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.
TIMESTAMP display sizes are discarded.
Also note that TIMESTAMP columns are
NOT NULL by default.
Columns that are part of a PRIMARY KEY
are made NOT NULL even if not declared
that way.
Trailing spaces are automatically deleted from
ENUM and SET member
values when the table is created.
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
available type.
If strict SQL mode is not enabled, a
VARCHAR column with a length
specification greater than 65535 is converted to
TEXT, and a VARBINARY
column with a length specification greater than 65535 is
converted to BLOB. Otherwise, an error
occurs in either of these cases.
Specifying the CHARACTER SET binary
attribute for a character data type causes the column to be
created as the corresponding binary data type:
CHAR becomes BINARY,
VARCHAR becomes
VARBINARY, and TEXT
becomes BLOB. For the
ENUM and 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
);
To see whether MySQL used a data type other than the one you
specified, issue a DESCRIBE or SHOW
CREATE TABLE statement after creating or altering the
table.
Certain other data type changes can occur if you compress a table using myisampack. See Section 13.4.3.3, “Compressed Table Characteristics”.

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