mysql - MySQL-具有IF条件的联接表

我有以下查询，希望使用IF条件进行修改：

if(specimen.snop_axis = 'M', join morphology
    on morphology.morphology_code = specimen.snop_code, join functions on functions.functions_code = specimen.snop_code)

select *
  from specimen
  join topography_index
    on substring(specimen.topography_index, 2, 2) =
       topography_index.topography_index_code
  join morphology
    on morphology.morphology_code = specimen.snop_code
  join functions
    on functions.functions_code = specimen.snop_code
  left join specimen_image_lookup
    on specimen_image_lookup.specimen_fk = specimen.specimen_pk
  left join image
    on image.image_pk = specimen_image_lookup.image_fk
 where specimen.specimen_pk = '$specimen'

如果条件该如何修改此查询以合并此查询？也就是说，IF语句需要替换：

join morphology
    on morphology.morphology_code = specimen.snop_code
  join functions
    on functions.functions_code = specimen.snop_code

最佳答案

在两个表的连接条件中包括specimen.snop_axis的逻辑（等于或不等于），如下所示：

SELECT
      *
FROM specimen
      JOIN topography_index
                  ON SUBSTRING(specimen.topography_index, 2, 2) =
                        topography_index.topography_index_code
      LEFT JOIN morphology
                  ON specimen.snop_axis = 'M' AND morphology.morphology_code = specimen.snop_code
      LEFT JOIN functions
                  ON specimen.snop_axis <>'M' AND functions.functions_code = specimen.snop_code
      LEFT JOIN specimen_image_lookup
                  ON specimen_image_lookup.specimen_fk = specimen.specimen_pk
      LEFT JOIN image
                  ON image.image_pk = specimen_image_lookup.image_fk
WHERE specimen.specimen_pk = '$specimen'
;

关于mysql - MySQL-具有IF条件的联接表，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/26091845/