对一个表的复杂SQL查询

Question

There is a table:

event |id |timestamp
---------------------
event1|001|21-03-15
event2|001|22-03-15
event1|002|23-03-15
event2|002|24-03-15

What should be a request to display the result:

id |event1  |event2  |
----------------------
001|21-03-15|22-03-15|
002|23-03-15|24-03-15|

I think you first need to make a selection of unique id :

SELECT id FROM test GROUP BY id;

And then something like this:

SELECT timestamp
FROM   ... 
WHERE id IN (SELECT id FROM test GROUP BY id) AND event='event1';

Events are known in advance ('event1', 'event2') . If there are recurring events under one id, with different or the same timestamp, add columns to the result, for example:

id |event1  |event2  |event1  |event2  |
----------------------------------------
001|21-03-15|22-03-15|23-03-15|23-03-15|
002|23-03-15|24-03-15|NULL    |NULL    |

Answer 1

You are looking for a simple "pivot" or "crosstab" trick:

SELECT id
     , min(CASE event WHEN 'event1' THEN timestamp END) AS event1
     , min(CASE event WHEN 'event2' THEN timestamp END) AS event2
FROM   test
GROUP  BY id
ORDER  BY id;

SQL Fiddle.

Answer 2

I found a solution, query is looking like this:

SELECT A.id,GROUP_CONCAT(B.timestamp) AS event1, GROUP_CONCAT(C.timestamp) AS event2 FROM (select distinct id from test) A
   LEFT JOIN test B ON B.id=A.id and B.event='event1'
   LEFT JOIN test C ON C.id=A.id and C.event='event2' GROUP BY A.id

Answer 3

You can join records that have event1 with records that have event2 :

SELECT t1.id as id, t1.timestamp as event1, t2.timestamp as event2 
FROM test t1 LEFT JOIN test t2
    ON t1.id = t2.id AND t1.event = 'event1' and t2.event = 'event2'

The query assumes that you always have event1 , so LEFT JOIN was used.

If you need to handle cases where only event2 is available, you can emulate FULL OUTER JOIN as described in Full Outer Join in MySQL