MySQL -带有IF条件的连接表

Question

我有以下查询，我想用一个IF条件修改这个查询，如下所示：

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'

如何修改此查询以包含此if条件？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

Answer 1

将specimen.snop_axis (等于或，不等于)的逻辑包含到两个表的联接条件中，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'
;

Answer 2

SQL查询只能返回一组固定的列。您可以使用动态SQL来做您想做的事情，但不能使用常规的SQL查询。

当样本不匹配时，您可以添加额外的列，并让它们成为NULL：

from条款将继续：

left join
image
on image.image_pk = specimen_image_lookup.image_fk left join
morphology
on morphology.morphology_code = specimen.snop_code and specimen.snop_axis = 'M' left join
. . . 

我不太清楚你的join是如何继续的。但是如果包含额外的条件，那么不匹配的样本将具有列的NULL值。