How to delete master detail records using one sql? - sql

Question

Using Delphi 7 and interbase 7
Is it possible to delete a master detail record and all of its nested detail records in one SQL statement?
Example:
Table1
ID - Integer
TITLE - Varchar(80)
Table2
ID - Integer
Table1_ID - Integer
TITLE - Varchar(80)
Table3
ID - Integer
Table2_ID - Integer
TITLE - Varchar(80)
I would like to delete ID 10 from Table1, and all of its matching records (Table1_ID) in table 2, and all its matching records (Table2_ID) in table 3
If i can't do this in one sql, how do i do it in multiple sqls (correct sequence to call statements)?

Answer 1

Yo can do it witn some SQL's deleteing in order, records on Table3, table2 and table1. All in one transaction to do it like "unique operation".
One alternative is use Triggers for delete record related on table2 when you delete one record on table1, and equivalent in table2 to delete related records on table3.
Another (if DB let you) is use ON CASCADE DELETE (or similar) to delete related records on a tabla2 and table3, when you delete a record un table1 (see help or documentation on your SGBD/Database).
Excuse-me for mistakes with English. It's not my natural language.
Regards.

Answer 2

You can use foreign keys to cascade deletes to child records when the parent record is deleted.
The following command should create the foreign key to delete records in Table2 (Table1_ID) when the record in Table1 (ID) is deleted.
ALTER TABLE TABLE2 ADD CONSTRAINT FK_TABLE2_TABLE1
FOREIGN KEY (Table1_ID) REFERENCES TABLE1 (ID)
ON DELETE CASCADE;
This way the database engine takes care of deletes in child tables so all you need to do is
delete from TABLE1 where ID = :ID_VALUE
Regards,

Answer 3

DELETE FROM Table3 WHERE Table2_ID IN (SELECT ID FROM Table_2 WHERE Table1_ID=xxxx)
DELETE FROM Table2 WHERE Table1_ID=xxxx
DELETE FROM Table1 WHERE ID=xxxx

Answer 4

You cannot do that in a single statement but instead you can use transactions in Delphi make sure that either all or none of the statements are executed.
If you are using BDE, then drop TDatabase component and set the default properties and write the following code.
try
Database1.StartTransaction;
//Execute first query
//Execute second query
//Execute third query
Database1.Commit;
except on E: Exception do
Database1.Rollback;
end;
If you are using ADO then use ADOConnection1.BeginTrans; ADOConnection1.CommitTrans; ADOConnection1.RollbackTrans; statements

Answer 5

If you create foreign key references with cascade option, deleting the parent record will also delete all details (unless other restrictions prevent this).
SQL:
ALTER TABLE Table2 ADD CONSTRAINT Table2_Table1_ID
FOREIGN KEY(Table1_ID) REFERENCES Table1(ID) ON DELETE CASCADE
This solution does not require the cooperation of the client applications, the server will keep the data model consistent.

Answer 6

In addition to the answers already given with multiple SQL statements, foreign key relationships and triggers. In InterBase you can also write a stored procedure to delete the master and detail records. Then you will only need one SQL statement in your program.
Below two kind of stored procedures you could use in this situation.
The first one is almost the same as HeartWave answer, but then in a stored procedure.
CREATE PROCEDURE DELETEMASTERDETAIL_WITHOUTINFO(
pMasterID INTEGER)
RETURNS (
rResult INTEGER)
AS
declare variable vTable2ID integer;
begin
/* don't return information about deleted records */
rResult = 0;
for select id
from table2
where table1_id = :pMasterID
into :vTable2ID do
begin
delete from table3
where table2_id = :vTable2ID;
end
delete from table2
where table1_id = :pMasterID;
delete from table1
where id = :pMasterID;
rResult = rResult + 1;
suspend;
end
The SQL statement to call this stored procedure is:
select rresult
from deletemasterdetail_withoutinfo(:pMasterID)
The second one will return information about the amount of deleted records per table. I don't know if you need it, but perhaps it is helpfull for someone else. If the ID field in Table1 is the primary key the first for select statement is a bit overkill.
CREATE PROCEDURE DELETEMASTERDETAIL_WITHINFO(
pMasterID INTEGER)
RETURNS (
rTable1Deleted INTEGER,
rTable2Deleted INTEGER,
rTable3Deleted INTEGER)
AS
declare variable vTable1ID integer;
declare variable vTable2ID integer;
declare variable vTable3ID integer;
begin
/* return information about deleted records */
rTable1Deleted = 0;
rTable2Deleted = 0;
rTable3Deleted = 0;
for select id
from table1
where id = :pMasterID
into :vTable1ID do
begin
for select id
from table2
where table1_id = :vTable1ID
into :vTable2ID do
begin
for select id
from table3
where table2_id = :vTable2ID
into :vTable3ID do
begin
rTable3Deleted = rTable3Deleted + 1;
delete from table3
where id = :vTable3ID;
end
rTable2Deleted = rTable2Deleted + 1;
delete from table2
where id = :vTable2ID;
end
rTable1Deleted = rTable1Deleted + 1;
delete from table1
where id = :vTable1ID;
end
suspend;
end
The SQL statement to call this stored procedure is:
select rtable1deleted, rtable2deleted, rtable3deleted
from deletemasterdetail_withinfo(:pMasterID)
BTW. I almost always use at least one return parameter in SP's. This will allow to use a Query component to call the stored procedure.
If there are no result parameters a Stored Procedure component must be used to execute the SP.