Question

I have two tables in the database:

First Table: Students

Field           Type          Null    Key   Default
id              int(11)       NO      PRI   NULL
class_id        int(11)       NO      PRI   NULL
admission_id    int(11)       NO      PRI   NULL
school_id       int(11)       NO      PRI   NULL
name            varchar(64)   YES           NULL
admin_date      datetime      YES           NULL

Second Table: Admission

Field           Type          Null    Key   Default
id              int(11)       NO      PRI   NULL
class_id        int(11)       NO      PRI   1
school_id       int(11)       NO      PRI   NULL
name            varchar(64)   YES           NULL
admin_date      datetime      YES           NULL

Now I want to fill in the class_id column of Admission table which I introduced later into the table. By default all the class_id are set to 1 and hence need to be updated according to student.
What I was doing :

   Update Admission
   set class_id = (select class_id from Students where Students.school_id = Admission.school_id);

and i get an error message : ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails

Can you help me in place where I’m going wrong or any other alternative solution to fill in column.
Thanks in advance

Answer 1

Try this ::

Update Admission
inner join Students on (Students.school_id = Admission.school_id)
   set Admission.class_id = Students.class_id