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11.3.1.1. TIMESTAMP Properties as of MySQL 4.1

Note: In older versions of MySQL (prior to 4.1), the properties of the TIMESTAMP data type differed significantly in many ways from what is described in this section. If you need to convert older TIMESTAMP data to work with MySQL 5.1, be sure to see the MySQL 3.23, 4.0, 4.1 Reference Manual for details.

TIMESTAMP columns are displayed in the same format as DATETIME columns. In other words, the display width is fixed at 19 characters, and the format is YYYY-MM-DD HH:MM:SS.

The MySQL server can be also be run with the MAXDB SQL mode enabled. When the server runs with this mode enabled, TIMESTAMP is identical with DATETIME. That is, if this mode is enabled at the time that a table is created, TIMESTAMP columns are created as DATETIME columns. As a result, such columns use DATETIME display format, have the same range of values, and there is no automatic initialization or updating to the current date and time.

To enable MAXDB mode, set the server SQL mode to MAXDB at startup using the --sql-mode=MAXDB server option or by setting the global sql_mode variable at runtime:

mysql> SET GLOBAL sql_mode=MAXDB;

A client can cause the server to run in MAXDB mode for its own connection as follows:

mysql> SET SESSION sql_mode=MAXDB;

Note that the information in the following discussion applies to TIMESTAMP columns only for tables not created with MAXDB mode enabled, because such columns are created as DATETIME columns.

MySQL does not accept timestamp values that include a zero in the day or month column or values that are not a valid date. The sole exception to this rule is the special value '0000-00-00 00:00:00'.

You have considerable flexibility in determining when automatic TIMESTAMP initialization and updating occur and which column should have those behaviors:

  • For one TIMESTAMP column in a table, you can assign the current timestamp as the default value and the auto-update value. It is possible to have the current timestamp be the default value for initializing the column, for the auto-update value, or both. It is not possible to have the current timestamp be the default value for one column and the auto-update value for another column.

  • You can specify which TIMESTAMP column to automatically initialize or update to the current date and time. This need not be the first TIMESTAMP column.

The following rules govern initialization and updating of TIMESTAMP columns:

  • If a DEFAULT value is specified for the first TIMESTAMP column in a table, it is not ignored. The default can be CURRENT_TIMESTAMP or a constant date and time value.

  • DEFAULT NULL is the same as DEFAULT CURRENT_TIMESTAMP for the first TIMESTAMP column. For any other TIMESTAMP column, DEFAULT NULL is treated as DEFAULT 0.

  • Any single TIMESTAMP column in a table can be used as the one that is initialized to the current timestamp or updated automatically.

  • In a CREATE TABLE statement, the first TIMESTAMP column can be declared in any of the following ways:

    • With both DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP clauses, the column has the current timestamp for its default value, and is automatically updated.

    • With neither DEFAULT nor ON UPDATE clauses, it is the same as DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP.

    • With a DEFAULT CURRENT_TIMESTAMP clause and no ON UPDATE clause, the column has the current timestamp for its default value but is not automatically updated.

    • With no DEFAULT clause and with an ON UPDATE CURRENT_TIMESTAMP clause, the column has a default of 0 and is automatically updated.

    • With a constant DEFAULT value, the column has the given default. If the column has an ON UPDATE CURRENT_TIMESTAMP clause, it is automatically updated, otherwise not.

    In other words, you can use the current timestamp for both the initial value and the auto-update value, or either one, or neither. (For example, you can specify ON UPDATE to enable auto-update without also having the column auto-initialized.)

  • Any of CURRENT_TIMESTAMP, CURRENT_TIMESTAMP(), or NOW() can be used in the DEFAULT and ON UPDATE clauses. They all mean “the current timestamp.

    The order of the DEFAULT and ON UPDATE attributes does not matter. If both DEFAULT and ON UPDATE are specified for a TIMESTAMP column, either can precede the other. For example, these statements are equivalent:

    CREATE TABLE t (ts TIMESTAMP);
    CREATE TABLE t (ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP
                                 ON UPDATE CURRENT_TIMESTAMP);
    CREATE TABLE t (ts TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
                                 DEFAULT CURRENT_TIMESTAMP);
    
  • To specify automatic default or updating for a TIMESTAMP column other than the first one, you must suppress the automatic initialization and update behaviors for the first TIMESTAMP column by explicitly assigning it a constant DEFAULT value (for example, DEFAULT 0 or DEFAULT '2003-01-01 00:00:00'). Then, for the other TIMESTAMP column, the rules are the same as for the first TIMESTAMP column, except that if you omit both of the DEFAULT and ON UPDATE clauses, no automatic initialization or updating occurs.

    Example. These statements are equivalent:

    CREATE TABLE t (
        ts1 TIMESTAMP DEFAULT 0,
        ts2 TIMESTAMP DEFAULT CURRENT_TIMESTAMP
                      ON UPDATE CURRENT_TIMESTAMP);
    CREATE TABLE t (
        ts1 TIMESTAMP DEFAULT 0,
        ts2 TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
                      DEFAULT CURRENT_TIMESTAMP);
    

You can set the current time zone on a per-connection basis, as described in Section 5.10.8, “MySQL Server Time Zone Support”. TIMESTAMP values are stored in UTC, being converted from the current time zone for storage, and converted back to the current time zone upon retrieval. As long as the time zone setting remains constant, you get back the same value you store. If you store a TIMESTAMP value, and then change the time zone and retrieve the value, the retrieved value is different than the value you stored. This occurs because the same time zone was not used for conversion in both directions. The current time zone is available as the value of the time_zone system variable.

You can include the NULL attribute in the definition of a TIMESTAMP column to allow the column to contain NULL values. For example:

CREATE TABLE t
(
  ts1 TIMESTAMP NULL DEFAULT NULL,
  ts2 TIMESTAMP NULL DEFAULT 0,
  ts3 TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP
);

If the NULL attribute is not specified, setting the column to NULL sets it to the current timestamp. Note that a TIMESTAMP column which allows NULL values will not take on the current timestamp except under one of the following conditions:

  • Its default value is defined as CURRENT_TIMESTAMP

  • NOW() or CURRENT_TIMESTAMP is inserted into the column

In other words, a TIMESTAMP column defined as NULL will auto-update only if it is created using a definition such as the following:

CREATE TABLE t (ts TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP);

Otherwise — that is, if the TIMESTAMP column is defined to allow NULL values but not using DEFAULT TIMESTAMP, as shown here…

CREATE TABLE t1 (ts TIMESTAMP NULL DEFAULT NULL);
CREATE TABLE t2 (ts TIMESTAMP NULL DEFAULT '0000-00-00 00:00:00');

…then you must explicitly insert a value corresponding to the current date and time. For example:

INSERT INTO t1 VALUES (NOW());
INSERT INTO t2 VALUES (CURRENT_TIMESTAMP);

 
 
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