Question

I have this table of prices

type | year | price
===================
A    | 2002 | 200
B    | 2002 | 120
C    | 2002 | 126
B    | 1999 | 100
A    | 2004 | 202
A    | 2005 | 206

I need a SELECT query to produce 2d array of prices of type A and B in every year they appear, having NULL if there is no data for the given type.

year | price A | price B
========================
1999 | NULL    | 100
2002 | 200     | 120
2004 | 202     | NULL
2005 | 206     | NULL

Answer 1

This is basically a PIVOT but MySQL does not have a PIVOT function so you can replicate it using a CASE statement with an aggregate function. If you have a known number of type values, then you can hard-code the values:

select year,
  avg(case when type='A' then price end) PriceA,
  avg(case when type='B' then price end) PriceB
from yourtable
group by year

See SQL Fiddle with Demo

Result:

| YEAR | PRICEA | PRICEB |
--------------------------
| 1999 | (null) |    100 |
| 2002 |    200 |    120 |
| 2004 |    202 | (null) |
| 2005 |    206 | (null) |

If you have an unknown number of types then you can use a prepared statement:

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'avg(case when type = ''',
      type,
      ''' then price end) AS Price',
      type
    )
  ) INTO @sql
FROM yourtable;

SET @sql = CONCAT('SELECT year, ', @sql, ' 
                  FROM yourtable 
                   GROUP BY year');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

See SQL Fiddle with Demo