MySQL Reference Manual :: 13.1.5.1 Silent Column Specification Changes

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 Chapter 14, MySQL Storage Engines and Table Types.

From MySQL 4.1.0 to MySQL 5.0.2, 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. Note that this conversion results in a change in behavior with regard to treatment of trailing spaces.

Similar conversions occur for BINARY and VARBINARY, except that they are converted to a BLOB type.

From MySQL 5.0.3 on, CHAR and BINARY columns with a length specification greater than 255 are not silently converted. Instead, an error occurs. From MySQL 5.0.6 on, silent conversion of VARCHAR and VARBINARY columns with a length specification greater than 65,535 does not occur if strict SQL mode is enabled. Instead, an error occurs.

For a specification of DECIMAL(M,D), if M is not larger than D, it is adjusted upward. For example, DECIMAL(10,10) becomes DECIMAL(11,10).

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.

Before MySQL 4.1.6, 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 member values when the table is created.

MySQL maps certain column types used by other SQL database vendors to MySQL types. See Section 11.7, “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 uses the available type.