled, the server handles TIMESTAMP as follows instead:如果我们在启动的时候在配置文件中指定了explicit_defaults_for_timestamp=1，mysql会按照如下的方式处理TIMESTAMP 列

• TIMESTAMP columns not explicitly declared as NOT NULL permit NULL values. Setting such a column to NULL sets it to NULL, not the current timestamp. 此时如果TIMESTAMP列没有显式的指定not null属性，那么默认的该列可以为null，此时向该列中插入null值时，会直接记录null，而不是current timestamp。

• No TIMESTAMP column is assigned the DEFAULT CURRENT_TIMESTAMP or ON UPDATE CURRENT_TIMESTAMP attributes automatically. Those attributes must be explicitly specified. 不会自动的为表中的第一个TIMESTAMP列加上DEFAULT CURRENT_TIMESTAMP 和ON UPDATE CURRENT_TIMESTAMP属性，除非你在建表的时候显式的指明

• TIMESTAMP columns declared as NOT NULL and without an explicit DEFAULT clause are treated as having no default value. For inserted rows that specify no explicit value for such a column, the result depends on the SQL mode. If strict SQL mode is enabled, an error occurs. If strict SQL mode is not enabled, the column is assigned the implicit default of '0000-00-00 00:00:00' and a warning occurs. This is similar to how MySQL treats other temporal types such as DATETIME.如果TIMESTAMP列被加上了not null属性，并且没有指定默认值。这时如果向表中插入记录，但是没有给该TIMESTAMP列指定值的时候，如果strict sql_mode被指定了，那么会直接报错。如果strict sql_mode没有被指定，那么会向该列中插入'0000-00-00 00:00:00'并且产生一个warning

Note

explicit_defaults_for_timestamp is itself deprecated because its only purpose is to permit control over now-deprecated TIMESTAMP behaviors that will be removed in a future MySQL release. When that removal occurs,explicit_defaults_for_timestamp will have no purpose and will be removed as well.  >>explicit_defaults_for_timestamp参数主要用来控制TIMESTAMP数据类型跟其他数据类型不一致的特性，但是TIMESTAMP的这个特性在将来会被废弃，所以explicit_defaults_for_timestamp参数也会在将来被废弃