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Question

我有 2 张桌子，看起来像：

Table A
key1 key2 key3
1    a    a
2    a    a
3    a    a

Table B
key1 key2 key3
1    xxx  zzz
2    yyy  www

现在我想在 A 和 B 中有一个大表，其中 A.key1 匹配 B.key1 粘贴 B 的内容，否则如果没有 key1 匹配粘贴默认值

例如

Key1 Key2 Key3 Key5 Key6
1    a    a    xxx  zzz            
2    a    a    yyy  www               
3    a    a    0    0       

什么是查询？

Answer 1

你需要左连接：

select
  a.key1,
  a.key2,
  a.key3,
  ifnull(b.key2, 0) as key4,
  ifnull(b.key3, 3) as key5
from
  tableA a
left join
  tableB b on b.key1=a.key1

Answer 2

select * from a left outer join b on a.key1 = b.key1

（这将为没有来自 b 的匹配行的 b 列提供空值）

Answer 3

select key1, key2, key3, 
    case key5 when null then default(key5) else key5 as key5,  
    case key6 when null then default(key6) else key6 as key6
from a
left outer join b on a.key1 = b.key1

编辑

SQLFiddle 示例

select a.key1, a.key2, a.key3, 
       ifnull(b.key2, 0) as key5,  
       ifnull(b.key2, 3) as key6
from a
left outer join b on a.key1 = b.key1