11.3.4 Automatic Initialization and Updating for TIMESTAMP and DATETIME

TIMESTAMP and DATETIME columns can be automatically initializated and updated to the current date and time (that is, the current timestamp).

For any TIMESTAMP or DATETIME column in a table, you can assign the current timestamp as the default value, the auto-update value, or both:

In addition, if the explicit_defaults_for_timestamp system variable is disabled, you can initialize or update any TIMESTAMP (but not DATETIME) column to the current date and time by assigning it a NULL value, unless it has been defined with the NULL attribute to permit NULL values.

To specify automatic properties, use the DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP clauses in column definitions. The order of the clauses does not matter. If both are present in a column definition, either can occur first. Any of the synonyms for CURRENT_TIMESTAMP have the same meaning as CURRENT_TIMESTAMP. These are CURRENT_TIMESTAMP(), NOW(), LOCALTIME, LOCALTIME(), LOCALTIMESTAMP, and LOCALTIMESTAMP().

Use of DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP is specific to TIMESTAMP and DATETIME. The DEFAULT clause also can be used to specify a constant (nonautomatic) default value (for example, DEFAULT 0 or DEFAULT '2000-01-01 00:00:00').

TIMESTAMP or DATETIME column definitions can specify the current timestamp for both the default and auto-update values, for one but not the other, or for neither. Different columns can have different combinations of automatic properties. The following rules describe the possibilities:

TIMESTAMP and DATETIME columns have no automatic properties unless they are specified explicitly, with this exception: If the explicit_defaults_for_timestamp system variable is disabled, the first TIMESTAMP column has both DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP if neither is specified explicitly. To suppress automatic properties for the first TIMESTAMP column, use one of these strategies:

Consider these table definitions:

The tables have these properties:

If a TIMESTAMP or DATETIME column definition includes an explicit fractional seconds precision value anywhere, the same value must be used throughout the column definition. This is permitted:

This is not permitted:

TIMESTAMP Initialization and the NULL Attribute

If the explicit_defaults_for_timestamp system variable is disabled, TIMESTAMP columns by default are NOT NULL, cannot contain NULL values, and assigning NULL assigns the current timestamp. To permit a TIMESTAMP column to contain NULL, explicitly declare it with the NULL attribute. In this case, the default value also becomes NULL unless overridden with a DEFAULT clause that specifies a different default value. DEFAULT NULL can be used to explicitly specify NULL as the default value. (For a TIMESTAMP column not declared with the NULL attribute, DEFAULT NULL is invalid.) If a TIMESTAMP column permits NULL values, assigning NULL sets it to NULL, not to the current timestamp.

The following table contains several TIMESTAMP columns that permit NULL values:

A TIMESTAMP column that permits NULL values does not take on the current timestamp at insert time except under one of the following conditions:

In other words, a TIMESTAMP column defined to permit NULL values auto-initializes only if its definition includes DEFAULT CURRENT_TIMESTAMP:

If the TIMESTAMP column permits NULL values but its definition does not include DEFAULT CURRENT_TIMESTAMP, you must explicitly insert a value corresponding to the current date and time. Suppose that tables t1 and t2 have these definitions:

To set the TIMESTAMP column in either table to the current timestamp at insert time, explicitly assign it that value. For example:

If the explicit_defaults_for_timestamp system variable is enabled, TIMESTAMP columns permit NULL values only if declared with the NULL attribute. Also, TIMESTAMP columns do not permit assigning NULL to assign the current timestamp, whether declared with the NULL or NOT NULL attribute. To assign the current timestamp, set the column to CURRENT_TIMESTAMP or a synonym such as NOW().