Question

我想根据列优先级

加入2个表

离。假设Table1有六列（Col1，Col2，Col3，Col4，Col5，Col6）如果我想将表1加入table2（Col1，Col2，Col3，Col4，Col5，Col7），它应该

否则

Select Table2.col7
where 
first check col1 , col2 and col3 if match found no need go check more 
second check col1 , col2  if match found no need go check more 
third check col1  if match found no need go check more 
last ignore all col1 , col2 and col3 
AND Table1.Col4=Table2.Col4
AND Table1.Col5=Table2.Col5

我可能不清楚我的话，如果有任何疑虑请大声说出来

Answer 1

Select t2.col7
  from Table1 t1 inner join Table2 t2
    on
  case 
    when t1.col1 = t2.col1 then 1
    when t1.col2 = t2.col2 then 1
    when t1.col3 = t2.col3 then 1
    when t1.Col4=t2.Col4
       and t1.Col5=t2.Col5 then 1
    else 0 end = 1
  ;

Answer 2

你不能告诉SQL首先尝试加入某个条件，以防它找不到匹配继续搜索。您可以做的是加入所有允许的组合（在您的情况下匹配col4和col5）然后对您的匹配进行排名（这样col1和col2和col3的匹配被认为是最好的等等）。然后只保留最佳匹配：

select col7
from
(
  select 
    t1.*, 
    t2.*,
    row_number() over 
    (
      partition by t1.col4, t1.col5
      order by case 
        when t2.col1 = t1.col1 and t2.col2 = t1.col2 and t2.col3 = t1.col3 then 1
        when t2.col1 = t1.col1 and t2.col2 = t1.col2 then 2
        when t2.col1 = t1.col1 then 3
        else 4
    ) as rn
  from table1 t1
  join table2 t2 on t2.col4 = t1.col4 and t2.col5 = t1.col5
)
where rn = 1;

基于列优先级加入

2 个答案: