In some cases, MySQL silently changes column specifications from
those given in a
CREATE TABLE or
ALTER TABLE statement. These
might be changes to a data type, to attributes associated with a
data type, or to an index specification.
All changes are subject to the internal row-size limit of 65,535 bytes, which may cause some attempts at data type changes to fail. See Section D.7.4, “Limits on Table Column Count and Row Size”.
Some silent column specification changes include modifications to attribute or index specifications:
TIMESTAMP display sizes are
Also note that
NOT NULL by default.
Columns that are part of a
NOT NULL even if not declared
MySQL maps certain data types used by other SQL database vendors to MySQL types. See Section 11.9, “Using Data Types from Other Database Engines”.
If you include a
USING clause to specify
an index type that is not legal for a given storage engine,
but there is another index type available that the engine
can use without affecting query results, the engine uses the
Possible data type changes are given in the following list. If a version number is given, the change occurs only up to the versions listed. After that, an error occurs if a column cannot be created using the specified data type.
Before MySQL 5.0.3, if any column in a table has a variable
length, the entire row becomes variable-length as a result.
Therefore, if a table contains any variable-length columns
CHAR columns longer than
three characters are changed to
VARCHAR columns. This does
not affect how you use the columns in any way; in MySQL,
VARCHAR is just a different
way to store characters. MySQL performs this conversion
because it saves space and makes table operations faster.
See Chapter 14, Storage Engines.
Before MySQL 5.0.3, a
VARCHAR column with a length
specification greater than 255 is converted to the smallest
TEXT type that can hold
values of the given length. For example,
VARCHAR(500) is converted to
VARCHAR(200000) is converted to
conversions occur for
VARBINARY, except that
they are converted to a
Note that these conversions result in a change in behavior with regard to treatment of trailing spaces.
As of MySQL 5.0.3, a
BINARY column with a length
specification greater than 255 is not silently converted.
Instead, an error occurs. From MySQL 5.0.6 on, silent
VARBINARY columns with a
length specification greater than 65535 does not occur if
strict SQL mode is enabled. Instead, an error occurs.
Before MySQL 5.0.10, for a specification of
M is not larger than
D, it is adjusted upward. For
DECIMAL(11,10). As of MySQL 5.0.10,
DECIMAL(10,10) is created as specified.
CHARACTER SET binary
attribute for a character data type causes the column to be
created as the corresponding binary data type:
BLOB. For the
SET data types, this does not
occur; they are created as declared. Suppose that you
specify a table using this definition:
CREATE TABLE t ( c1 VARCHAR(10) CHARACTER SET binary, c2 TEXT CHARACTER SET binary, c3 ENUM('a','b','c') CHARACTER SET binary );
The resulting table has this definition:
CREATE TABLE t ( c1 VARBINARY(10), c2 BLOB, c3 ENUM('a','b','c') CHARACTER SET binary );
Certain other data type changes can occur if you compress a table using myisampack. See Section 18.104.22.168, “Compressed Table Characteristics”.