Saturday, January 7, 2012

MySQL Data Type Default Values When Creating Tables

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.
As of MySQL 5.0.2, 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. This is the same as before 5.0.2.
If the column cannot take NULL as the 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 aNOT NULL column (because PRIMARY KEY columns must be NOT NULL), but also assigns it a DEFAULT clause using the implicit default value. To prevent this, include an explicit NOT NULL in the definition of any PRIMARY KEYcolumn.
For data entry for 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 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:
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:
For a given table, you can use the SHOW CREATE TABLE statement to see which columns have an explicitDEFAULT clause.
SERIAL DEFAULT VALUE in the definition of an integer column is an alias for NOT NULL AUTO_INCREMENT UNIQUE.

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