11.7 Data Type Default Values

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.

Handling of Explicit Defaults as of MySQL 8.0.13

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, for TIMESTAMP and DATETIME columns, you can specify the CURRENT_TIMESTAMP function as the default, without enclosing parentheses. See Section 11.3.5, “Automatic Initialization and Updating for TIMESTAMP and DATETIME”.

The BLOB, TEXT, GEOMETRY, and JSON 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:

This is permitted (literal default specified as expression):
1. CREATE TABLE t2 (b BLOB DEFAULT ('abc'));
This produces an error (literal default not specified as expression):
1. 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.

Literals, built-in functions (both deterministic and nondeterministic), and operators are permitted.
Subqueries, parameters, variables, stored functions, and user-defined functions are not permitted.
An expression default value cannot depend on a column that has the AUTO_INCREMENT attribute.
An expression default value for one column can refer to other table columns, with the exception that references to generated columns or columns with expression default values must be to columns that occur earlier in the table definition. That is, expression default values cannot contain forward references to generated columns or columns with expression default values.
The ordering constraint also applies to the use of ALTER TABLE to reorder table columns. If the resulting table would have an expression default value that contains a forward reference to a generated column or column with an expression default value, the statement fails.

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 and CREATE 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 as INSERT, UPDATE, and ALTER TABLE.

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 of DEFAULT(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, an ER_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. See Section 12.2, “Type Conversion in Expression Evaluation”.

Handling of Explicit Defaults Prior to MySQL 8.0.13

With one exception, the default value specified in a DEFAULT clause must be a literal 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, for TIMESTAMP and DATETIME columns, you can specify CURRENT_TIMESTAMP as the default. See Section 11.3.5, “Automatic Initialization and Updating for TIMESTAMP and DATETIME”.

The BLOB, TEXT, GEOMETRY, and JSON data types cannot be assigned a default value.

Handling of Implicit Defaults

If a data type specification 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 a value, MySQL defines the column with no explicit DEFAULT clause. Exception: If the column is defined as part of a PRIMARY KEY but not explicitly as NOT NULL, MySQL creates it as a NOT NULL column (because PRIMARY KEY columns must be NOT NULL).

For data entry into a NOT NULL column that has no explicit DEFAULT clause, 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 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.
If strict mode is not enabled, MySQL sets the column to the implicit default value for the column data type.

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:

See Section 5.1.11, “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:

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. This is also true for TIMESTAMP if the explicit_defaults_for_timestamp system variable is enabled (see Section 5.1.8, “Server System Variables”). Otherwise, for the first TIMESTAMP column in a table, the default value is the current date and time. See Section 11.3, “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.

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Document created the 26/06/2006, last modified the 26/10/2018
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References

Manuel MySQL : https://dev.mysql.com/

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