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Posts Tagged ‘UPDATE’

DML | Is SELECT a DML?

July 26, 2011 1 comment

DML or Data Manipulation Langauge as the term suggest represents those SQL statements that manipulates the data in a database. Thus these langauges allows users to INSERT, UPDATE & DELETE the data in a particular database. Other than this the much debatable SELECT statement may or may not be considered as DML upon its usage.

A simple SELECT statement which fetches data from a table is a read-only language and cannot be called as DML.

But a modified version of SELECT i.e. ‘SELECT INTO’ can fall into the DML segment. The ‘SELECT INTO’ can be used to create a Table and insert records fetched from the SELECT statement.

Also, we can manipulate the data for reporting purpose while retriving by using the SELECT statement. The data in underlying tables is unchanged but on the frontend you get a view of modified data, like:

USE [AdventureWorks]
GO

SELECT ContactID, Title,
	FirstName, MiddleName, LastName,
	FirstName + ' ' + ISNULL(MiddleName,'') + ' ' + LastName as [FullName] -- Name manipulated by joining 3 part names.
FROM Person.Contact

SELECT SalesOrderID, SalesOrderDetailID,
	OrderQty, UnitPrice,
	OrderQty * UnitPrice as [TotalPrice] -- Manipulated cost by calculating it for total items purchased.
FROM Sales.SalesOrderDetail

Thus SELECT also comes under DML and following is the list of all DMLs:
1. SELECT {COLUMN LIST} [INTO {TABLE_NAME}] [WHERE {WHERE condition}]
2. INSERT INTO {TABLE_NAME} VALUES (SET of Values)
3. UPDATE {TABLE_NAME} SET [WHERE {WHERE condition}]
4. DELETE FROM {TABLE_NAME} [WHERE {WHERE condition}]

UPDATE statement conflicted with the REFERENCE constraint

December 21, 2010 9 comments

Just replied on MSDN forum’s following post: http://social.msdn.microsoft.com/Forums/en-US/transactsql/thread/289c4ca5-6ab2-4ed6-b446-0300f5c7dc88

When tables are referenced by Foreign Key constraints it gets difficult to UPDATE the key columns.
An example shows this issue and how one can perform UPDATE on FK columns.

USE [tempdb]
GO

-- CREATE test tables
CREATE TABLE dept (d_id INT PRIMARY KEY, dept VARCHAR(50))

CREATE TABLE emp (sn INT PRIMARY KEY, ename VARCHAR(50),
	d_fk INT CONSTRAINT fk_dept_d_fk FOREIGN KEY (d_fk) REFERENCES dept(d_id))

-- INSERT test data
INSERT INTO dept
SELECT 1, 'finance' UNION
SELECT 2, 'hr'

INSERT INTO emp
SELECT 1, 'Manoj', 1 UNION
SELECT 2, 'Saurabh', 2 UNION
SELECT 3, 'Kanchan', 1 UNION
SELECT 4, 'Pooja', 2

-- SELECT the INSERTed data
SELECT * from dept
d_id	dept
1	finance
2	hr	-- DeptID needs to be updated to 3
SELECT * from emp
sn	ename	d_fk
1	Manoj	1
2	Saurabh	2	-- DeptID should also be updated here to 3
3	Kanchan	1
4	Pooja	2	-- DeptID should also be updated here to 3
-- Now lets UPDATE the FK column values
UPDATE dept SET d_id=3 WHERE d_id=2
Error Message:
Msg 547, Level 16, State 0, Line 1
The UPDATE statement conflicted with the REFERENCE constraint "fk_dept_d_fk".
The conflict occurred in database "tempdb", table "dbo.emp", column 'd_fk'.
The statement has been terminated.
UPDATE emp SET d_fk=3 WHERE d_fk=2
Error Message:
Msg 547, Level 16, State 0, Line 1
The UPDATE statement conflicted with the FOREIGN KEY constraint "fk_dept_d_fk".
The conflict occurred in database "tempdb", table "dbo.dept", column 'd_id'.
The statement has been terminated.

We can disable the FK constraints by ALTER statement before the UPDATE and enable them after the UPDATE, like:

-- Disable FK Constraint
ALTER TABLE emp NOCHECK CONSTRAINT fk_dept_d_fk

-- Perform UPDATE
UPDATE dept SET d_id=3 WHERE d_id=2
UPDATE emp SET d_fk=3 WHERE d_fk=2

-- Enable FK Constraint
ALTER TABLE emp WITH CHECK CHECK CONSTRAINT fk_dept_d_fk

-- Following SELECTS will show the new values in updated records:
SELECT * from dept
d_id	dept
1	finance
3	hr	-- Updated record
SELECT * from emp
sn	ename	d_fk
1	Manoj	1
2	Saurabh	3	-- Updated record
3	Kanchan	1
4	Pooja	3	-- Updated record
-- Final Cleanup, DROP the tables.
DROP TABLE emp
DROP TABLE dept

More info: http://www.mssqlcity.com/Articles/General/using_constraINTs.htm
MSDN BOL: http://msdn.microsoft.com/en-us/library/10cetyt6(v=VS.80).aspx

OUTPUT clause and MERGE statement

November 25, 2010 1 comment

Just responded to a post in MSDN forum, link: http://social.msdn.microsoft.com/Forums/en-US/transactsql/thread/c06e1db4-7fd6-43c4-8569-5335d555dac8

Accroding to MS-BOL, OUTPUT clause returns information from, or expressions based on, each row affected by an INSERT, UPDATE, DELETE, or MERGE statement. These results can be returned to the processing application for use in such things as confirmation messages, archiving, and other such application requirements. The results can also be inserted into a table or table variable. Additionally, you can capture the results of an OUTPUT clause in a nested INSERT, UPDATE, DELETE, or MERGE statement, and insert those results into a target table or view.

Lets go with a self descriptive example:

–> OUTPUT with INSERT

create table manoj (sn int, ename varchar(50))
insert into manoj
OUTPUT INSERTED.*
values (1,'manoj'), (2,'hema'), (3,'kanchan'), (4,'pankaj')
This gives me following output instead of normal message (N row(s) affected):
sn      ename
1	manoj
2	hema
3	kanchan
4	pankaj
select * from manoj
This gives me the same output as above:
sn      ename
1	manoj
2	hema
3	kanchan
4	pankaj

–> OUTPUT with DELETE

delete from manoj
OUTPUT DELETED.*
where sn = 4
This gives me following output:
sn      ename
4	pankaj
select * from manoj
Now the result set is changed to:
sn      ename
1	manoj
2	hema
3	kanchan

–> OUTPUT with UPDATE

update manoj
set ename = 'pankaj'
OUTPUT DELETED.*, INSERTED.*
from manoj
where sn = 2
This gives me following output:
sn      ename   sn     ename
2	hema	2	pankaj
select * from manoj
Now the result set is changed to:
sn      ename
1	manoj
2	pankaj
3	kanchan

–> OUTPUT with MERGE

According to MS-BOL, MERGE performs insert, update, or delete operations on a target table based on the results of a join with a source table. For example, you can synchronize two tables by inserting, updating, or deleting rows in one table based on differences found in the other table.

create table manoj2 (sn int, ename varchar(50))

insert into manoj2
values (1,'manoj'), (2,'hema'), (3,'kanchan'), (4,'pankaj'), (5,'saurabh')

select * from manoj2
This gives me following output instead of normal message (N row(s) affected):
sn      ename
1	manoj
2	hema
3	kanchan
4	pankaj
MERGE manoj AS TARGET
USING (SELECT sn, ename FROM manoj2) AS SOURCE
ON (TARGET.sn = SOURCE.sn)
WHEN MATCHED THEN
	UPDATE SET TARGET.ename = SOURCE.ename
WHEN NOT MATCHED BY TARGET THEN
	INSERT (sn, ename) VALUES (sn, ename)
OUTPUT $action, DELETED.*, INSERTED.*;
This gives me following output:
$action sn      ename   sn      ename
INSERT	NULL	NULL	4	pankaj
INSERT	NULL	NULL	5	saurabh
UPDATE	1	manoj	1	manoj
UPDATE	2	pankaj	2	hema
UPDATE	3	kanchan	3	kanchan
select * from manoj
Now the result set is changed to:
sn      ename
1	manoj
2	hema
3	kanchan
4	pankaj
5	saurabh

–> Final cleanup

drop table manoj
drop table manoj2

Plz note: An UPDATE, INSERT, or DELETE statement that has an OUTPUT clause will return rows to the client even if the statement encounters errors and is rolled back. The result should not be used if any error occurs when you run the statement.

MS BOL:-

- On MERGE: http://technet.microsoft.com/en-us/library/bb510625.aspx

- On OUTPUT: http://technet.microsoft.com/en-us/library/ms177564.aspx

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