SQL Server 2008将两个表合并为一个，但得到重复的行

Question

I need to combine two tables by SQL server 2008.

table1:

 col  col1  col2_a
 yyy  ddd   1589

table2:

 col  col1  col2_b
 yyy  ddd   6231

The tables have same values for col and col1, only col2 are different. I need to merge them into one table.

SELECT a.col , a.col1, a.col2_a, b.col2_b
 FROM table1 as a, table2 as b
 WHERE a.col = b.col AND a.col1 = b.col1

But, this give me 4 duplicated rows for each col and col1 combination.

Any help would be appreciated.

I expect :

 col  col1  cola  colb
 yyy  ddd   1589  6231

Answer 1

If you just plain want to eliminate exact duplicate rows , you can simply use DISTINCT ;

SELECT DISTINCT a.col , a.col1, a.col2_a, b.col2_b
FROM table1 as a
JOIN table2 as b
  ON a.col = b.col AND a.col1 = b.col1

If you instead want only one value (for example the max one) from table2 even though several rows match, you can select rows from table1 and only the max matching value of b.col2 from table2;

SELECT DISTINCT a.col , a.col1, a.col2_a, MAX(b.col2_b) col2_b
FROM table1 as a
JOIN table2 as b
  ON a.col = b.col AND a.col1 = b.col1
GROUP BY a.col, a.col1, a.col2