I think this may do what you are trying to do. If the search term is in both tables then the matching row only from Table A displays, otherwise ALL the rows in Table A are displayed.

IF OBJECT_ID('tempdb..#TableA') IS NOT NULL

DROP TABLE #TableA

IF OBJECT_ID('tempdb..#LinkTable') IS NOT NULL

DROP TABLE #LinkTable

IF OBJECT_ID('tempdb..#TableB') IS NOT NULL

DROP TABLE #TableB

CREATE TABLE #TableA (

[ID] INT NOT NULL,

[Val] VARCHAR(10) NULL,

PRIMARY KEY (ID),

UNIQUE (ID))

CREATE TABLE #LinkTable(

[ID] INT IDENTITY(1,1) NOT NULL,

[TableA_ID] INT NULL,

[TableB_ID] INT NULL,

PRIMARY KEY (ID),

UNIQUE (ID))

CREATE TABLE #TableB (

[ID] INT NOT NULL,

[Val] VARCHAR(10) NULL,

PRIMARY KEY (ID),

UNIQUE (ID))

INSERT INTO #TableA

SELECT 1,'Apples'

UNION

SELECT 2,'Pears'

UNION

SELECT 3,'Oranges'

UNION

SELECT 4,'Grapes'

INSERT INTO #TableB

SELECT 1,'Oranges'

UNION

SELECT 2,'Onions'

UNION

SELECT 3,'Apples'

UNION

SELECT 4,'Squash'

INSERT INTO #LinkTable

SELECT 1,3

UNION

SELECT 3,1

DECLARE @strSearch VARCHAR(50)

SET @strSearch = 'Apples'

--SET @strSearch = 'Pears'

--SET @strSearch = 'Oranges'

--SET @strSearch = 'Onions'

--SET @strSearch = ''

SELECT

a.ID

,a.Val

FROM

#TableA AS a

LEFT OUTER JOIN

#LinkTable AS lt

ON a.ID = lt.TableA_ID

LEFT OUTER JOIN

#TableB AS b

ON lt.TableB_ID = b.ID

WHERE

--if @strSearch in A and in B then matching row from A

((a.Val = ISNULL(NULLIF(@strSearch,''),0)

AND (b.Val = ISNULL(NULLIF(@strSearch,''),0)))

--if @strSearch not A or not B then ALL rows from A

OR

((NOT EXISTS (SELECT ID FROM #TableA WHERE val = @strSearch)

OR NOT EXISTS (SELECT ID FROM #TableB WHERE val = @strSearch))

AND EXISTS (SELECT ID FROM #TableA)))