IF EXISTS, THEN SELECT ELSE INSERT AND THEN SELECT - sql

Question

How do you say the following in Microsoft SQL Server 2005:
IF EXISTS (SELECT * FROM Table WHERE FieldValue='') THEN
SELECT TableID FROM Table WHERE FieldValue=''
ELSE
INSERT INTO TABLE(FieldValue) VALUES('')
SELECT TableID FROM Table WHERE TableID=SCOPE_IDENTITY()
END IF
What I'm trying to do is to see if there is a blank fieldvalue already, and if there is then return that TableID, else insert a blank fieldvalue and return the corresponding primary key.

Answer 1

You need to do this in transaction to ensure two simultaneous clients won't insert same fieldValue twice:
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
BEGIN TRANSACTION
DECLARE #id AS INT
SELECT #id = tableId FROM table WHERE fieldValue=#newValue
IF #id IS NULL
BEGIN
INSERT INTO table (fieldValue) VALUES (#newValue)
SELECT #id = SCOPE_IDENTITY()
END
SELECT #id
COMMIT TRANSACTION
you can also use Double-checked locking to reduce locking overhead
DECLARE #id AS INT
SELECT #id = tableID FROM table (NOLOCK) WHERE fieldValue=#newValue
IF #id IS NULL
BEGIN
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
BEGIN TRANSACTION
SELECT #id = tableID FROM table WHERE fieldValue=#newValue
IF #id IS NULL
BEGIN
INSERT INTO table (fieldValue) VALUES (#newValue)
SELECT #id = SCOPE_IDENTITY()
END
COMMIT TRANSACTION
END
SELECT #id
As for why ISOLATION LEVEL SERIALIZABLE is necessary, when you are inside a serializable transaction, the first SELECT that hits the table creates a range lock covering the place where the record should be, so nobody else can insert the same record until this transaction ends.
Without ISOLATION LEVEL SERIALIZABLE, the default isolation level (READ COMMITTED) would not lock the table at read time, so between SELECT and UPDATE, somebody would still be able to insert. Transactions with READ COMMITTED isolation level do not cause SELECT to lock. Transactions with REPEATABLE READS lock the record (if found) but not the gap.

Answer 2

IF EXISTS (SELECT 1 FROM Table WHERE FieldValue='')
BEGIN
SELECT TableID FROM Table WHERE FieldValue=''
END
ELSE
BEGIN
INSERT INTO TABLE(FieldValue) VALUES('')
SELECT SCOPE_IDENTITY() AS TableID
END
See here for more information on IF ELSE
Note: written without a SQL Server install handy to double check this but I think it is correct
Also, I've changed the EXISTS bit to do SELECT 1 rather than SELECT * as you don't care what is returned within an EXISTS, as long as something is
I've also changed the SCOPE_IDENTITY() bit to return just the identity assuming that TableID is the identity column

Answer 3

You were close:
IF EXISTS (SELECT * FROM Table WHERE FieldValue='')
SELECT TableID FROM Table WHERE FieldValue=''
ELSE
BEGIN
INSERT INTO TABLE (FieldValue) VALUES ('')
SELECT TableID FROM Table WHERE TableID=SCOPE_IDENTITY()
END

Answer 4

You just have to change the structure of the if...else..endif somewhat:
if exists(select * from Table where FieldValue='') then begin
select TableID from Table where FieldValue=''
end else begin
insert into Table (FieldValue) values ('')
select TableID from Table where TableID = scope_identity()
end
You could also do:
if not exists(select * from Table where FieldValue='') then begin
insert into Table (FieldValue) values ('')
end
select TableID from Table where FieldValue=''
Or:
if exists(select * from Table where FieldValue='') then begin
select TableID from Table where FieldValue=''
end else begin
insert into Table (FieldValue) values ('')
select scope_identity() as TableID
end

Answer 5

It sounds like your table has no key. You should be able to simply try the INSERT: if it’s a duplicate then the key constraint will bite and the INSERT will fail. No worries: you just need to ensure the application doesn't see/ignores the error. When you say 'primary key' you presumably mean IDENTITY value. That's all very well but you also need a key constraint (e.g. UNIQUE) on your natural key.
Also, I wonder whether your procedure is doing too much. Consider having separate procedures for 'create' and 'read' actions respectively.

Answer 6

DECLARE #t1 TABLE (
TableID int IDENTITY,
FieldValue varchar(20)
)
--<< No empty string
IF EXISTS (
SELECT *
FROM #t1
WHERE FieldValue = ''
) BEGIN
SELECT TableID
FROM #t1
WHERE FieldValue=''
END
ELSE BEGIN
INSERT INTO #t1 (FieldValue) VALUES ('')
SELECT SCOPE_IDENTITY() AS TableID
END
--<< A record with an empty string already exists
IF EXISTS (
SELECT *
FROM #t1
WHERE FieldValue = ''
) BEGIN
SELECT TableID
FROM #t1
WHERE FieldValue=''
END
ELSE BEGIN
INSERT INTO #t1 (FieldValue) VALUES ('')
SELECT SCOPE_IDENTITY() AS TableID
END

Answer 7

create schema tableName authorization dbo
go
IF OBJECT_ID ('tableName.put_fieldValue', 'P' ) IS NOT NULL
drop proc tableName.put_fieldValue
go
create proc tableName.put_fieldValue(#fieldValue int) as
declare #tableid int = 0
select #tableid = tableid from table where fieldValue=''
if #tableid = 0 begin
insert into table(fieldValue) values('')
select #tableid = scope_identity()
end
return #tableid
go
declare #tablid int = 0
exec #tableid = tableName.put_fieldValue('')