Question

我有多个select语句涉及许多表和联接。所有select语句都具有相同的标题。我正在尝试将所有这些组合成一个结果集。那么哪种方法更好的SQL UNION或MERGE？我知道UNION是A + B.因此，如果表A中的列为NULL并且它在表B中具有值，那么UNION会给我两行吗？所以，如果我想根据id将所有行组合成一行，我应该使用MERGE吗？我可以选择在SQL或SSIS中执行此操作。

SELECT ID, NAME, VitalName as VitalName FROM TABLE A
UNION
SELECT ID, NAME, VitalReadings as VitalName FROM TABLE B

表A

+----+------+-----------+
| ID | Name | VitalName |
+----+------+-----------+
|  1 | AAA  | HeartRate |
|  2 |      | Systolic  |
|  3 |      | Diastolic |
+----+------+-----------+

表B

+----+------+---------------+
| ID | Name | VitalReadings |
+----+------+---------------+
|  1 | AAA  | HeartRate     |
|  2 | BBB  | Systolic      |
+----+------+---------------+

预期结果

+----+------+---------------+
| ID | Name | VitalName     |
+----+------+---------------+
|  1 | AAA  | HeartRate     |
|  2 | BBB  | Systolic      |
|  3 |      | Diastolic     |
+----+------+---------------+

Answer 1

UNION和MERGE完全不同的概念，都不能解决您的问题。但可以使用FULL JOIN来实现此目的。

DECLARE @TableA TABLE (ID INT,  Name VARCHAR(10), VitalName VARCHAR(10))
INSERT INTO @TableA VALUES
(1,'AAA','HeartRate'),
(2,NULL,'Systolic'),
(3,NULL,'Diastolic')

DECLARE @TableB TABLE ( ID INT,  Name VARCHAR(10), VitalReadings VARCHAR(10))

INSERT INTO @TableB VALUES
(1,'AAA','HeartRate'),
(2,'BBB','Systolic')


SELECT 
    A.ID, 
    COALESCE(A.Name, B.Name) Name,
    COALESCE(A.VitalName, B.VitalReadings) VitalName
FROM 
    @TableA A 
    FULL JOIN @TableB B ON A.ID = B.ID

结果：

ID          Name       VitalName
----------- ---------- ----------
1           AAA        HeartRate
2           BBB        Systolic
3           NULL       Diastolic

Answer 2

GROUP BY UNION的结果。使用MAX()返回NAME：

select ID, MAX(NAME), VitalName 
from
(
    SELECT ID, NAME, VitalName as VitalName FROM TABLE A
    UNION ALL
    SELECT ID, NAME, VitalReadings as VitalName FROM TABLE B
) dt
group by ID, VitalName

Answer 3

使用union，然后使用select distinct删除重复。

e.g。 SELECT DISTINCT * FROM (SELECT ID, NAME, VitalName as VitalName FROM TABLE_A UNION SELECT ID, NAME, VitalReadings as VitalName FROM TABLE_B) as TABLE_C

SQL UNION和MERGE

3 个答案: