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原文地址:https://dev.mysql.com/doc/refman/5.6/en/timestamp-initialization.html
As of MySQL 5.6.5, TIMESTAMP and DATETIME columns can be automatically initializated and updated to the current date and time (that is, the current timestamp). Before 5.6.5, this is true only for TIMESTAMP, and for at most one TIMESTAMP column per table. The following notes first describe automatic initialization and updating for MySQL 5.6.5 and up, then the differences for versions preceding 5.6.5.
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:
An auto-initialized column is set to the current timestamp for inserted rows that specify no value for the column.
An auto-updated column is automatically updated to the current timestamp when the value of any other column in the row is changed from its current value. An auto-updated column remains unchanged if all other columns are set to their current values. To prevent an auto-updated column from updating when other columns change, explicitly set it to its current value. To update an auto-updated column even when other columns do not change, explicitly set it to the value it should have (for example, set it to CURRENT_TIMESTAMP).
In addition, you can initialize or update any TIMESTAMP column to the current date and time by assigning it a NULLvalue, 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_TIMESTAMPclauses 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 asCURRENT_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 andDATETIME. 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‘.
The following examples use DEFAULT 0, a default that can produce warnings or errors depending on whether strict SQL mode or the NO_ZERO_DATE SQL mode is enabled. Be aware that the TRADITIONAL SQL mode includes strict mode and NO_ZERO_DATE. See Section 5.1.7, “Server SQL Modes”.
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:
With both DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP, the column has the current timestamp for its default value and is automatically updated to the current timestamp.
CREATE TABLE t1 ( ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, dt DATETIME DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP );
With a DEFAULT clause but no ON UPDATE CURRENT_TIMESTAMP clause, the column has the given default value and is not automatically updated to the current timestamp.
The default depends on whether the DEFAULT clause specifies CURRENT_TIMESTAMP or a constant value. WithCURRENT_TIMESTAMP, the default is the current timestamp.
CREATE TABLE t1 ( ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP, dt DATETIME DEFAULT CURRENT_TIMESTAMP );
With a constant, the default is the given value. In this case, the column has no automatic properties at all.
CREATE TABLE t1 ( ts TIMESTAMP DEFAULT 0, dt DATETIME DEFAULT 0 );
With an ON UPDATE CURRENT_TIMESTAMP clause and a constant DEFAULT clause, the column is automatically updated to the current timestamp and has the given constant default value.
CREATE TABLE t1 ( ts TIMESTAMP DEFAULT 0 ON UPDATE CURRENT_TIMESTAMP, dt DATETIME DEFAULT 0 ON UPDATE CURRENT_TIMESTAMP );
With an ON UPDATE CURRENT_TIMESTAMP clause but no DEFAULT clause, the column is automatically updated to the current timestamp but does not have the current timestamp for its default value.
The default in this case is type dependent. TIMESTAMP has a default of 0 unless defined with the NULL attribute, in which case the default is NULL.
CREATE TABLE t1 ( ts1 TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, -- default 0 ts2 TIMESTAMP NULL ON UPDATE CURRENT_TIMESTAMP -- default NULL );
DATETIME has a default of NULL unless defined with the NOT NULL attribute, in which case the default is 0.
CREATE TABLE t1 ( dt1 DATETIME ON UPDATE CURRENT_TIMESTAMP, -- default NULL dt2 DATETIME NOT NULL ON UPDATE CURRENT_TIMESTAMP -- default 0 );
TIMESTAMP and DATETIME columns have no automatic properties unless they are specified explicitly, with this exception: By default, 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 TIMESTAMPcolumn, use one of these strategies:
Enable the explicit_defaults_for_timestamp system variable. If this variable is enabled, the DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP clauses that specify automatic initialization and updating are available, but are not assigned to any TIMESTAMP column unless explicitly included in the column definition.
Alternatively, if explicit_defaults_for_timestamp is disabled (the default), do either of the following:
Define the column with a DEFAULT clause that specifies a constant default value.
Specify the NULL attribute. This also causes the column to permit NULL values, which means that you cannot assign the current timestamp by setting the column to NULL. Assigning NULL sets the column to NULL.
Consider these table definitions:
CREATE TABLE t1 (
ts1 TIMESTAMP DEFAULT 0,
ts2 TIMESTAMP DEFAULT CURRENT_TIMESTAMP
ON UPDATE CURRENT_TIMESTAMP);
CREATE TABLE t2 (
ts1 TIMESTAMP NULL,
ts2 TIMESTAMP DEFAULT CURRENT_TIMESTAMP
ON UPDATE CURRENT_TIMESTAMP);
CREATE TABLE t3 (
ts1 TIMESTAMP NULL DEFAULT 0,
ts2 TIMESTAMP DEFAULT CURRENT_TIMESTAMP
ON UPDATE CURRENT_TIMESTAMP);
The tables have these properties:
In each table definition, the first TIMESTAMP column has no automatic initialization or updating.
The tables differ in how the ts1 column handles NULL values. For t1, ts1 is NOT NULL and assigning it a value ofNULL sets it to the current timestamp. For t2 and t3, ts1 permits NULL and assigning it a value of NULL sets it toNULL.
t2 and t3 differ in the default value for ts1. For t2, ts1 is defined to permit NULL, so the default is also NULL in the absence of an explicit DEFAULT clause. For t3, ts1 permits NULL but has an explicit default of 0.
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:
CREATE TABLE t1 ( ts TIMESTAMP(6) DEFAULT CURRENT_TIMESTAMP(6) ON UPDATE CURRENT_TIMESTAMP(6) );
This is not permitted:
CREATE TABLE t1 ( ts TIMESTAMP(6) DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP(3) );
Before MySQL 5.6.5, support for automatic initialization and updating is more limited:
DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP cannot be used with DATETIME columns.
DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP can be used with at most one TIMESTAMPcolumn per table. 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 choose whether to use these properties and which TIMESTAMP column should have them. It need not be the first one in a table that is automatically initialized or updated to the current timestamp. To specify automatic initialization or updating for a different TIMESTAMP column, you must suppress the automatic properties for the first one, as previously described. Then, for the other TIMESTAMP column, the rules for the DEFAULT and ON UPDATE clauses are the same as for the first TIMESTAMP column, except that if you omit both clauses, no automatic initialization or updating occurs.
By default, TIMESTAMP columns 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 TIMESTAMPcolumn not declared with the NULL attribute, DEFAULT NULL is invalid.) If a TIMESTAMP column permits NULLvalues, assigning NULL sets it to NULL, not to the current timestamp.
The following table contains several TIMESTAMP columns that permit NULL values:
CREATE TABLE t ( ts1 TIMESTAMP NULL DEFAULT NULL, ts2 TIMESTAMP NULL DEFAULT 0, ts3 TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP );
A TIMESTAMP column that permits NULL values does not take on the current timestamp at insert time except under one of the following conditions:
Its default value is defined as CURRENT_TIMESTAMP and no value is specified for the column
CURRENT_TIMESTAMP or any of its synonyms such as NOW() is explicitly inserted into the column
In other words, a TIMESTAMP column defined to permit NULL values auto-initializes only if its definition includesDEFAULT CURRENT_TIMESTAMP:
CREATE TABLE t (ts TIMESTAMP NULL 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:
CREATE TABLE t1 (ts TIMESTAMP NULL DEFAULT ‘0000-00-00 00:00:00‘); CREATE TABLE t2 (ts TIMESTAMP NULL DEFAULT NULL);
To set the TIMESTAMP column in either table to the current timestamp at insert time, explicitly assign it that value. For example:
INSERT INTO t1 VALUES (NOW()); INSERT INTO t2 VALUES (CURRENT_TIMESTAMP);
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原文地址：http://www.cnblogs.com/davidwang456/p/4747069.html
