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 areNOT NULL
by default.Columns that are part of a
PRIMARY KEY
are madeNOT NULL
even if not declared that way.Trailing spaces are automatically deleted from
ENUM
andSET
member values when the table is created.MySQL maps certain data types used by other SQL database vendors to MySQL types. See Section 10.8, “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 toTEXT
, and aVARBINARY
column with a length specification greater than 65535 is converted toBLOB
. 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
becomesBINARY
,VARCHAR
becomesVARBINARY
, andTEXT
becomesBLOB
. For theENUM
andSET
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 13.5.3.3, “Compressed Table Characteristics”.