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MySQL 5.7 Reference Manual  /  ...  /  Silent Column Specification Changes

13.1.18.7 Silent Column Specification Changes

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.

All changes are subject to the internal row-size limit of 65,535 bytes, which may cause some attempts at data type changes to fail. See Section C.10.4, “Limits on Table Column Count and Row Size”.

  • 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 11.10, “Using Data Types from Other Database Engines”.

  • If you include a USING clause to specify an index type that is not permitted 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 15.2.3.3, “Compressed Table Characteristics”.


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