I believe this option is better for performance.

SELECT ID,

SUM( CASE WHEN Type = 'a' THEN Amount ELSE 0 END) AS Amount_A,

SUM( CASE WHEN Type = 'b' THEN Amount ELSE 0 END) AS Amount_B

FROM MyTable

GROUP BY ID

ORDER BY ID

For more information, you can check Cross Tabs and Pivots, Part 1 – Converting Rows to Columns[/url]