using case and if in sql server query

SELECT '(X' + x.paramno + ')' + x.xParameterName AS Xparametername, '(Y' + y.paramno + ') ' + y.yParameterName AS Yparametername,
'UnitNo' + u.paramno + ' (' + u.uParameterName + ')' AS Unitparametername
FROM (SELECT CASE substring(parametername, 1, 1) WHEN 'X' THEN substring(parametername, 2, 100) WHEN 'Y' THEN substring(parametername, 2, 100)
WHEN ('U') THEN substring(parametername, 7, 100) ELSE NULL END AS paramno, SUBSTRING(ParameterName, 1, 1) AS indicator,
CAST(ParamValueInt AS varchar(10)) AS xParameterName
FROM insite.dc_Eval_CoordinateInput
WHERE (SUBSTRING(ParameterName, 1, 1) = 'X')) AS x INNER JOIN
(SELECT CASE substring(parametername, 1, 1) WHEN 'X' THEN substring(parametername, 2, 100) WHEN 'Y' THEN substring(parametername, 2,
100) WHEN ('U') THEN substring(parametername, 7, 100) ELSE NULL END AS paramno, SUBSTRING(ParameterName, 1, 1) AS indicator,
CAST(ParamValueInt AS varchar(10)) AS yParameterName
FROM insite.dc_Eval_CoordinateInput
WHERE (SUBSTRING(ParameterName, 1, 1) = 'Y')) AS y ON x.paramno = y.paramno INNER JOIN
(SELECT CASE substring(parametername, 1, 1) WHEN 'X' THEN substring(parametername, 2, 100) WHEN 'Y' THEN substring(parametername, 2,
100) WHEN ('U') THEN substring(parametername, 7, 100) ELSE NULL END AS paramno, SUBSTRING(ParameterName, 1, 1) AS indicator,
CAST(ParamValueInt AS varchar(10)) AS uParameterName
FROM insite.dc_Eval_CoordinateInput
WHERE (SUBSTRING(ParameterName, 1, 1) = 'U')) AS u ON x.paramno = u.paramno