如何在SQL中合并超过两（3）个表

Question

In a Facebook-like notification system I have three tables, like this....

table1

________________________________
| id |  value_1  |  timestamp  |
|----|-----------|-------------|
| id |  value21  |  2014-05-21 |
| id |  value22  |  2014-05-22 |
|____|___________|_____________|

table2

________________________________
| id |  value_2  |  timestamp  |
|----|-----------|-------------|
| id |  value30  |  2014-05-30 |
|____|___________|_____________|

table3

________________________________
| id |  value_3  |  timestamp  |
|----|-----------|-------------|
| id |  value01  |  2014-05-01 |
| id |  value27  |  2014-05-27 |
|____|___________|_____________|

To return all these data in one row, I used SQL union...

(SELECT value_1  FROM table1)
UNION
(SELECT value_2  FROM table2)
UNION
(SELECT value_3  FROM table3)

...but it brings it all together in one column. Instead of that, I need the result to be in a table like this, in order of Timestamp

________________________________________________________
| id |  value_1  |  value_2  |  value_3  |  timestamp  |
|----|-----------|-----------|-----------|-------------|
| id |  null     |  null     |  value01  |  2014-05-01 |
| id |  value21  |  null     |  null     |  2014-05-21 |
| id |  value22  |  null     |  null     |  2014-05-22 |
| id |  null     |  null     |  value27  |  2014-05-27 |
| id |  null     |  value30  |  null     |  2014-05-30 |
|____|___________|___________|___________|_____________|

Is it possible to do, only with SQL without handing over the job to the PHP engine? Any ideas appreciated.

Answer 1

You can do this using union all and order by :

SELECT id, value_1, NULL as value_2, NULL as value_3, timestamp
FROM table1
UNION ALL
SELECT id, NULL, value_2, NULL, timestamp
FROM table2
UNION ALL
SELECT id, NULL, NULL, value_3, timestamp
FROM table3
ORDER BY timestamp;

Note that union all is more efficient than union because it does not return duplicates.

Answer 2

You need to add dummy columns in each part of the union:

SELECT value_1
     , cast(null as <type_of_value2>) as value_2
     , cast(null as <type_of_value3>) as value_3
     , ts 
FROM table1 
UNION 
SELECT cast(null as <type_of_value1>) as value_1
     , value_2
     , cast(null as <type_of_value3>) as value_3 
     , ts
FROM table2 
UNION 
SELECT cast(null as <type_of_value1>) as value_1
     , cast(null as <type_of_value2>) as value_2
     , value_3
     , ts 
FROM table3
ORDER BY ts