Question

I have two table a and b

表a

   ID 
    a
    b
    c

表b

   ID           Value
   a             1   
   b             2
   default       0

所以我想在完全匹配时在ID上连接两个表，否则使用默认值期望的结果

   ID           Value
   a             1   
   b             2
   c             0

Answer 1

为此目的使用LEFT OUTER JOIN，如

select t1.ID, COALESCE(t2.Value, 0) as Value
from tablea t1 
left join tableb t2 on t1.ID = t2.ID;

Answer 2

试试这个：

SELECT a.ID, b.Value
FROM a
INNER JOIN b
ON a.ID = b.ID
UNION ALL
SELECT a.ID, b.Value
FROM a
CROSS JOIN b
WHERE
    a.id <> b.id

这将为您提供所有匹配的ID，然后CROSS JOIN查询的UNION应该为不匹配的ID提供默认值。

Answer 3

试试这个：

   SELECT t1.ID, 
          COALESCE(t2.Value, (SELECT Value FROM tableb t3 WHERE ID ='default')) AS Value
     FROM tablea t1 
LEFT JOIN tableb t2 on t1.ID = t2.ID;

在完全匹配时加入其他明智的加入默认值

3 个答案: