小弟这里想把诊断分为入院诊断和出院诊断分开显示 但是区分他们的标记在另外一个表中现在要把他们一起显示出来，请问一下怎么才能办到呢

红框标记的写法是不对的，至少在sqlserver和mysql中是语法错误的，应该这样写：
CASE MRC_DiagnosType.DTYP_Desc WHEN '入院诊断' THEN MRC_ICDDx.MRCID_Code ELSE NULL END AS 入院诊断编码,
CASE MRC_DiagnosType.DTYP_Desc WHEN '入院诊断' THEN MRC_ICDDx.MRCID_Desc ELSE NULL END AS 入院诊断名称,
CASE MRC_DiagnosType.DTYP_Desc WHEN '出院诊断' THEN MRC_ICDDx.MRCID_Code ELSE NULL END AS 出院诊断编码,
CASE MRC_DiagnosType.DTYP_Desc WHEN '出院诊断' THEN MRC_ICDDx.MRCID_Desc ELSE NULL END AS 出院诊断名称,