SQL语法连接表

Question

我有一个SQL语法问题。

这里是我的表：

default_caps: 
+-------+------------+------+-----+---------+----------------+ 
| Field | Type  | Null | Key | Default | Extra   | 
+-------+------------+------+-----+---------+----------------+ 
| idx | int(11) | NO | PRI | NULL | auto_increment | 
| tab | int(11) | NO |  | NULL |    | 
| cap | tinyint(1) | NO |  | 0  |    | 
| value | double  | YES |  | NULL |    | 
+-------+------------+------+-----+---------+----------------+ 

convertEntries: 
+----------------+------------+------+-----+---------+----------------+ 
| Field   | Type  | Null | Key | Default | Extra   | 
+----------------+------------+------+-----+---------+----------------+ 
| idx   | int(11) | NO | PRI | NULL | auto_increment | 
| convertTable | int(3)  | NO |  | 0  |    | 
| convertEntry | int(2)  | NO |  | 0  |    | 
| cap   | tinyint(1) | NO |  | 1  |    | 
| capTable  | int(2)  | NO |  | 1  |    | 
+----------------+------------+------+-----+---------+----------------+ 

我想选择convertEntries行，结果中的每一行中，我想包括default_caps.value这样default_caps.tab等于convertEntries.capTable，和default_caps.cap等于convertEntries.cap。

所以，如果default_caps看起来是这样的：

+-----+-----+-----+---------+ 
| idx | tab | cap | value | 
+-----+-----+-----+---------+ 
| 1 | 1 | 0 | 8650.75 | 
| 2 | 1 | 1 |  50 | 
| 3 | 1 | 7 |  350 | 
| 2 | 2 | 0 | 9000 | 
| 2 | 2 | 1 |  100 | 
| 3 | 2 | 7 |  200 | 
+-----+-----+-----+---------+ 

而且convertEntries看起来像这样（省略了一些列）：

+-----+----------+-----+ 
| idx | capTable | cap | 
+-----+----------+-----+ 
| 1 |  1 | 1 | 
| 2 |  1 | 1 | 
| 3 |  1 | 1 | 
| 4 |  1 | 1 | 
| 5 |  1 | 1 | 
| 6 |  1 | 1 | 
| 7 |  1 | 7 | 
| 8 |  1 | 7 | 
| 9 |  1 | 7 | 
| 10 |  1 | 0 | 
| 11 |  1 | 0 | 
| 12 |  1 | 0 | 
+-----+----------+-----+ 

我想这样的结果：

+-----+----------+-----+---------+ 
| idx | capTable | cap | value | 
+-----+----------+-----+---------+ 
| 1 |  1 | 1 |  50 | 
| 2 |  1 | 1 |  50 | 
| 3 |  1 | 1 |  50 | 
| 4 |  1 | 1 |  50 | 
| 5 |  1 | 1 |  50 | 
| 6 |  1 | 1 |  50 | 
| 7 |  1 | 7 |  350 | 
| 8 |  1 | 7 |  350 | 
| 9 |  1 | 7 |  350 | 
| 10 |  1 | 0 | 8650.75 | 
| 11 |  1 | 0 | 8650.75 | 
| 12 |  1 | 0 | 8650.75 | 
+-----+----------+-----+---------+ 

谢谢！

Answer 1

SELECT convertEntries.idx, convertEntries.capTable, convertEntries.cap, 
    default_caps.value 
FROM convertEntries 
LEFT JOIN default_caps 
    ON convertEntries.cap = default_caps.cap 
    AND convertEntries.capTable = default_caps.tab 

Answer 2

select 
    a.idx, 
    a.cap, 
    a.capTable, 
    b.Value 
from convertEntries a 
inner join default_caps b on a.tab = b.captable 
          and a.cap = b.cap 

Answer 3

这是一个简单的加入，依我看

From ConvertEntries e 
    Left Join default_caps c on e.tab=c.captable and e.cap=c.cap 

我做它作为一个左连接，因为架构很可能有数据的问题。如果您内部连接并且default_caps中不存在适当的查找，则该记录将被忽略。

Answer 4

我会建议使用更有意义的列名，但是：

SELECT convertEntries.idx 
    , convertEntries.capTable 
    , convertEntries.cap 
    , default_caps.value 
FROM convertEntries 
    INNER JOIN default_caps 
    ON default_caps.tab = convertEntries.capTable 
     AND default_caps.cap = convertEntries.cap