Erwin Brandstetter

一 标准SQL 版本使用 boolean logic :

SELECT company_name
     , COUNT(action = 'EMAIL' OR NULL) AS "Email"
     , COUNT(action = 'PRINT' AND pagecount = 1 OR NULL) AS "Print 1 pages"
     , COUNT(action = 'PRINT' AND pagecount = 2 OR NULL) AS "Print 2 pages"
     , COUNT(action = 'PRINT' AND pagecount = 3 OR NULL) AS "Print 3 pages"
FROM   tbl
GROUP  BY company_name;

SQL Fiddle.

怎么用?

TRUE OR NULL 产量 TRUE .
FALSE OR NULL 产量 NULL .
NULL OR NULL 产量 无效的 .
和 COUNT 仅统计非空值。VoRE