SQL JOIN only when a condition is true

Question

I have 3 variables in my SQL stored procedure and want to add a join only if the 3rd variable is not null.

This is how i am trying to do it but it doesn't work. It gives the following error on the line pointed out:

Incorrect syntax near '{'

ALTER PROCEDURE [dbo].[Search] 
@one NVARCHAR(50), @two NVARCHAR(50), @three NVARCHAR(50)

SELECT  cinfo.ID,
    cinfo.Nam,
    cinfo.INAM,
    cinfo.CA,
    cinfo.Form,
    cinfo.Std,
    cval.Prop,
    cval.Cons,
    sc.Accep

From dbo.Info AS cinfo
Inner JOIN dbo.values AS cval
    ON cinfo.ID = cval.ID
INNER JOIN dbo.Sources AS sc
    ON (cval.sID = sc.sID AND sc.Accept = 'A')
IF @three IS NOT NULL{               **<---------------------**
LEFT JOIN dbo.Synonym AS synm
    ON cinfo.ID = synm.ID}

where (cinfo.NAM LIKE '%'+@one+'%' OR cinfo.CAS LIKE '%'+@two+'%' OR 
  synm.SynonymID LIKE '%'+@three+'%') AND
    (cval.PropID = '1' OR
    cval.PropID = '2' OR
    cval.PropID = '3' OR
    cval.PropID = '4' OR)

Answer 1

Couldn't you just use an and on the join? The join will be attempted but if @three is null then no joins to synonym would occur.... or do you really need to ignore the join for performance reasons? if so Dynamic SQL is the only way I could see to make it work...

ALTER PROCEDURE [dbo].[Search] 
@one NVARCHAR(50), @two NVARCHAR(50), @three NVARCHAR(50)

SELECT  cinfo.ID,
    cinfo.Nam,
    cinfo.INAM,
    cinfo.CA,
    cinfo.Form,
    cinfo.Std,
    cval.Prop,
    cval.Cons,
    sc.Accep

From dbo.Info AS cinfo
Inner JOIN dbo.values AS cval
    ON cinfo.ID = cval.ID
INNER JOIN dbo.Sources AS sc
    ON (cval.sID = sc.sID AND sc.Accept = 'A')
LEFT JOIN dbo.Synonym AS synm
    ON cinfo.ID = synm.ID
   and @three IS NOT NULL               **<---------------------**

where (cinfo.NAM LIKE '%'+@one+'%' OR cinfo.CAS LIKE '%'+@two+'%' OR 
  synm.SynonymID LIKE '%'+@three+'%') AND
    (cval.PropID = '1' OR
    cval.PropID = '2' OR
    cval.PropID = '3' OR
    cval.PropID = '4' OR)

Answer 2

This is not correct syntax, but you could do achieve this in few ways, the dumbest way to do is add the Boolean check on the top.

IF @three IS NOT NULL
BEGIN
--FULL QUERY that JOIN dbo.Synonym 
END 

IF @Three IS NULL
BEGIN
--FULL QUERY that does not JOIN dbo.Synonym 
END

Or you could use case..when to control, it does the same thing as multiple if

Answer 3

You need to give that condition while joining itself.

ALTER PROCEDURE [dbo].[Search] 
@one NVARCHAR(50), @two NVARCHAR(50), @three NVARCHAR(50)

SELECT  cinfo.ID,
    cinfo.Nam,
    cinfo.INAM,
    cinfo.CA,
    cinfo.Form,
    cinfo.Std,
    cval.Prop,
    cval.Cons,
    sc.Accep

From dbo.Info AS cinfo
Inner JOIN dbo.values AS cval
    ON cinfo.ID = cval.ID
INNER JOIN dbo.Sources AS sc
    ON (cval.sID = sc.sID AND sc.Accept = 'A')
       LEFT JOIN dbo.Synonym AS synm
    ON (cinfo.ID = synm.ID and @three is not null) **<---------------------**

where (cinfo.NAM LIKE '%'+@one+'%' OR cinfo.CAS LIKE '%'+@two+'%' OR 
  synm.SynonymID LIKE '%'+@three+'%') AND
    (cval.PropID = '1' OR
    cval.PropID = '2' OR
    cval.PropID = '3' OR
    cval.PropID = '4' OR)

Answer 4

You can achieve this by building dynamic query like following

ALTER PROCEDURE [dbo].[Search] 
@one NVARCHAR(50), @two NVARCHAR(50), @three NVARCHAR(50)

DECLARE @query varchar(2000)=' SELECT  cinfo.ID,
    cinfo.Nam,
    cinfo.INAM,
    cinfo.CA,
    cinfo.Form,
    cinfo.Std,
    cval.Prop,
    cval.Cons,
    sc.Accep

From dbo.Info AS cinfo Inner JOIN dbo.values AS cval ON cinfo.ID = cval.ID
INNER JOIN dbo.Sources AS sc ON (cval.sID = sc.sID AND sc.Accept = ''A'')'

IF @three IS NOT NULL
BEGIN
   SET @query = @query + ' LEFT JOIN dbo.Synonym AS synm ON cinfo.ID = synm.ID '
END

SET @query = @query + 'where (cinfo.NAM LIKE ''%' + @one + '%'' OR cinfo.CAS LIKE ''%' + @two + '%'' OR 
  synm.SynonymID LIKE ''%' + @three + '%'') AND
    (cval.PropID = ''1'' OR
    cval.PropID = ''2'' OR
    cval.PropID = ''3'' OR
    cval.PropID = ''4'')'

EXEC(@query)

Answer 5

What about:

ALTER PROCEDURE [dbo].[Search] 
@one NVARCHAR(50), @two NVARCHAR(50), @three NVARCHAR(50)

SELECT  cinfo.ID,
    cinfo.Nam,
    cinfo.INAM,
    cinfo.CA,
    cinfo.Form,
    cinfo.Std,
    cval.Prop,
    cval.Cons,
    sc.Accep

From dbo.Info AS cinfo
Inner JOIN dbo.values AS cval
    ON cinfo.ID = cval.ID
INNER JOIN dbo.Sources AS sc
    ON (cval.sID = sc.sID AND sc.Accept = 'A')
LEFT JOIN dbo.Synonym AS synm
    ON (cinfo.ID = synm.ID AND @three IS NOT NULL)

where (cinfo.NAM LIKE '%'+@one+'%' OR cinfo.CAS LIKE '%'+@two+'%' OR 
  synm.SynonymID LIKE '%'+@three+'%') AND
    (cval.PropID = '1' OR
    cval.PropID = '2' OR
    cval.PropID = '3' OR
    cval.PropID = '4' OR)

Answer 6

replace IF @three with this :

 LEFT JOIN (select * from dbo.Synonym  where @three IS NOT NULL) as synm ON cinfo.ID = synm.ID

and verify ALTER procedure syntax and LIKE syntax.

Answer 7

You cannot use { for if condition just remove '{'

ALTER PROCEDURE [dbo].[Search] 
@one NVARCHAR(50), @two NVARCHAR(50), @three NVARCHAR(50)

SELECT  cinfo.ID,
    cinfo.Nam,
    cinfo.INAM,
    cinfo.CA,
    cinfo.Form,
    cinfo.Std,
    cval.Prop,
    cval.Cons,
    sc.Accep

From dbo.Info AS cinfo
Inner JOIN dbo.values AS cval
    ON cinfo.ID = cval.ID
INNER JOIN dbo.Sources AS sc
    ON (cval.sID = sc.sID AND sc.Accept = 'A')
**IF @three IS NOT NULL**           Remove this line
LEFT JOIN dbo.Synonym AS synm
    ON cinfo.ID = synm.ID}

where (cinfo.NAM LIKE '%'+@one+'%' OR cinfo.CAS LIKE '%'+@two+'%' OR 
  synm.SynonymID LIKE '%'+@three+'%') AND
    (cval.PropID = '1' OR
    cval.PropID = '2' OR
    cval.PropID = '3' OR
    cval.PropID = '4' OR)

Try to write Like IF @three IS NOT NULL ALTER PROCEDURE [dbo].[Search] @one NVARCHAR(50), @two NVARCHAR(50), @three NVARCHAR(50)

SELECT  cinfo.ID,
    cinfo.Nam,
    cinfo.INAM,
    cinfo.CA,
    cinfo.Form,
    cinfo.Std,
    cval.Prop,
    cval.Cons,
    sc.Accep

From dbo.Info AS cinfo
Inner JOIN dbo.values AS cval
    ON cinfo.ID = cval.ID
INNER JOIN dbo.Sources AS sc
    ON (cval.sID = sc.sID AND sc.Accept = 'A')

where (cinfo.NAM LIKE '%'+@one+'%' OR cinfo.CAS LIKE '%'+@two+'%' OR 
  synm.SynonymID LIKE '%'+@three+'%') AND
    (cval.PropID = '1' OR
    cval.PropID = '2' OR
    cval.PropID = '3' OR
    cval.PropID = '4' OR)

end else ALTER PROCEDURE [dbo].[Search] @one NVARCHAR(50), @two NVARCHAR(50), @three NVARCHAR(50)

SELECT  cinfo.ID,
    cinfo.Nam,
    cinfo.INAM,
    cinfo.CA,
    cinfo.Form,
    cinfo.Std,
    cval.Prop,
    cval.Cons,
    sc.Accep

From dbo.Info AS cinfo
Inner JOIN dbo.values AS cval
    ON cinfo.ID = cval.ID
INNER JOIN dbo.Sources AS sc
    ON (cval.sID = sc.sID AND sc.Accept = 'A')
LEFT JOIN dbo.Synonym AS synm
    ON cinfo.ID = synm.ID}

where (cinfo.NAM LIKE '%'+@one+'%' OR cinfo.CAS LIKE '%'+@two+'%' OR 
  synm.SynonymID LIKE '%'+@three+'%') AND
    (cval.PropID = '1' OR
    cval.PropID = '2' OR
    cval.PropID = '3' OR
    cval.PropID = '4' OR).. rest code

end