左连接表没有重复

Question

我有两张桌子 -

T1：

col_1        col_2        col_3
10001        apple        3
10001        orange       2
10001        grapes       5

T2：

col_1        col_2        col_3        col_4
10001        apple        3            123
10001        orange       2            345

哪个SQL查询会加入这两个表;给我这样的结果：

col_1        col_2        col_3        col_4
10001        apple        3            123
10001        orange       2            345
10001        grapes       5     

Answer 1

连接表，然后使用COALESCE获取第一个非空数据列

SELECT COALESCE(t1.col_1,t2.col_1) col_1,
   COALESCE(t1.col_2,t2.col_2) col_2,
   COALESCE(t1.col_3,t2.col_3) col_3,
   COALESCE(t2.col_4,'') col_4,
FROM t1
FULL JOIN t2 ON t1 ON t1.col_1 = t2.col1 AND t1.col_2 = t2.col_2 AND t1.col_3 = t2.col_3

Answer 2

(SELECT col_1, col_2, col_3 
FROM t1)

UNION DISTINCT

(SELECT col_1, col_2, col_3 
FROM t2)

Answer 3

SELECT t1.col_1, t1.col_2, t1.col3, t2.col_4
FROM t1
LEFT OUTER JOIN t2 ON t1.col_1 = t2.col_1 AND t1.col_2 = t2.col_2 AND t1.col_3 = t2.col_3

Answer 4

SELECT t1.*, 
       col_4 
FROM   t1 
       LEFT JOIN t2 using(col_2); 

如果col_1和col_2标识唯一条目，则将最后一行更改为：

LEFT JOIN t2 using(col_1,col_2);

如果col_1，col_2，col_3标识唯一条目，则将最后一行更改为：

LEFT JOIN t2 using(col_1,col_2,col_3);