The DEFAULT
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
value
NOW()
or
CURRENT_DATE
. The exception is
that you can specify
CURRENT_TIMESTAMP
as the default
for a TIMESTAMP
column. See
Section 10.3.1.1, “TIMESTAMP
Properties”.
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, or an
UPDATE
statement sets the column
to NULL
, 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 nontransactional 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 5.1.8, “Server SQL Modes”.
For a given table, you can use the SHOW
CREATE TABLE
statement to see which columns have an
explicit DEFAULT
clause.
Implicit defaults are defined as follows:
For numeric types, the default is 0
, with
the exception that for integer or floating-point types
declared with the AUTO_INCREMENT
attribute, the default is the next value in the sequence.
For date and time types other than
TIMESTAMP
, the default is the
appropriate “zero” value for the type. For the
first TIMESTAMP
column in a
table, the default value is the current date and time. See
Section 10.3, “Date and Time Types”.
For string types other than
ENUM
, the default value is
the empty string. For ENUM
,
the default is the first enumeration value.
SERIAL DEFAULT VALUE
in the definition of an
integer column is an alias for NOT NULL AUTO_INCREMENT
UNIQUE
.
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