11.1.4. Data Type Default Values
The DEFAULT value
clause in a data type specification indicates a default value
for a column. With one exception, the default value must be a
constant; it cannot be a function or an expression. This means,
for example, that you cannot set the default for a date column
to be the value of a function such as NOW()
or CURRENT_DATE. The exception is that you
can specify CURRENT_TIMESTAMP as the default
for a TIMESTAMP column. See
Section 11.3.1.1, “TIMESTAMP Properties as of MySQL 4.1”.
BLOB and TEXT columns
cannot be assigned a default value.
If a column definition includes no explicit
DEFAULT value, MySQL determines the default
value as follows:
If the column can take NULL as a value, the
column is defined with an explicit DEFAULT
NULL clause.
If the column cannot take NULL as the value,
MySQL defines the column with no explicit
DEFAULT clause. For data entry, if an
INSERT or REPLACE
statement includes no value for the column, MySQL handles the
column according to the SQL mode in effect at the time:
If strict SQL mode is not enabled, MySQL sets the column to
the implicit default value for the column data type.
If strict mode is enabled, an error occurs for transactional
tables and the statement is rolled back. For
non-transactional tables, an error occurs, but if this
happens for the second or subsequent row of a multiple-row
statement, the preceding rows will have been inserted.
Suppose that a table t is defined as follows:
CREATE TABLE t (i INT NOT NULL);
In this case, i has no explicit default, so
in strict mode each of the following statements produce an error
and no row is inserted. When not using strict mode, only the
third statement produces an error; the implicit default is
inserted for the first two statements, but the third fails
because DEFAULT(i) cannot produce a value:
INSERT INTO t VALUES();
INSERT INTO t VALUES(DEFAULT);
INSERT INTO t VALUES(DEFAULT(i));
See Section 5.2.5, “The Server SQL Mode”.
For a given table, you can use the SHOW CREATE
TABLE statement to see which columns have an explicit
DEFAULT clause.
