根据表中的值将MySQL行拆分为多行

Question

I need to split my MySQL table rows into multiple rows based on the number of students applying for the course with certain course code.

basically, I need to separate the poorly designed MySQL table into rows so each student ends up in its designated row generated with the relevant course_code and unique id (id_new). Notice that the total number of studens in course1_students and course2_students will obviously be equal to the number of rows in the needed table.

Appreciate any help!

Answer 1

To split each row into two rows, get two copies of each row in the FROM via cross join of the poorly designed table with a two-row constant table. To get courseX_student rows for each row of that table in the FROM, cross join with a table of integers >= 0 on course_students. Insert the following rows into a table with auto_incremented id_new. (I'll assume that a courseX_students value of 0 produces no output rows. Because you haven't given the exact meaning of the tables.)

select id,name,1 as course_students,
    case which
    when 1 then course1_code
    when 2 then course2_code
    end as course_code
from numbers
cross join (select 1 as which union select 2) w
cross join poorly
where n <
    case which
    when 1 then course1_students
    when 2 then course2_students
    end

Answer 2

CREATE TABLE my_new_table AS
SELECT id
     , name
     , course1_students course_students
     , course1_code course_code 
  FROM my_table
 UNION
SELECT id
     , name
     , course2_students
     , course2_code 
  FROM my_table;

 ALTER TABLE my_new_table ADD PRIMARY KEY(id,course_code);

id_new is redundant (and name probably belongs in a separate table)