用alter table語句：

完整用法：
ALTER [IGNORE] TABLE tbl_name

alter_specification [, alter_specification] ...

alter_specification:

table_option ...
| ADD [COLUMN] column_definition [FIRST | AFTER col_name ]
| ADD [COLUMN] (column_definition,...)
| ADD {INDEX|KEY} [index_name] [index_type] (index_col_name,...)
| ADD [CONSTRAINT [symbol]]

PRIMARY KEY [index_type] (index_col_name,...)
| ADD [CONSTRAINT [symbol]]

UNIQUE [INDEX|KEY] [index_name] [index_type] (index_col_name,...)
| ADD [FULLTEXT|SPATIAL] [INDEX|KEY] [index_name] (index_col_name,...)
| ADD [CONSTRAINT [symbol]]

FOREIGN KEY [index_name] (index_col_name,...)

[reference_definition]
| ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
| CHANGE [COLUMN] old_col_name column_definition

[FIRST|AFTER col_name]
| MODIFY [COLUMN] column_definition [FIRST | AFTER col_name]
| DROP [COLUMN] col_name
| DROP PRIMARY KEY
| DROP {INDEX|KEY} index_name
| DROP FOREIGN KEY fk_symbol
| DISABLE KEYS
| ENABLE KEYS
| RENAME [TO] new_tbl_name
| ORDER BY col_name [, col_name] ...
| CONVERT TO CHARACTER SET charset_name [COLLATE collation_name]
| [DEFAULT] CHARACTER SET charset_name [COLLATE collation_name]
| DISCARD TABLESPACE
| IMPORT TABLESPACE

index_col_name:

col_name [(length)] [ASC | DESC]

index_type:

USING {BTREE | HASH}

刪除 主鍵 和外鍵約束:

ALTER TABLE mytablename
DROP PRIMARY KEY
DROP FOREIGN KEY fk_symbol;

還有就是怎么樣查詢 約束;

你可以通過查看該表的 創建語句來 查看 約束：

SHOW CREATE TABLE mytablename;

得到ddl語句；