Default Values

The DEFAULT value clause in a data type specification indicates a default value for a column.

You can set default values for all data types. Typically, default values must be constants and cannot be functions or expressions, but there are some exceptions:

• For time types, you can use NOW, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP functions as default values for TIMESTAMP and DATETIME columns.
• For integer types, you can use the NEXT VALUE FOR function to set the next value of a sequence as the default value for a column, and use the RAND() function to generate a random floating-point value as the default value for a column.
• For string types, you can use the UUID() function to generate a universally unique identifier (UUID) as the default value for a column.
• For binary types, you can use the UUID_TO_BIN() function to convert a UUID to the binary format and set the converted value as the default value for a column.
• Starting from v8.0.0, TiDB additionally supports specifying the default values for BLOB, TEXT, and JSON data types, but you can only use expressions to set the default values for them.

If a column definition includes no explicit DEFAULT value, TiDB 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, TiDB defines the column with no explicit DEFAULT clause.

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, TiDB 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.
• If strict mode is not enabled, TiDB sets the column to the implicit default value for the column data type.

Implicit defaults are defined as follows:

• For numeric types, the default is 0. If 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 TIMESTAMP, the default value is the current date and time.
• For string types other than ENUM, the default value is the empty string. For ENUM, the default is the first enumeration value.

Specify expressions as default values

Starting from 8.0.13, MySQL supports specifying expressions as default values in the DEFAULT clause. For more information, see Explicit default handling as of MySQL 8.0.13.

Starting from v8.0.0, TiDB additionally supports specifying the following expressions as default values in the DEFAULT clause.

• UPPER(SUBSTRING_INDEX(USER(), '@', 1))
• REPLACE(UPPER(UUID()), '-', '')
• DATE_FORMAT expressions in the following formats:
  • DATE_FORMAT(NOW(), '%Y-%m')
  • DATE_FORMAT(NOW(), '%Y-%m-%d')
  • DATE_FORMAT(NOW(), '%Y-%m-%d %H.%i.%s')
  • DATE_FORMAT(NOW(), '%Y-%m-%d %H:%i:%s')
• STR_TO_DATE('1980-01-01', '%Y-%m-%d')

Starting from v8.0.0, TiDB additionally supports assigning default values to BLOB, TEXT, and JSON data types. However, you can only use expressions to set the default values for these data types. The following is an example of BLOB:

CREATE TABLE t2 (b BLOB DEFAULT (RAND()));