Question

I basically created some tables to play around with: I have Two main tables, and a Many-Many join table. Here is the DDL: (I am using HSQLDB)

CREATE TABLE PERSON (     PERSON_ID INTEGER GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,      NAME VARCHAR(50), MAIN_PERSON_ID INTEGER )  CREATE TABLE JOB (     JOB_ID INTEGER GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,      NAME VARCHAR(50) ) CREATE TABLE JOB_PERSON (     PERSON_ID INTEGER,      JOB_ID INTEGER ) ALTER TABLE JOB_PERSON ADD      CONSTRAINT FK_PERSON_JOB FOREIGN KEY(PERSON_ID)      REFERENCES PERSON ON DELETE CASCADE ON UPDATE CASCADE   ALTER TABLE JOB_PERSON ADD     CONSTRAINT FK_JOB_PERSON FOREIGN KEY(JOB_ID)      REFERENCES JOB ON DELETE CASCADE ON UPDATE CASCADE  ALTER TABLE PERSON ADD     CONSTRAINT FK_PERSON_PERSON FOREIGN KEY(MAIN_PERSON_ID)      REFERENCES PERSON ON DELETE CASCADE ON UPDATE CASCADE  insert into person values(null,'Arthur', null); insert into person values(null,'James',0); insert into job values(null, 'Programmer') insert into job values(null, 'Manager') insert into job_person values(0,0); insert into job_person values(0,1); insert into job_person values(1,1); 

I want to create a delete statement that deletes orphans from JOB (if there exists only one entry in the join table for a specific job) based on the PERSON.PERSON_ID.

In pseudo language:

delete from job where job_person.job_id=job.job_id  AND count(job_person.job_id)=1 AND job_person.person_id=X 

Where X is some person_id. I have tried a lot of different ways; I think it is the ‘COUNT’ part that is causing problems. I am an SQL rookie, so any help would be much appreciated.

Answer 1

I’m not following.

You cannot delete JOB rows which have JOB_PERSON rows (even one) because of your FK contraints. Thus there is no way to delete JOB rows based on PERSON rows.

JOB_PERSON rows have to be deleted before either a JOB or PERSON can be deleted.

If you want to delete all JOB rows with no JOB_PERSON, then one way is:

DELETE FROM JOB WHERE JOB_ID NOT IN (     SELECT JOB_ID     FROM JOB_PERSON ) 

If you want to delete all JOB_PERSON rows for a particular person and then all orphans, do it in two steps:

DELETE FROM JOB_PERSON WHERE PERSON_ID = X  DELETE FROM JOB WHERE JOB_ID NOT IN (     SELECT JOB_ID     FROM JOB_PERSON ) 

If you want to delete only the orphan JOBs previously linked to X, you will need to hold those in a temporary table before the first delete.

INSERT INTO TEMP_TABLE SELECT JOB.JOB_ID FROM JOB INNER JOIN JOB_PERSON     ON JOB_PERSON.JOB_ID = JOB.JOB_ID WHERE JOB_PERSON.PERSON_ID = X  DELETE FROM PERSON WHERE PERSON_ID = X  -- YOUR CASCADING DELETE DOES THIS: /* DELETE FROM JOB_PERSON WHERE PERSON_ID = X */  -- Now clean up (only) new orphans on the other side DELETE FROM JOB WHERE JOB_ID NOT IN (     SELECT JOB_ID     FROM JOB_PERSON ) AND JOB_ID IN (     SELECT JOB_ID     FROM TEMP_TABLE )