mysql：多表联合查询保留空值问题

[bao86]

现有三表A、B、C，三表均只有一字段i，A表里有记录{1,2,3,4,5,6}，B表有记录{1,2,3}，C表有记录{4,5,6}。
怎样才能查询得到记录类似：

A.i B.i C.i
1 1 -1
2 2 -1
3 3 -1
4 -1 4
5 -1 5
6 -1 6

注：所用数据库为mysql，建表及插入数据语句如下：

CREATE TABLE A(i INT);
INSERT A VALUES (1),(2),(3),(4),(5),(6);
CREATE TABLE B(i INT);
INSERT B VALUES (1),(2),(3);
CREATE TABLE C(i INT);
INSERT C VALUES (4),(5),(6);

[wwwwb]

select a.*,IFNULL(b.i,-1) ,IFNULL(c.i,-1)


from a left join b on a.i=b.i
left join c on a.i=b.i

[bao86]

[Quote=引用 1 楼 wwwwb 的回复:]
select a.*,IFNULL(b.i,-1) ,IFNULL(c.i,-1)


from a left join b on a.i=b.i
left join c on a.i=b.i
[/Quote]

用if方法可以实现：代码如下：

SELECT DISTINCT a.i AS 'a.i',IF(b.i!=a.i,-1,b.i) AS 'b.i',IF(c.i!=a.i,-1,c.i) AS 'c.i' 
FROM a,b,c 
WHERE a.i=b.i AND a.i=c.i OR a.i=c.i AND a.i!=b.i OR a.i=b.i AND a.i!=c.i

能否直接用查询条件而不是用if或ifnull进行筛选得到目标结果呢？

[lxq19851204]

Select A.i,if(B.i<>'',B.i,-1) as "B.i",if(c.i<>'',c.i,-1) as "C.i"
From A Left Join B on A.i=B.i
Left JOin C on A.i=C.i

[wwwwb]

用LEFT JOIN 是比较保险的，你用的是内连接

[bao86]

谢谢！一楼和三楼。

[ACMAIN_CHM]

mysql> select a.i,ifnull(b.i,-1) ,ifnull(c.i,-1)
    -> from a left join b on a.i=b.i
    ->  left join c on a.i=c.i;
+------+----------------+----------------+
| i    | ifnull(b.i,-1) | ifnull(c.i,-1) |
+------+----------------+----------------+
|    1 |              1 |             -1 |
|    2 |              2 |             -1 |
|    3 |              3 |             -1 |
|    4 |             -1 |              4 |
|    5 |             -1 |              5 |
|    6 |             -1 |              6 |
+------+----------------+----------------+
6 rows in set (0.06 sec)

mysql>