6.5.3.1 Silent Column Specification Changes

In some cases, MySQL silently changes a column specification from that given in a CREATE TABLE statement. (This may also occur with ALTER TABLE.):

• 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 is 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 7 MySQL Table Types.
• From version 4.1.0, if a CHAR or VARCHAR field with a length specification greater than 255 is converted to TEXT. This is a compatibility feature.
• 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 may be assigned NULL values.
• MySQL maps certain column types used by other SQL database vendors to MySQL types. See section 6.2.5 Using Column Types from Other Database Engines.

If you want to see whether MySQL used a column type other than the one you specified, issue a DESCRIBE tbl_name statement after creating or altering your table.

Certain other column type changes may occur if you compress a table using myisampack. See section 7.1.2.3 Compressed Table Characteristics.

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