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In some cases, MySQL silently changes column specifications from
those given in a CREATE TABLE or ALTER TABLE statement:
VARCHAR columns with a length less than four are changed to
CHAR.
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 (VARCHAR, TEXT, or BLOB),
all CHAR columns longer than three characters are changed to
VARCHAR columns. This doesn't 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 section MySQL Storage Engines and Table Types.
From MySQL 4.1.0 on, a CHAR or 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 TEXT, and
VARCHAR(200000) is converted to MEDIUMTEXT.
This is a compatibility feature.
TIMESTAMP display sizes are discarded from MySQL 4.1 on, due
to changes made to the TIMESTAMP column 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.
You cannot store a literal NULL in a TIMESTAMP column; setting
it to NULL sets it to the current date and time. Because
TIMESTAMP columns behave this way, the NULL and NOT NULL
attributes do not apply in the normal way and are ignored if you specify
them. DESCRIBE tbl_name always reports that a TIMESTAMP
column can be assigned NULL values.
Columns that are part of a PRIMARY KEY are made NOT NULL even if
not declared that way.
Starting from MySQL 3.23.51, trailing spaces are automatically deleted from
ENUM and SET values when the table is created.
MySQL maps certain column types used by other SQL database vendors to MySQL types. See section Using Column Types from Other Database Engines.
If you include a USING clause to specify an index type that is not
legal for a storage engine, but there is another index type available that
the engine can use without affecting query results, the engine will use the
available type.
To see whether MySQL used a column type other
than the one you specified, issue a DESCRIBE or SHOW
CREATE TABLE statement after creating or altering your table.
Certain other column type changes can occur if you compress a table
using myisampack. See section Compressed Table Characteristics.
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