用于从其他表中查找数据的 SQL / Oracle SQL 代码

Question

如何编写查询以从其他表中获取现有表中每一列的数据。

Answer 1

你可以在system_code上left join三次：

select
    d.id,
    s_cat.full_name cat_code_full_name,
    s_group.full_name group_code_full_name,
    s_other.full_name other_code_full_name
from data_table d 
left join system_code s_cat
    on s_cat.value = d.cat_code and s.code = 1
left join system_code s_group
    on s_group.value = d.group_code and s_group.code = 2
left join system_code s_other
    on s_other.value = d.other_code and s_other.code = 3

为避免重复连接，另一种解决方案是进行条件聚合：

select
    d.id,
    max(case when s.value = d.cat_code   and s.code = 1 then s.full_name end) cat_code_full_name,
    max(case when s.value = d.group_code and s.code = 2 then s.full_name end) group_code_full_name,
    max(case when s.value = d.other_code and s.code = 3 then s.full_name end) other_code_full_name
from data_table d 
left join system_code s on s.value in (d.cat_code, d.group_code, d.other_code)
gtoup by d.id

Answer 2

我想您在这里面临的问题是，如何获取所有 3 列的全名。方法1是三次加入SYSTEM_CODE表——

SELECT DT.ID
      ,SC1.FULL_NAME CAT_CODE_FULL_NAME
      ,SC2.FULL_NAME GROUP_CODE_FULL_NAME
      ,SC3.FULL_NAME OTHER_CODE_FULL_NAME
      ,DT.PRODUCT
FROM DATA_TABLE DT
JOIN SYSTEM_CODE SC1 ON SC1.VALUE = DT.CAT_CODE
JOIN SYSTEM_CODE SC2 ON SC2.VALUE = DT.CAT_CODE
JOIN SYSTEM_CODE SC3 ON SC3.VALUE = DT.CAT_CODE