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.
Possible data type changes are given in the following list.
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 13, Storage Engines.
From MySQL 4.1.0 onward, 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.
From MySQL 4.1.2 on, specifying the
binary attribute for a character data type causes
the column to be created as the corresponding binary data
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 );
For a specification of
M is not larger than
D, it is adjusted upward. For
Other silent column specification changes include modifications to attribute or index specifications:
TIMESTAMP display sizes are
discarded from MySQL 4.1 on, due to changes made to the
TIMESTAMP data type in that
version. Before MySQL 4.1,
TIMESTAMP display sizes must
be even and in the range from 2 to 14. If you specify a
display size of 0 or greater than 14, the size is coerced to
14. Odd-valued sizes in the range from 1 to 13 are coerced
to the next higher even number.
Also note that, in MySQL 4.1 and later,
TIMESTAMP columns are
NOT NULL by default.
Before MySQL 4.1.6, you cannot store a literal
NULL in a
TIMESTAMP column; setting it
NULL sets it to the current date and
columns behave this way, the
NOT NULL attributes do not apply in the
normal way and are ignored if you specify them.
TIMESTAMP column can
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 10.7, “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
Certain other data type changes can occur if you compress a table using myisampack. See Section 188.8.131.52, “Compressed Table Characteristics”.