1.查看所有约束

SELECT 
    CONSTRAINT_NAME, 
    TABLE_NAME, 
    CONSTRAINT_TYPE 
FROM 
    information_schema.TABLE_CONSTRAINTS 
WHERE 
    TABLE_NAME = '表名字';

2.删除约束

ALTER TABLE bus_qualification DROP INDEX unique_item_name;

3.添加约束

ALTER TABLE bus_qualification ADD UNIQUE KEY unique_item_name_company (item_name, item_company);

4.查看具体约束

SELECT 
    COLUMN_NAME, 
    CONSTRAINT_NAME 
FROM 
    information_schema.KEY_COLUMN_USAGE 
WHERE 
    TABLE_NAME = '表名';