Check if a row exists, otherwise insert - sql

Question

I need to write a T-SQL stored procedure that updates a row in a table. If the row doesn't exist, insert it. All this steps wrapped by a transaction.
This is for a booking system, so it must be atomic and reliable. It must return true if the transaction was committed and the flight booked.
I'm sure on how to use ##rowcount. This is what I've written until now. Am I on the right road?
-- BEGIN TRANSACTION (HOW TO DO?)
UPDATE Bookings
SET TicketsBooked = TicketsBooked + #TicketsToBook
WHERE FlightId = #Id AND TicketsMax < (TicketsBooked + #TicketsToBook)
-- Here I need to insert only if the row doesn't exists.
-- If the row exists but the condition TicketsMax is violated, I must not insert
-- the row and return FALSE
IF ##ROWCOUNT = 0
BEGIN
INSERT INTO Bookings ... (omitted)
END
-- END TRANSACTION (HOW TO DO?)
-- Return TRUE (How to do?)

Answer 1

I assume a single row for each flight? If so:
IF EXISTS (SELECT * FROM Bookings WHERE FLightID = #Id)
BEGIN
--UPDATE HERE
END
ELSE
BEGIN
-- INSERT HERE
END
I assume what I said, as your way of doing things can overbook a flight, as it will insert a new row when there are 10 tickets max and you are booking 20.

Answer 2

Take a look at MERGE command. You can do UPDATE, INSERT & DELETE in one statement.
Here is a working implementation on using MERGE
- It checks whether flight is full before doing an update, else does an insert.
if exists(select 1 from INFORMATION_SCHEMA.TABLES T
where T.TABLE_NAME = 'Bookings')
begin
drop table Bookings
end
GO
create table Bookings(
FlightID int identity(1, 1) primary key,
TicketsMax int not null,
TicketsBooked int not null
)
GO
insert Bookings(TicketsMax, TicketsBooked) select 1, 0
insert Bookings(TicketsMax, TicketsBooked) select 2, 2
insert Bookings(TicketsMax, TicketsBooked) select 3, 1
GO
select * from Bookings
And then ...
declare #FlightID int = 1
declare #TicketsToBook int = 2
--; This should add a new record
merge Bookings as T
using (select #FlightID as FlightID, #TicketsToBook as TicketsToBook) as S
on T.FlightID = S.FlightID
and T.TicketsMax > (T.TicketsBooked + S.TicketsToBook)
when matched then
update set T.TicketsBooked = T.TicketsBooked + S.TicketsToBook
when not matched then
insert (TicketsMax, TicketsBooked)
values(S.TicketsToBook, S.TicketsToBook);
select * from Bookings

Answer 3

Pass updlock, rowlock, holdlock hints when testing for existence of the row.
begin tran /* default read committed isolation level is fine */
if not exists (select * from Table with (updlock, rowlock, holdlock) where ...)
/* insert */
else
/* update */
commit /* locks are released here */
The updlock hint forces the query to take an update lock on the row if it already exists, preventing other transactions from modifying it until you commit or roll back.
The holdlock hint forces the query to take a range lock, preventing other transactions from adding a row matching your filter criteria until you commit or roll back.
The rowlock hint forces lock granularity to row level instead of the default page level, so your transaction won't block other transactions trying to update unrelated rows in the same page (but be aware of the trade-off between reduced contention and the increase in locking overhead - you should avoid taking large numbers of row-level locks in a single transaction).
See http://msdn.microsoft.com/en-us/library/ms187373.aspx for more information.
Note that locks are taken as the statements which take them are executed - invoking begin tran doesn't give you immunity against another transaction pinching locks on something before you get to it. You should try and factor your SQL to hold locks for the shortest possible time by committing the transaction as soon as possible (acquire late, release early).
Note that row-level locks may be less effective if your PK is a bigint, as the internal hashing on SQL Server is degenerate for 64-bit values (different key values may hash to the same lock id).

Answer 4

i'm writing my solution. my method doesn't stand 'if' or 'merge'. my method is easy.
INSERT INTO TableName (col1,col2)
SELECT #par1, #par2
WHERE NOT EXISTS (SELECT col1,col2 FROM TableName
WHERE col1=#par1 AND col2=#par2)
For Example:
INSERT INTO Members (username)
SELECT 'Cem'
WHERE NOT EXISTS (SELECT username FROM Members
WHERE username='Cem')
Explanation:
(1) SELECT col1,col2 FROM TableName WHERE col1=#par1 AND col2=#par2
It selects from TableName searched values
(2) SELECT #par1, #par2 WHERE NOT EXISTS
It takes if not exists from (1) subquery
(3) Inserts into TableName (2) step values

Answer 5

I finally was able to insert a row, on the condition that it didn't already exist, using the following model:
INSERT INTO table ( column1, column2, column3 )
(
SELECT $column1, $column2, $column3
WHERE NOT EXISTS (
SELECT 1
FROM table
WHERE column1 = $column1
AND column2 = $column2
AND column3 = $column3
)
)
which I found at:
http://www.postgresql.org/message-id/87hdow4ld1.fsf#stark.xeocode.com

Answer 6

This is something I just recently had to do:
set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[cjso_UpdateCustomerLogin]
(
#CustomerID AS INT,
#UserName AS VARCHAR(25),
#Password AS BINARY(16)
)
AS
BEGIN
IF ISNULL((SELECT CustomerID FROM tblOnline_CustomerAccount WHERE CustomerID = #CustomerID), 0) = 0
BEGIN
INSERT INTO [tblOnline_CustomerAccount] (
[CustomerID],
[UserName],
[Password],
[LastLogin]
) VALUES (
/* CustomerID - int */ #CustomerID,
/* UserName - varchar(25) */ #UserName,
/* Password - binary(16) */ #Password,
/* LastLogin - datetime */ NULL )
END
ELSE
BEGIN
UPDATE [tblOnline_CustomerAccount]
SET UserName = #UserName,
Password = #Password
WHERE CustomerID = #CustomerID
END
END

Answer 7

You could use the Merge Functionality to achieve. Otherwise you can do:
declare #rowCount int
select #rowCount=##RowCount
if #rowCount=0
begin
--insert....

Answer 8

INSERT INTO [DatabaseName1].dbo.[TableName1] SELECT * FROM [DatabaseName2].dbo.[TableName2]
WHERE [YourPK] not in (select [YourPK] from [DatabaseName1].dbo.[TableName1])

Answer 9

Full solution is below (including cursor structure). Many thanks to Cassius Porcus for the begin trans ... commit code from posting above.
declare #mystat6 bigint
declare #mystat6p varchar(50)
declare #mystat6b bigint
DECLARE mycur1 CURSOR for
select result1,picture,bittot from all_Tempnogos2results11
OPEN mycur1
FETCH NEXT FROM mycur1 INTO #mystat6, #mystat6p , #mystat6b
WHILE ##Fetch_Status = 0
BEGIN
begin tran /* default read committed isolation level is fine */
if not exists (select * from all_Tempnogos2results11_uniq with (updlock, rowlock, holdlock)
where all_Tempnogos2results11_uniq.result1 = #mystat6
and all_Tempnogos2results11_uniq.bittot = #mystat6b )
insert all_Tempnogos2results11_uniq values (#mystat6 , #mystat6p , #mystat6b)
--else
-- /* update */
commit /* locks are released here */
FETCH NEXT FROM mycur1 INTO #mystat6 , #mystat6p , #mystat6b
END
CLOSE mycur1
DEALLOCATE mycur1
go

Answer 10

Simple way to copy data from T1 to T2 and avoid duplicate in T2
--Insert a new record
INSERT INTO dbo.Table2(NoEtu, FirstName, LastName)
SELECT t1.NoEtuDos, t1.FName, t1.LName
FROM dbo.Table1 as t1
WHERE NOT EXISTS (SELECT (1) FROM dbo.Table2 AS t2
WHERE t1.FName = t2.FirstName
AND t1.LName = t2.LastName
AND t1.NoEtuDos = t2.NoEtu)

Answer 11

INSERT INTO table ( column1, column2, column3 )
SELECT $column1, $column2, $column3
EXCEPT SELECT column1, column2, column3
FROM table

Answer 12

The best approach to this problem is first making the database column UNIQUE
ALTER TABLE table_name ADD UNIQUE KEY
THEN INSERT IGNORE INTO table_name ,the value won't be inserted if it results in a duplicate key/already exists in the table.