I am using sqlite3.
I have one "currencies" table, and two tables that reference the currencies table using a foreign key, as follows:
CREATE TABLE currencies (
currency TEXT NOT NULL PRIMARY KEY
);
CREATE TABLE table1 (
currency TEXT NOT NULL PRIMARY KEY,
FOREIGN KEY(currency)
REFERENCES currencies(currency)
);
CREATE TABLE table2 (
currency TEXT NOT NULL PRIMARY KEY,
FOREIGN KEY(currency)
REFERENCES currencies(currency)
);
I would like to make sure that rows in the "currencies" table that are not referenced by any row from "table1" and "table2" will be removed automatically. This should behave like some kind of ref-counted object. When the reference count reaches zero, the relevant row from the "currencies" table should be erased.
What is the "SQL way" to solve this problem?
I am willing to redesign my tables if it could lead to an elegant solution.
I prefer to avoid solutions that require extra work from the application side, or solutions that require periodic cleanup.
Create an AFTER DELETE TRIGGER in each of table1 and table2:
CREATE TRIGGER remove_currencies_1 AFTER DELETE ON table1
BEGIN
DELETE FROM currencies
WHERE currency = OLD.currency
AND NOT EXISTS (SELECT 1 FROM table2 WHERE currency = OLD.currency);
END;
CREATE TRIGGER remove_currencies_2 AFTER DELETE ON table2
BEGIN
DELETE FROM currencies
WHERE currency = OLD.currency
AND NOT EXISTS (SELECT 1 FROM table1 WHERE currency = OLD.currency);
END;
Every time that you delete a row in either table1 or table2, the trigger involved will check the other table if it contains the deleted currency and if it does not contain it, it will be deleted from currencies.
See the demo.
There is no automatic way of doing this. The reverse can be handling using cascading delete foreign key references. The reverse is that when a currency is deleted all related rows are.
You could schedule a job daily running something like:
delete from currencies c
where not exists (select 1 from table1 t1 where t1.currency = c.currency) and
not exists (select 1 from table2 t2 where t2.currency = c.currency);
If you need an automatic way for doing that, then most dbms provide a trigger mechanism. You can create a trigger on update and delete operations that run the folowing query:
you can use a left join for that:
https://www.w3schools.com/sql/sql_join_left.asp
It return a row for all rows from the left table, even if there is no corresponding row in the right table, replacing the rows form the right with null. You can then check a not null right table field for null with is null. This will filter for the rows the have no counterpart in the right table.
For example:
SELECT currencies.currency FROM currencies LEFT JOIN table1 WHERE table1.currency IS NULL
will show the relevant rows for table1.
You can do the same with table two.
This will give you two queries, that shows which rows have no couterpart.
You can then use intersect on the result, so that you have the rows that have not couterpart in either:
SELECT * FROM query1 INTERSECT SELECT * FROM query2
Now you have the list of currencies to be deleted.
You can finish this by using a subqueried delete:
DELETE FROM currencies WHERE currency IN (SELECT ...)
Related
I didn't find a working solution for creating a "lookup column" in a Firebird database.
Here is an example:
Table1: Orders
[OrderID] [CustomerID] [CustomerName]
Table2: Customers
[ID] [Name]
When I run SELECT * FROM ORDERS I want to get OrderID, CustomerID and CustomerName....but CustomerName should automatically be computed by looking for the "CustomerID" in the "ID" column of "Customer" Table, returning the content of the "Name" column.
Firebird has calculated fields (generated always as/computed by), and these allow selecting from other tables (contrary to an earlier version of this answer, which stated that Firebird doesn't support this).
However, I suggest you use a view instead, as I think it performs better (haven't verified this, so I suggest you test this if performance is important).
Use a view
The common way would be to define a base table and an accompanying view that gathers the necessary data at query time. Instead of using the base table, people would query from the view.
create view order_with_customer
as
select orders.id, orders.customer_id, customer.name
from orders
inner join customer on customer.id = orders.customer_id;
Or you could just skip the view and use above join in your own queries.
Alternative: calculated fields
I label this as an alternative and not the main solution, as I think using a view would be the preferable solution.
To use calculated fields, you can use the following syntax (note the double parentheses around the query):
create table orders (
id integer generated by default as identity primary key,
customer_id integer not null references customer(id),
customer_name generated always as ((select name from customer where id = customer_id))
)
Updates to the customer table will be automatically reflected in the orders table.
As far as I'm aware, the performance of this option is less than when using a join (as used in the view example), but you might want to test that for yourself.
FB3+ with function
With Firebird 3, you can also create calculated fields using a trigger, this makes the expression itself shorter.
To do this, create a function that selects from the customer table:
create function lookup_customer_name(customer_id integer)
returns varchar(50)
as
begin
return (select name from customer where id = :customer_id);
end
And then create the table as:
create table orders (
id integer generated by default as identity primary key,
customer_id integer not null references customer(id),
customer_name generated always as (lookup_customer_name(customer_id))
);
Updates to the customer table will be automatically reflected in the orders table. This solution can be relatively slow when selecting a lot of records, as the function will be executed for each row individually, which is a lot less efficient than performing a join.
Alternative: use a trigger
However if you want to update the table at insert (or update) time with information from another table, you could use a trigger.
I'll be using Firebird 3 for my answer, but it should translate - with some minor differences - to earlier versions as well.
So assuming a table customer:
create table customer (
id integer generated by default as identity primary key,
name varchar(50) not null
);
with sample data:
insert into customer(name) values ('name1');
insert into customer(name) values ('name2');
And a table orders:
create table orders (
id integer generated by default as identity primary key,
customer_id integer not null references customer(id),
customer_name varchar(50) not null
)
You then define a trigger:
create trigger orders_bi_bu
active before insert or update
on orders
as
begin
new.customer_name = (select name from customer where id = new.customer_id);
end
Now when we use:
insert into orders(customer_id) values (1);
the result is:
id customer_id customer_name
1 1 name1
Update:
update orders set customer_id = 2 where id = 1;
Result:
id customer_id customer_name
1 2 name2
The downside of a trigger is that updating the name in the customer table will not automatically be reflected in the orders table. You would need to keep track of these dependencies yourself, and create an after update trigger on customer that updates the dependent records, which can lead to update/lock conflicts.
No need here a complex lookup field.
No need to add a persistant Field [CustomerName] on Table1.
As Gordon said, a simple Join is enough :
Select T1.OrderID, T2.ID, T2.Name
From Customers T2
Join Orders T1 On T1.IDOrder = T2.ID
That said, if you want to use lookup Fields (as we do it on a Dataset) with SQL you can use some thing like :
Select T1.OrderID, T2.ID,
( Select T3.YourLookupField From T3 where (T3.ID = T2.ID) )
From Customers T2 Join Orders T1 On T1.IDOrder = T2.ID
Regards.
When the user clicks a delete button in my UI to delete a product I need to do a fast check to see if it is a foreign key in any of the 4 tables (Table1, Table2, Table3, Table4). If it isn't then I can proceed with the delete. If it is in a single one of them I can't delete it.
Some of these tables have thousands of records and I already learned the hard way that using joins is not the best way because the query takes minutes to complete.
I figured union might be the best way but I am wondering if there is a way I can further enhance it. Or even possibly send back which tables it is involved in so I can give the user a descriptive message on why they can't delete the Product.
Here is what I have so far and it is really fast, but returns thousands of 1's when the product exists all over the place. I suppose I can just do a single or default and if not null then don't let them delete.
select 1
from (
select ProductId from Table1
union all
select ProductId from Table2
union all
select ProductId from Table3
union all
select ProductId from Table4
) tbl
where ProductId = 1000
Here is a method with exists and case:
select (case when exists (select 1 from table1 where productId = 1000) then 1
when exists (select 1 from table2 where productId = 1000) then 1
when exists (select 1 from table3 where productId = 1000) then 1
when exists (select 1 from table4 where productId = 1000) then 1
else 0
end)
One way is to not define any "on delete" clauses when creating the foreign keys. Then just go ahead and try to delete the record. If there is no foreign key in any table referencing that record, the delete will succeed and you go about business as normal. If there is a foreign key in any table referencing that record, the delete will fail. Catch the error and try to give the user a meaningful message: "This customer cannot be deleted as they have at least one order still pending."
This way the system itself will perform the check for you, which will assuredly be more efficient than any check you can perform at the SQL level.
I am currently trying to delete from Table A where a corresponding record is not being used in Table B. Table A has Section, SubSection, Code, Text as fields, where the first three are the Primary Key. Table B has ID, Section, SubSection, Code as fields, where all four are the Primary Key. There are more columns, but they are irrelevant to this question...just wanted to point that out before I get questioned on why all columns are part of the Primary Key for Table B. Pretty much Table A is a repository of all possible data that can be assigned to a entity, Table B is where they are assigned. I want to delete all records from table A that are not in use in Table B. I have tried the following with no success:
DELETE FROM Table A
WHERE NOT EXISTS (SELECT * from Table B
WHERE A.section = B.section
AND A.subsection = B.subsection
AND A.code = b.code)
If I do a Select instead of a delete, I get the subset I am looking for, but when I do a delete, I get an error saying that there is a syntax error at Table A. I would use a NOT IN statement, but with multiple columns being part of the Primary Key, I just don't see how that would work. Any help would be greatly appreciated.
In sql server,when using not exists, you need to set an alias for the table to be connected, and in the delete statement, to specify the table to delete rows from.
DELETE a FROM Table_A a
WHERE NOT EXISTS (SELECT * from Table_B b
WHERE a.section = b.section
AND a.subsection = b.subsection
AND a.code = b.code)
Please try :
DELETE FROM Table A
WHERE NOT EXISTS (SELECT 1 from Table B
WHERE A.section = B.section
AND A.subsection = B.subsection
AND A.code = b.code)
1 is just a placeholder, any constant/single non-null column will work.
Try something like this:
delete from Table_A
where (section, subsection, code) not in (select section,
subsection,
code
from Table_B)
Here want to delete rows with a duplicated column's value (Product) which will be then used as a primary key.
The column is of type nvarchar and we don't want to have 2 rows for one product.
The database is a large one with about thousands rows we need to remove.
During the query for all the duplicates, we want to keep the first item and remove the second one as the duplicate.
There is no primary key yet, and we want to make it after this activity of removing duplicates.
Then the Product columm could be our primary key.
The database is SQL Server CE.
I tried several methods, and mostly getting error similar to :
There was an error parsing the query. [ Token line number = 2,Token line offset = 1,Token in error = FROM ]
A method which I tried :
DELETE FROM TblProducts
FROM TblProducts w
INNER JOIN (
SELECT Product
FROM TblProducts
GROUP BY Product
HAVING COUNT(*) > 1
)Dup ON w.Product = Dup.Product
The preferred way trying to learn and adjust my code with something similar
(It's not correct yet):
SELECT Product, COUNT(*) TotalCount
FROM TblProducts
GROUP BY Product
HAVING COUNT(*) > 1
ORDER BY COUNT(*) DESC
--
;WITH cte -- These 3 lines are the lines I have more doubt on them
AS (SELECT ROW_NUMBER() OVER (PARTITION BY Product
ORDER BY ( SELECT 0)) RN
FROM Word)
DELETE FROM cte
WHERE RN > 1
If you have two DIFFERENT records with the same Product column, then you can SELECT the unwanted records with some criterion, e.g.
CREATE TABLE victims AS
SELECT MAX(entryDate) AS date, Product, COUNT(*) AS dups FROM ProductsTable WHERE ...
GROUP BY Product HAVING dups > 1;
Then you can do a DELETE JOIN between ProductTable and Victims.
Or also you can select Product only, and then do a DELETE for some other JOIN condition, for example having an invalid CustomerId, or EntryDate NULL, or anything else. This works if you know that there is one and only one valid copy of Product, and all the others are recognizable by the invalid data.
Suppose you instead have IDENTICAL records (or you have both identical and non-identical, or you may have several dupes for some product and you don't know which). You run exactly the same query. Then, you run a SELECT query on ProductsTable and SELECT DISTINCT all products matching the product codes to be deduped, grouping by Product, and choosing a suitable aggregate function for all fields (if identical, any aggregate should do. Otherwise I usually try for MAX or MIN). This will "save" exactly one row for each product.
At that point you run the DELETE JOIN and kill all the duplicated products. Then, simply reimport the saved and deduped subset into the main table.
Of course, between the DELETE JOIN and the INSERT SELECT, you will have the DB in a unstable state, with all products with at least one duplicate simply disappeared.
Another way which should work in MySQL:
-- Create an empty table
CREATE TABLE deduped AS SELECT * FROM ProductsTable WHERE false;
CREATE UNIQUE INDEX deduped_ndx ON deduped(Product);
-- DROP duplicate rows, Joe the Butcher's way
INSERT IGNORE INTO deduped SELECT * FROM ProductsTable;
ALTER TABLE ProductsTable RENAME TO ProductsBackup;
ALTER TABLE deduped RENAME TO ProductsTable;
-- TODO: Copy all indexes from ProductsTable on deduped.
NOTE: the way above DOES NOT WORK if you want to distinguish "good records" and "invalid duplicates". It only works if you have redundant DUPLICATE records, or if you do not care which row you keep and which you throw away!
EDIT:
You say that "duplicates" have invalid fields. In that case you can modify the above with a sorting trick:
SELECT * FROM ProductsTable ORDER BY Product, FieldWhichShouldNotBeNULL IS NULL;
Then if you have only one row for product, all well and good, it will get selected. If you have more, the one for which (FieldWhichShouldNeverBeNull IS NULL) is FALSE (i.e. the one where the FieldWhichShouldNeverBeNull is actually not null as it should) will be selected first, and inserted. All others will bounce, silently due to the IGNORE clause, against the uniqueness of Product. Not a really pretty way to do it (and check I didn't mix true with false in my clause!), but it ought to work.
EDIT
actually more of a new answer
This is a simple table to illustrate the problem
CREATE TABLE ProductTable ( Product varchar(10), Description varchar(10) );
INSERT INTO ProductTable VALUES ( 'CBPD10', 'C-Beam Prj' );
INSERT INTO ProductTable VALUES ( 'CBPD11', 'C Proj Mk2' );
INSERT INTO ProductTable VALUES ( 'CBPD12', 'C Proj Mk3' );
There is no index yet, and no primary key. We could still declare Product to be primary key.
But something bad happens. Two new records get in, and both have NULL description.
Yet, the second one is a valid product since we knew nothing of CBPD14 before now, and therefore we do NOT want to lose this record completely. We do want to get rid of the spurious CBPD10 though.
INSERT INTO ProductTable VALUES ( 'CBPD10', NULL );
INSERT INTO ProductTable VALUES ( 'CBPD14', NULL );
A rude DELETE FROM ProductTable WHERE Description IS NULL is out of the question, it would kill CBPD14 which isn't a duplicate.
So we do it like this. First get the list of duplicates:
SELECT Product, COUNT(*) AS Dups FROM ProductTable GROUP BY Product HAVING Dups > 1;
We assume that: "There is at least one good record for every set of bad records".
We check this assumption by positing the opposite and querying for it. If all is copacetic we expect this query to return nothing.
SELECT Dups.Product FROM ProductTable
RIGHT JOIN ( SELECT Product, COUNT(*) AS Dups FROM ProductTable GROUP BY Product HAVING Dups > 1 ) AS Dups
ON (ProductTable.Product = Dups.Product
AND ProductTable.Description IS NOT NULL)
WHERE ProductTable.Description IS NULL;
To further verify, I insert two records that represent this mode of failure; now I do expect the query above to return the new code.
INSERT INTO ProductTable VALUES ( "AC5", NULL ), ( "AC5", NULL );
Now the "check" query indeed returns,
AC5
So, the generation of Dups looks good.
I proceed now to delete all duplicate records that are not valid. If there are duplicate, valid records, they will stay duplicate unless some condition may be found, distinguishing among them one "good" record and declaring all others "invalid" (maybe repeating the procedure with a different field than Description).
But ay, there's a rub. Currently, you cannot delete from a table and select from the same table in a subquery ( http://dev.mysql.com/doc/refman/5.0/en/delete.html ). So a little workaround is needed:
CREATE TEMPORARY TABLE Dups AS
SELECT Product, COUNT(*) AS Duplicates
FROM ProductTable GROUP BY Product HAVING Duplicates > 1;
DELETE ProductTable FROM ProductTable JOIN Dups USING (Product)
WHERE Description IS NULL;
Now this will delete all invalid records, provided that they appear in the Dups table.
Therefore our CBPD14 record will be left untouched, because it does not appear there. The "good" record for CBPD10 will be left untouched because it's not true that its Description is NULL. All the others - poof.
Let me state again that if a record has no valid records and yet it is a duplicate, then all copies of that record will be killed - there will be no survivors.
To avoid this can may first SELECT (using the query above, the check "which should return nothing") the rows representing this mode of failure into another TEMPORARY TABLE, then INSERT them back into the main table after the deletion (using transactions might be in order).
Create a new table by scripting the old one out and renaming it. Also script all objects (indexes etc..) from the old table to the new. Insert the keepers into the new table. If you're database is in bulk-logged or simple recovery model, this operation will be minimally logged. Drop the old table and then rename the new one to the old name.
The advantage of this over a delete will be that the insert can be minimally logged. Deletes do double work because not only does the data get deleted, but the delete has to be written to the transaction log. For big tables, minimally logged inserts will be much faster than deletes.
If it's not that big and you have some downtime, and you have Sql Server Management studio, you can put an identity field on the table using the GUI. Now you have the situation like your CTE, except the rows themselves are truly distinct. So now you can do the following
SELECT MIN(table_a.MyTempIDField)
FROM
table_a lhs
join table_1 rhs
on lhs.field1 = rhs.field1
and lhs.field2 = rhs.field2 [etc]
WHERE
table_a.MyTempIDField <> table_b.MyTempIDField
GROUP BY
lhs.field1, rhs.field2 etc
This gives you all the 'good' duplicates. Now you can wrap this query with a DELETE FROM query.
DELETE FROM lhs
FROM table_a lhs
join table_b rhs
on lhs.field1 = rhs.field1
and lhs.field2 = rhs.field2 [etc]
WHERE
lhs.MyTempIDField <> rhs.MyTempIDField
and lhs.MyTempIDField not in (
SELECT MIN(lhs.MyTempIDField)
FROM
table_a lhs
join table_a rhs
on lhs.field1 = rhs.field1
and lhs.field2 = rhs.field2 [etc]
WHERE
lhs.MyTempIDField <> rhs.MyTempIDField
GROUP BY
lhs.field1, lhs.field2 etc
)
Try this:
DELETE FROM TblProducts
WHERE Product IN
(
SELECT Product
FROM TblProducts
GROUP BY Product
HAVING COUNT(*) > 1)
This suffers from the defect that it deletes ALL the records with a duplicated Product. What you probably want to do is delete all but one of each group of records with a given Product. It might be worthwhile to copy all the duplicates to a separate table first, and then somehow remove duplicates from that table, then apply the above, and then copy remaining products back to the original table.
Okay I have two tables
VOUCHERT with the following fields
ACTIVATIONCODE
SERIALNUMBER
VOUCHERDATADBID
UNAVAILABLEAT
UNAVAILABLEOPERATORDBID
AVAILABLEAT
AVAILABLEOPERATORDBID
ACTIVATIONCODENEW
EXT1
EXT2
EXT3
DENOMINATION -- I added this column into the table.
and the second table is VOUCHERDATAT with the following fields
VOUCHERDATADBID
BATCHID
VALUE
CURRENCY
VOUCHERGROUP
EXPIRYDATE
AGENT
EXT1
EXT2
EXT3
What I want to do is copy the corresponding VALUE from VOUCHERDATAT and put it into DENOMINATION of VOUCHERT. The linking between the two is VOUCHERDATADBID. How do I go about it?
It is not a 1:1 mapping. What I mean is there may be 1000 SERIALNUMBERS with a same VOUCHERDATADBID. And that VOUCHERDATADBID has only entry in VOUCHERDATAT, hence one value. Therefore, all serial numbers belonging to a certain VOUCHERDATADBID will have the same value.
Will JOINS work? What type of JOIN should I use? Or is UPDATE table the way to go?
Thanks for the help !!
Your problem is one of design. None of your tables are in any of the normal forms, not even in the first normal form (1NF). You should not add a column to the VOUCHERT table, but create a new table (pick the name) with the following columns: SERIALNUMBER, VALUE, VOUCHERDATADBID (maybe ACTIVATIONCODE too - need to know the primary key on VOUCHERT to be sure if ACTIVATIONCODE should be included in the new table). Normalization is the database design process that aims to resolve any possible INSERT/UPDATE/DELETE anomalies. This should solve your INSERT issue.
Hope this helps.
You can do a join between these two tables and you will get a 'view'. You can update this view like:
UPDATE (SELECT *
FROM VOUCHERT A JOIN VOUCHERDATAT B
ON A.VOUCHERDATADBID = B.VOUCHERDATADBID)
SET DENOMINATION = VALUE;
You may put outer join if you need.
VOUCHERDATADBID MUST BE PRIMARY KEY in VOUCHERDATAT and FOREIGN KEY in VOUCHERT, otherwise you will get an error:
ORA-01779: cannot modify a column which maps to a non key-preserved table
update (
select v.DENOMINATION
, vd.VALUE
from VOUCHERT v
join VOUCHERDATAT vd
on vd.VOUCHERDATADBID = v.VOUCHERDATADBID
) t
set t.DENOMINATION = t.Value
If the voucherdatadbid is not a primary key, this should work:
UPDATE vouchert
SET denomination =
(SELECT MAX(value)
FROM voucherdatat
WHERE voucherdatadbid = vouchert.voucherdatadbid);