13.6 Data Type Default Values (original) (raw)

Data type specifications can have explicit or implicit default values.

A DEFAULT _`value`_ clause in a data type specification explicitly indicates a default value for a column. Examples:

CREATE TABLE t1 (
  i     INT DEFAULT -1,
  c     VARCHAR(10) DEFAULT '',
  price DOUBLE(16,2) DEFAULT 0.00
);

SERIAL DEFAULT VALUE is a special case. In the definition of an integer column, it is an alias for NOT NULL AUTO_INCREMENT UNIQUE.

Some aspects of explicit DEFAULT clause handling are version dependent, as described following.

Explicit Default Handling

The default value specified in a DEFAULT clause can be a literal constant or an expression. With one exception, enclose expression default values within parentheses to distinguish them from literal constant default values. Examples:

CREATE TABLE t1 (
  -- literal defaults
  i INT         DEFAULT 0,
  c VARCHAR(10) DEFAULT '',
  -- expression defaults
  f FLOAT       DEFAULT (RAND() * RAND()),
  b BINARY(16)  DEFAULT (UUID_TO_BIN(UUID())),
  d DATE        DEFAULT (CURRENT_DATE + INTERVAL 1 YEAR),
  p POINT       DEFAULT (Point(0,0)),
  j JSON        DEFAULT (JSON_ARRAY())
);

The exception is that, forTIMESTAMP andDATETIME columns, you can specify the CURRENT_TIMESTAMP function as the default, without enclosing parentheses. SeeSection 13.2.5, “Automatic Initialization and Updating for TIMESTAMP and DATETIME”.

The BLOB,TEXT,GEOMETRY, andJSON data types can be assigned a default value only if the value is written as an expression, even if the expression value is a literal:

CREATE TABLE t2 (b BLOB DEFAULT ('abc'));  
CREATE TABLE t2 (b BLOB DEFAULT 'abc');

Expression default values must adhere to the following rules. An error occurs if an expression contains disallowed constructs.

Note

If any component of an expression default value depends on the SQL mode, different results may occur for different uses of the table unless the SQL mode is the same during all uses.

For CREATE TABLE ... LIKE andCREATE TABLE ... SELECT, the destination table preserves expression default values from the original table.

If an expression default value refers to a nondeterministic function, any statement that causes the expression to be evaluated is unsafe for statement-based replication. This includes statements such asINSERT andUPDATE. In this situation, if binary logging is disabled, the statement is executed as normal. If binary logging is enabled andbinlog_format is set toSTATEMENT, the statement is logged and executed but a warning message is written to the error log, because replicas might diverge. Whenbinlog_format is set toMIXED or ROW, the statement is executed as normal.

When inserting a new row, the default value for a column with an expression default can be inserted either by omitting the column name or by specifying the column as DEFAULT (just as for columns with literal defaults):

mysql> CREATE TABLE t4 (uid BINARY(16) DEFAULT (UUID_TO_BIN(UUID())));
mysql> INSERT INTO t4 () VALUES();
mysql> INSERT INTO t4 () VALUES(DEFAULT);
mysql> SELECT BIN_TO_UUID(uid) AS uid FROM t4;
+--------------------------------------+
| uid                                  |
+--------------------------------------+
| f1109174-94c9-11e8-971d-3bf1095aa633 |
| f110cf9a-94c9-11e8-971d-3bf1095aa633 |
+--------------------------------------+

However, the use ofDEFAULT(col_name) to specify the default value for a named column is permitted only for columns that have a literal default value, not for columns that have an expression default value.

Not all storage engines permit expression default values. For those that do not, anER_UNSUPPORTED_ACTION_ON_DEFAULT_VAL_GENERATED error occurs.

If a default value evaluates to a data type that differs from the declared column type, implicit coercion to the declared type occurs according to the usual MySQL type-conversion rules. SeeSection 14.3, “Type Conversion in Expression Evaluation”.

Implicit Default Handling

If a data type specification includes no explicitDEFAULT 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 a value, MySQL defines the column with no explicitDEFAULT clause.

For data entry into a NOT NULL column that has no explicit DEFAULT clause, if anINSERT orREPLACE statement includes no value for the column, or anUPDATE statement sets the column to NULL, MySQL handles the column according to the SQL mode in effect at the time:

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 7.1.11, “Server SQL Modes”.

For a given table, the SHOW CREATE TABLE statement displays which columns have an explicit DEFAULT clause.

Implicit defaults are defined as follows: