SQLite 3.27

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CREATE TABLE projects(
  "project_number" TEXT,
  "project_manager" TEXT,
  main_project_number TEXT
);

✓
insert into projects ("project_manager", "project_number")
values ("Bob", "11204568-001"), ("Bill", "11204568-002"), ("Jack", "11204568-003"), ("Fred", "11204569"), ("Nancy", "11204569-003");
✓
SELECT * FROM projects


project_number
project_manager
main_project_number


11204568-001
Bob
null

11204568-002
Bill
null

11204568-003
Jack
null

11204569
Fred
null

11204569-003
Nancy
null


update projects 
set main_project_number = 
    CASE WHEN instr("project_number", "-") > 0 THEN substr("project_number", 0, instr("project_number", "-"))
         ELSE "project_number"
    END
✓
SELECT * FROM projects


project_number
project_manager
main_project_number


11204568-001
Bob
11204568

11204568-002
Bill
11204568

11204568-003
Jack
11204568

11204569
Fred
11204569

11204569-003
Nancy
11204569

project_number	project_manager	main_project_number
11204568-001	Bob	null
11204568-002	Bill	null
11204568-003	Jack	null
11204569	Fred	null
11204569-003	Nancy	null

project_number	project_manager	main_project_number
11204568-001	Bob	11204568
11204568-002	Bill	11204568
11204568-003	Jack	11204568
11204569	Fred	11204569
11204569-003	Nancy	11204569