if exists (select top 1 Column1 from Table1 where Column2 = @value)
begin
...
end
else
begin
...
end

P>if exists (select top 1 Column1 from Table1 where Column2 = @value)
P>begin
P>...
P>end
P>else
P>begin
P>...
P>end
P>

if exists (select Column1 from Table1 where Column2 = @value)

I'm running SQL Server 2000 and need to see whether a particular value exists in a table. Which is more efficient, using an EXISTS check or a SELECT COUNT(*)?

A Using the T-SQL EXISTS keyword to perform an existence check is almost always faster than using COUNT(*). EXISTS can stop as soon as the logical test proves true, but COUNT(*) must count every row, even after it knows one row has passed the test.

if exists (select 1 from Table1 where Column2 = @value)

___>if exists (select 1 from Table1 where Column2 = @value)
___>

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE TABLE [dbo].[tblExample](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [ParentId] [int] NULL,
 CONSTRAINT [PK_tblExample] PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

ALTER TABLE [dbo].[tblExample]  WITH CHECK ADD  CONSTRAINT [FK_tblExample_tblExample1] FOREIGN KEY([ParentId])
REFERENCES [dbo].[tblExample] ([Id])
GO

ALTER TABLE [dbo].[tblExample] CHECK CONSTRAINT [FK_tblExample_tblExample1]
GO

if exists (select 1 from tblExample
           where ParentId is null)
           print 'Ok'
else
    print 'error'

  |--Compute Scalar(DEFINE:([Expr1004]=CASE WHEN [Expr1005] THEN (1) ELSE (0) END))
       |--Nested Loops(Left Semi Join, DEFINE:([Expr1005] = [PROBE VALUE]))
            |--Constant Scan
            |--Index Seek(OBJECT:([saad_test2].[dbo].[tblExample].[IX_tblExample]), SEEK:([saad_test2].[dbo].[tblExample].[ParentId]=NULL) ORDERED FORWARD)

P>>  |--Compute Scalar(DEFINE:([Expr1004]=CASE WHEN [Expr1005] THEN (1) ELSE (0) END))
P>>       |--Nested Loops(Left Semi Join, DEFINE:([Expr1005] = [PROBE VALUE]))
P>>            |--Constant Scan
P>>            |--Index Seek(OBJECT:([saad_test2].[dbo].[tblExample].[IX_tblExample]), SEEK:([saad_test2].[dbo].[tblExample].[ParentId]=NULL) ORDERED FORWARD)
P>>

StmtText    StmtId    NodeId    Parent    PhysicalOp    LogicalOp    Argument    DefinedValues    EstimateRows    EstimateIO    EstimateCPU    AvgRowSize    TotalSubtreeCost    OutputList    Warnings    Type    Parallel    EstimateExecutions
if exists (select 1 from tblExample where ParentId is not null)    1    1    0    NULL    NULL    1    NULL    1    NULL    NULL    NULL    0,003288537    NULL    NULL    COND WITH QUERY    0    NULL
  |--Compute Scalar(DEFINE:([Expr1004]=CASE WHEN [Expr1005] THEN (1) ELSE (0) END))    1    2    1    Compute Scalar    Compute Scalar    DEFINE:([Expr1004]=CASE WHEN [Expr1005] THEN (1) ELSE (0) END)    [Expr1004]=CASE WHEN [Expr1005] THEN (1) ELSE (0) END    1    0    1E-07    11    0,003288537    [Expr1004]    NULL    PLAN_ROW    0    1
       |--Nested Loops(Left Semi Join, DEFINE:([Expr1005] = [PROBE VALUE]))    1    3    2    Nested Loops    Left Semi Join    DEFINE:([Expr1005] = [PROBE VALUE])    [Expr1005] = [PROBE VALUE]    1    0    4,18E-06    9    0,003288437    [Expr1005]    NULL    PLAN_ROW    0    1
            |--Constant Scan    1    4    3    Constant Scan    Constant Scan    NULL    NULL    1    0    1,157E-06    9    1,157E-06    NULL    NULL    PLAN_ROW    0    1
            |--Index Seek(OBJECT:([saad_test3].[dbo].[tblExample].[IX_tblExample]), SEEK:([saad_test3].[dbo].[tblExample].[ParentId] IsNotNull) ORDERED FORWARD)    1    5    3    Index Seek    Index Seek    OBJECT:([saad_test3].[dbo].[tblExample].[IX_tblExample]), SEEK:([saad_test3].[dbo].[tblExample].[ParentId] IsNotNull) ORDERED FORWARD    NULL    1    0,2557176    0,2173036    9    0,0032831    NULL    NULL    PLAN_ROW    0    1
   print 'ok'    2    6    1    NULL    NULL    2    NULL    NULL    NULL    NULL    NULL    NULL    NULL    NULL    PRINT    0    NULL
   else  print 'error    3    7    1    NULL    NULL    5    NULL    NULL    NULL    NULL    NULL    NULL    NULL    NULL    PRINT    0    NULL
'    0    8    0    NULL    NULL    NULL    NULL    NULL    NULL    NULL    NULL    NULL    NULL    NULL    TEXT    0    NULL