我想从表a中得到行，唯一的\u键列在表b中，同时，唯一的\u键在表c中不存在。所以如果我写表格，

table a 
+----+------+-----------+
| id | Name | int_value |
+----+------+-----------+
| a1 | aa   |         1 |
| a2 | bb   |         2 |
| a3 | cc   |         5 |
+----+------+-----------+
table b
+----+------+-----------+
| id | Name |unique_key |
+----+------+-----------+
| a1 | aa   |        u1 |
| a2 | bb   |        u2 |
| a3 | cc   |        u5 |
+----+------+-----------+
table c 
+----+------+-----------+
| id | Name |unique_key |
+----+------+-----------+
| c1 | aa   |        u1 |
| c2 | bb   |        u2 |
| c3 | cc   |        u3 |
+----+------+-----------+

因此，我想

id name int_value b.unique_key
a3 cc   5         u5

因为表c中不存在u5。我尝试了这些sql，但都没有得到正确的结果。你会怎么做？

SELECT a.*, b.unique_key
FROM a
LEFT JOIN ( SELECT id FROM b ) b
ON a.id = b.id
LEFT JOIN ( SELECT unique_key FROM c ) c
ON b.unique_key <> c.unique_key;

SELECT a.*, b.unique_key, c.unique_key
FROM a
LEFT JOIN ( SELECT id FROM b ) b
ON a.id = b.id
LEFT JOIN ( SELECT unique_key FROM c ) c
ON b.unique_key = c.unique_key
WHERE c.unique_key IS NULL;