[sql] How to delete rows in tables that contain foreign keys to other tables

Suppose there is a main table containing a primary key and there is another table which contains a foreign key to this main table. So if we delete the row of main table it will delete the child table also.

How do I write this query?

From your question, I think it is safe to assume you have CASCADING DELETES turned on.
All that is needed in that case is

DELETE FROM MainTable
WHERE PrimaryKey = ???

You database engine will take care of deleting the corresponding referencing records.

If you have multiply rows to delete and you don't want to alter the structure of your tables you can use cursor. 1-You first need to select rows to delete(in a cursor) 2-Then for each row in the cursor you delete the referencing rows and after that delete the row him self.

Ex:

--id is primary key of MainTable
    declare @id int
    set @id = 1
    declare theMain cursor for select FK from MainTable where MainID = @id
    declare @fk_Id int
    open theMain
    fetch next from theMain into @fk_Id
    while @@fetch_status=0
    begin
        --fkid is the foreign key 
        --Must delete from Main Table first then child.
        delete from MainTable where fkid = @fk_Id
        delete from ReferencingTable where fkid = @fk_Id
        fetch next from theMain into @fk_Id
    end
    close theMain
    deallocate theMain

hope is useful

Need to set the foreign key option as on delete cascade... in tables which contains foreign key columns.... It need to set at the time of table creation or add later using ALTER table

First, as a one-time data-scrubbing exercise, delete the orphaned rows e.g.

DELETE 
  FROM ReferencingTable 
 WHERE NOT EXISTS (
                   SELECT * 
                     FROM MainTable AS T1
                    WHERE T1.pk_col_1 = ReferencingTable.pk_col_1
                  );

Second, as a one-time schema-alteration exercise, add the ON DELETE CASCADE referential action to the foreign key on the referencing table e.g.

ALTER TABLE ReferencingTable DROP 
   CONSTRAINT fk__ReferencingTable__MainTable;

ALTER TABLE ReferencingTable ADD 
   CONSTRAINT fk__ReferencingTable__MainTable 
      FOREIGN KEY (pk_col_1)
      REFERENCES MainTable (pk_col_1)
      ON DELETE CASCADE;

Then, forevermore, rows in the referencing tables will automatically be deleted when their referenced row is deleted.

You can alter a foreign key constraint with delete cascade option as shown below. This will delete chind table rows related to master table rows when deleted.

ALTER TABLE MasterTable
ADD CONSTRAINT fk_xyz 
FOREIGN KEY (xyz) 
REFERENCES ChildTable (xyz) ON DELETE CASCADE