How to refer primary key as Foreign to various table

Arunvairavan V :

I have a table called User Which contain user_id as primary key I have other tables like Doctor ,patient,staff ...etc which contain column called created by which reffer to user_id of user table.

Here is my issue I cannot use same constrain name across different table for example

ALTER TABLE `emr@default`.`staff` 
ADD CONSTRAINT `FK_created_by`
  FOREIGN KEY (`created_by`)
  REFERENCES `emr@default`.`users` (`user_id`)
  ON DELETE CASCADE;

is created successfully

when I try with other table

ALTER TABLE `emr@default`.patient 
ADD CONSTRAINT `FK_created_by`
  FOREIGN KEY (`created_by`)
  REFERENCES `emr@default`.`users` (`user_id`)
  ON DELETE CASCADE;

generating error Error Code: 1022. Can't write; duplicate key in table '#sql-1_7a64'

if I try

ALTER TABLE `emr@default`.patient 
ADD CONSTRAINT `FK_created_by1`
  FOREIGN KEY (`created_by`)
  REFERENCES `emr@default`.`users` (`user_id`)
  ON DELETE CASCADE;

is creating fk

my Doubt is can we use same constrain name.if not can I create different constrain name that will not effect anythin

Gordon Linoff :

Constraint names can have conflicts between tables. That is why I include both the table and column names in the constraint name:

ALTER TABLE `emr@default`.patient 
ADD CONSTRAINT `FK_patient_created_by`
  FOREIGN KEY (`created_by`)
  REFERENCES `emr@default`.`users` (`user_id`)
  ON DELETE CASCADE;

ALTER TABLE `emr@default`.`staff` 
ADD CONSTRAINT `FK_staff_created_by`
  FOREIGN KEY (`created_by`)
  REFERENCES `emr@default`.`users` (`user_id`)
  ON DELETE CASCADE;

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