Use of MYSQL Foreign Key -- modify (ADD) Foreign Key method, Foreign Key names cannot have repeated

Use of MYSQL Foreign Key -- modify (ADD) Foreign Key method, Foreign Key names cannot have repeated 1. caseNumber in case_detail created in wms today cannot be associated with case_head. An error is prompted several times after the result is created, which takes a long time. Keyword: mysql createtable errno 121 150 error table creation error foreign key 2. the statement for adding a foreign key is [SQL] ALTER TABLE 'setting _ group' ADD CONSTRAINT 'location _ ibfk_2 'FOREIGN KEY ('id ') REFERENCES 'setting _ action' ('id') on delete cascade on update cascade; Note:. the foreign key name cannot be important; B. if the primary key name is incorrect, an error is reported. 3. When modifying the data, you must first clear the data and add it. When data exists internally, the primary key cannot be modified or added.