PostgreSQL: how to combine multiple rows?

I have a table like this to save the results of a medical checkup and the date of the report sent and the result. Actually the date sent is based on the clinic_visit date. A client can have one or more reports (date may varies)

---------------------------------------
|  client_id  |  date_sent  | result |
---------------------------------------
| 1           |   2001      |    A   |
| 1           |   2002      |    B   |
| 2           |   2002      |    D   |
| 3           |   2001      |    A   |
| 3           |   2003      |    C   |
| 3           |   2005      |    E   |
| 4           |   2002      |    D   |
| 4           |   2004      |    E   |
| 5           |   2004      |    B   |
---------------------------------------

I want to extract the following report from the above data.

---------------------------------------------------
|  client_id  |  result1  |  result2  |   resut3  |
---------------------------------------------------
|      1      |    A      |    B      |           |
|      2      |    D      |           |           |
|      3      |    A      |    C开发者_运维问答      |     E     |
|      4      |    D      |    E      |           |
|      5      |    B      |           |           |
---------------------------------------------------

I'm working on Postgresql. the "crosstab" function won't work here because the "date_sent" is not consistent for each client.

Can anyone please give a rough idea how it should be queried?

I suggest the following approach:

SELECT client_id, array_agg(result) AS results
    FROM labresults
    GROUP BY client_id;

It's not exactly the same output format, but it will give you the same information much faster and cleaner.

If you want the results in separate columns, you can always do this:

SELECT client_id,
       results[1] AS result1,
       results[2] AS result2,
       results[3] AS result3
FROM
(
    SELECT client_id, array_agg(result) AS results
        FROM labresults
        GROUP BY client_id 
) AS r
ORDER BY client_id;

although that will obviously introduce a hardcoded number of possible results.

While I was reading about "simulating row_number", I tried to figure out another way to do this.

SELECT client_id,  
       MAX( CASE seq WHEN 1 THEN result ELSE '' END ) AS result1,  
       MAX( CASE seq WHEN 2 THEN result ELSE '' END ) AS result2,  
       MAX( CASE seq WHEN 3 THEN result ELSE '' END ) AS result3,  
       MAX( CASE seq WHEN 4 THEN result ELSE '' END ) AS result4,  
       MAX( CASE seq WHEN 5 THEN result ELSE '' END ) AS result5  
FROM ( SELECT p1.client_id, 
              p1.result,  
              ( SELECT COUNT(*)  
                FROM labresults p2  
                WHERE p2.client_id = p1.client_id  
                AND p2.result <= p1.result )  
       FROM labresults p1 
) D ( client_id, result, seq )  
GROUP BY client_id;  

but the query took 10 minutes (500,000 ms++). for 30,000 records. This is too long..